# Event Management

While Xlib allows the reading and processing of events anywhere in an application, widgets in the X Toolkit neither directly read events nor grab the server or pointer. Widgets register procedures that are to be called when an event or class of events occurs in that widget.

A typical application consists of startup code followed by an event loop that reads events and dispatches them by calling the procedures that widgets have registered. The default event loop provided by the Intrinsics is .

The event manager is a collection of functions to perform the following tasks:

- Add or remove event sources other than X server events (in particular, timer interrupts, file input, or POSIX signals).

- Query the status of event sources.

- Add or remove procedures to be called when an event occurs for a particular widget.

- Enable and disable the dispatching of user-initiated events (keyboard and pointer events) for a particular widget.

- Constrain the dispatching of events to a cascade of pop-up widgets.

- Register procedures to be called when specific events arrive.

- Register procedures to be called when the Intrinsics will block.

- Enable safe operation in a multi-threaded environment.

Most widgets do not need to call any of the event handler functions explicitly. The normal interface to X events is through the higher-level translation manager, which maps sequences of X events, with modifiers, into procedure calls. Applications rarely use any of the event manager routines besides .

## Adding and Deleting Additional Event Sources

While most applications are driven only by X events, some applications need to incorporate other sources of input into the Intrinsics event-handling mechanism. The event manager provides routines to integrate notification of timer events and file data pending into this mechanism.

The next section describes functions that provide input gathering from files. The application registers the files with the Intrinsics read routine. When input is pending on one of the files, the registered callback procedures are invoked.

### Adding and Removing Input Sources

To register a new file as an input source for a given application context, use .

XtInputId `XtAppAddInput` XtAppContext app_context int source XtPointer condition XtInputCallbackProc proc XtPointer client_data

app_context

  Specifies the application context that identifies the application.

source

  Specifies the source file descriptor on a POSIX-based system or other operating-system-dependent device specification.

condition

  Specifies the mask that indicates a read, write, or exception condition or some other operating-system-dependent condition.

proc

  Specifies the procedure to be called when the condition is found.

client_data

  Specifies an argument passed to the specified procedure when it is called.

The function registers with the Intrinsics read routine a new source of events, which is usually file input but can also be file output. Note that file should be loosely interpreted to mean any sink or source of data. also specifies the conditions under which the source can generate events. When an event is pending on this source, the callback procedure is called.

The legal values for the condition argument are operating-system-dependent. On a POSIX-based system, source is a file number and the condition is some union of the following:

XtInputReadMask

  Specifies that proc is to be called when source has data to be read.

XtInputWriteMask

  Specifies that proc is to be called when source is ready for writing.

XtInputExceptMask

  Specifies that proc is to be called when source has exception data.

Callback procedure pointers used to handle file events are of type .

typedef void `(*XtInputCallbackProc)` XtPointer client_data int *source XtInputId *id

client_data

  Passes the client data argument that was registered for this procedure in `XtAppAddInput`.

source

  Passes the source file descriptor generating the event.

id

  Passes the id returned from the corresponding call.

See for information regarding the use of in multiple threads.

To discontinue a source of input, use .

void `XtRemoveInput` XtInputId id

id

  Specifies the id returned from the corresponding call.

The function causes the Intrinsics read routine to stop watching for events from the file source specified by id.

See for information regarding the use of in multiple threads.

### Adding and Removing Blocking Notifications

Occasionally it is desirable for an application to receive notification when the Intrinsics event manager detects no pending input from file sources and no pending input from X server event sources and is about to block in an operating system call.

To register a hook that is called immediately prior to event blocking, use .

XtBlockHookId `XtAppAddBlockHook` XtAppContext app_context XtBlockHookProc proc XtPointer client_data

app_context

  Specifies the application context that identifies the application.

proc

  Specifies the procedure to be called before blocking.

client_data

  Specifies an argument passed to the specified procedure when it is called.

The function registers the specified procedure and returns an identifier for it. The hook procedure proc is called at any time in the future when the Intrinsics are about to block pending some input.

The procedure pointers used to provide notification of event blocking are of type .

typedef void *`XtBlockHookProc` XtPointer client_data

client_data

  Passes the client data argument that was registered for this procedure in `XtAppAddBlockHook`.

To discontinue the use of a procedure for blocking notification, use .

void `XtRemoveBlockHook` XtBlockHookId id

id

  Specifies the identifier returned from the corresponding call to .

The function removes the specified procedure from the list of procedures that are called by the Intrinsics read routine before blocking on event sources.

### Adding and Removing Timeouts

The timeout facility notifies the application or the widget through a callback procedure that a specified time interval has elapsed. Timeout values are uniquely identified by an interval id.

To register a timeout callback, use .

XtIntervalId `XtAppAddTimeOut` XtAppContext app_context unsigned long interval XtTimerCallbackProc proc XtPointer client_data

app_context

  Specifies the application context for which the timer is to be set.

interval

  Specifies the time interval in milliseconds.

proc

  Specifies the procedure to be called when the time expires.

client_data

  Specifies an argument passed to the specified procedure when it is called.

The function creates a timeout and returns an identifier for it. The timeout value is set to interval. The callback procedure proc is called when or is next called after the time interval elapses, and then the timeout is removed.

Callback procedure pointers used with timeouts are of type .

typedef void *`XtTimerCallbackProc` XtPointer client_data XtIntervalId *timer

client_data

  Passes the client data argument that was registered for this procedure in `XtAppAddTimeOut`.

timer

  Passes the id returned from the corresponding call.

See for information regarding the use of in multiple threads.

To clear a timeout value, use .

void `XtRemoveTimeOut` XtIntervalId timer

timer

  Specifies the id for the timeout request to be cleared.

The function removes the pending timeout. Note that timeouts are automatically removed once they trigger.

Please refer to Section 7.12 for information regarding the use of in multiple threads.

### Adding and Removing Signal Callbacks

The signal facility notifies the application or the widget through a callback procedure that a signal or other external asynchronous event has occurred. The registered callback procedures are uniquely identified by a signal id.

Prior to establishing a signal handler, the application or widget should call and store the resulting identifier in a place accessible to the signal handler. When a signal arrives, the signal handler should call to notify the Intrinsics that a signal has occurred. To register a signal callback use .

XtSignalId `XtAppAddSignal` XtAppContext app_context XtSignalCallbackProc proc XtPointer client_data

app_context

  Specifies the application context that identifies the application.

proc

  Specifies the procedure to be called when the signal is noticed.

client_data

  Specifies an argument passed to the specified procedure when it is called.

The callback procedure pointers used to handle signal events are of type .

typedef void `(*XtSignalCallbackProc)` XtPointer client_data XtSignalId *id

client_data

  Passes the client data argument that was registered for this procedure in .

id

  Passes the id returned from the corresponding call.

To notify the Intrinsics that a signal has occurred, use .

void `XtNoticeSignal` XtSignalId id

id

  Specifies the id returned from the corresponding call.

On a POSIX-based system, is the only Intrinsics function that can safely be called from a signal handler. If is invoked multiple times before the Intrinsics are able to invoke the registered callback, the callback is only called once. Logically, the Intrinsics maintain “pending” flag for each registered callback. This flag is initially `False` and is set to `True` by . When or (with a mask including `XtIMSignal`) is called, all registered callbacks with “pending” `True` are invoked and the flags are reset to `False`.

If the signal handler wants to track how many times the signal has been raised, it can keep its own private counter. Typically the handler would not do any other work; the callback does the actual processing for the signal. The Intrinsics never block signals from being raised, so if a given signal can be raised multiple times before the Intrinsics can invoke the callback for that signal, the callback must be designed to deal with this. In another case, a signal might be raised just after the Intrinsics sets the pending flag to `False` but before the callback can get control, in which case the pending flag will still be `True` after the callback returns, and the Intrinsics will invoke the callback again, even though all of the signal raises have been handled. The callback must also be prepared to handle this case.

To remove a registered signal callback, call .

void `XtRemoveSignal` XtSignalId id

id

  Specifies the id returned by the corresponding call to .

The client should typically disable the source of the signal before calling . If the signal could have been raised again before the source was disabled and the client wants to process it, then after disabling the source but before calling the client can test for signals with and process them by calling with the mask `XtIMSignal`.

## Constraining Events to a Cascade of Widgets

Modal widgets are widgets that, except for the input directed to them, lock out user input to the application.

When a modal menu or modal dialog box is popped up using , user events (keyboard and pointer events) that occur outside the modal widget should be delivered to the modal widget or ignored. In no case will user events be delivered to a widget outside the modal widget.

Menus can pop up submenus, and dialog boxes can pop up further dialog boxes to create a pop-up cascade. In this case, user events may be delivered to one of several modal widgets in the cascade.

Display-related events should be delivered outside the modal cascade so that exposure events and the like keep the application's display up-to-date. Any event that occurs within the cascade is delivered as usual. The user events delivered to the most recent spring-loaded shell in the cascade when they occur outside the cascade are called remap events and are `KeyPress`, `KeyRelease`, `ButtonPress`, and `ButtonRelease`. The user events ignored when they occur outside the cascade are `MotionNotify` and `EnterNotify`. All other events are delivered normally. In particular, note that this is one way in which widgets can receive `LeaveNotify` events without first receiving `EnterNotify` events; they should be prepared to deal with this, typically by ignoring any unmatched `LeaveNotify` events.

uses the and functions to constrain user events to a modal cascade and subsequently to remove a grab when the modal widget is popped down.

To constrain or redirect user input to a modal widget, use .

void `XtAddGrab` Widget w Boolean exclusive Boolean spring_loaded

w

  Specifies the widget to add to the modal cascade. Must be of class Core or any subclass thereof.

exclusive

  Specifies whether user events should be dispatched exclusively to this widget or also to previous widgets in the cascade.

spring_loaded

  Specifies whether this widget was popped up because the user pressed a pointer button.

The function appends the widget to the modal cascade and checks that exclusive is `True` if spring_loaded is `True`. If this condition is not met, generates a warning message.

The modal cascade is used by when it tries to dispatch a user event. When at least one modal widget is in the widget cascade, first determines if the event should be delivered. It starts at the most recent cascade entry and follows the cascade up to and including the most recent cascade entry added with the exclusive parameter `True`.

This subset of the modal cascade along with all descendants of these widgets comprise the active subset. User events that occur outside the widgets in this subset are ignored or remapped. Modal menus with submenus generally add a submenu widget to the cascade with exclusive `False`. Modal dialog boxes that need to restrict user input to the most deeply nested dialog box add a subdialog widget to the cascade with exclusive `True`. User events that occur within the active subset are delivered to the appropriate widget, which is usually a child or further descendant of the modal widget.

Regardless of where in the application they occur, remap events are always delivered to the most recent widget in the active subset of the cascade registered with spring_loaded `True`, if any such widget exists. If the event occurred in the active subset of the cascade but outside the spring-loaded widget, it is delivered normally before being delivered also to the spring-loaded widget. Regardless of where it is dispatched, the Intrinsics do not modify the contents of the event.

To remove the redirection of user input to a modal widget, use .

void `XtRemoveGrab` Widget w

w

  Specifies the widget to remove from the modal cascade.

The function removes widgets from the modal cascade starting at the most recent widget up to and including the specified widget. It issues a warning if the specified widget is not on the modal cascade.

### Requesting Key and Button Grabs

The Intrinsics provide a set of key and button grab interfaces that are parallel to those provided by Xlib and that allow the Intrinsics to modify event dispatching when necessary. X Toolkit applications and widgets that need to passively grab keys or buttons or actively grab the keyboard or pointer should use the following Intrinsics routines rather than the corresponding Xlib routines.

To passively grab a single key of the keyboard, use .

void `XtGrabKey` Widget widget KeyCode keycode Modifiers modifiers Boolean owner_events int pointer_mode int keyboard_mode

widget

  Specifies the widget in whose window the key is to be grabbed. Must be of class Core or any subclass thereof.

keycode
modifiers
owner_events
pointer_mode
keyboard_mode

  Specify arguments to `XGrabKey`; see Section 12.2 in Xlib — C Language X Interface.

calls `XGrabKey` specifying the widget's window as the grab window if the widget is realized. The remaining arguments are exactly as for `XGrabKey`. If the widget is not realized, or is later unrealized, the call to `XGrabKey` is performed (again) when the widget is realized and its window becomes mapped. In the future, if is called with a `KeyPress` event matching the specified keycode and modifiers (which may be `AnyKey` or `AnyModifier`, respectively) for the widget's window, the Intrinsics will call with the timestamp from the `KeyPress` event if either of the following conditions is true:

- There is a modal cascade and the widget is not in the active subset of the cascade and the keyboard was not previously grabbed, or

- `XFilterEvent` returns `True`.

To cancel a passive key grab, use .

void `XtUngrabKey` Widget widget KeyCode keycode Modifiers modifiers

widget

  Specifies the widget in whose window the key was grabbed.

keycode
modifiers

  Specify arguments to `XUngrabKey`; see Section 12.2 in Xlib — C Language X Interface.

The procedure calls `XUngrabKey` specifying the widget's window as the ungrab window if the widget is realized. The remaining arguments are exactly as for `XUngrabKey`. If the widget is not realized, removes a deferred request, if any, for the specified widget, keycode, and modifiers.

To actively grab the keyboard, use .

int `XtGrabKeyboard` Widget widget Boolean owner_events int pointer_mode int keyboard_mode Time time

widget

  Specifies the widget for whose window the keyboard is to be grabbed. Must be of class Core or any subclass thereof.

owner_events
pointer_mode
keyboard_mode
time

  Specify arguments to `XGrabKeyboard`; see Section 12.2 in Xlib — C Language X Interface.

If the specified widget is realized, calls `XGrabKeyboard` specifying the widget's window as the grab window. The remaining arguments and return value are exactly as for `XGrabKeyboard`. If the widget is not realized, immediately returns `GrabNotViewable`. No future automatic ungrab is implied by .

To cancel an active keyboard grab, use .

void `XtUngrabKeyboard` Widget widget Time time

widget

  Specifies the widget that has the active keyboard grab.

time

  Specifies the additional argument to `XUngrabKeyboard`; see Section 12.2 in Xlib — C Language X Interface.

calls `XUngrabKeyboard` with the specified time.

To passively grab a single pointer button, use .

void `XtGrabButton` Widget widget int button Modifiers modifiers Boolean owner_events unsigned int event_mask int pointer_mode int keyboard_mode Window confine_to Cursor cursor

widget

  Specifies the widget in whose window the button is to be grabbed. Must be of class Core or any subclass thereof.

button
modifiers
owner_events
event_mask
pointer_mode
keyboard_mode
confine_to
cursor

  Specify arguments to `XGrabButton`; see Section 12.1 in Xlib — C Language X Interface.

calls `XGrabButton` specifying the widget's window as the grab window if the widget is realized. The remaining arguments are exactly as for `XGrabButton`. If the widget is not realized, or is later unrealized, the call to `XGrabButton` is performed (again) when the widget is realized and its window becomes mapped. In the future, if is called with a `ButtonPress` event matching the specified button and modifiers (which may be `AnyButton` or `AnyModifier`, respectively) for the widget's window, the Intrinsics will call with the timestamp from the `ButtonPress` event if either of the following conditions is true:

- There is a modal cascade and the widget is not in the active subset of the cascade and the pointer was not previously grabbed, or

- `XFilterEvent` returns `True`.

To cancel a passive button grab, use .

void `XtUngrabButton` Widget widget unsigned int button Modifiers modifiers

widget

  Specifies the widget in whose window the button was grabbed.

button
modifiers

  Specify arguments to `XUngrabButton`; see Section 12.1 in Xlib — C Language X Interface.

The procedure calls `XUngrabButton` specifying the widget's window as the ungrab window if the widget is realized. The remaining arguments are exactly as for `XUngrabButton`. If the widget is not realized, removes a deferred request, if any, for the specified widget, button, and modifiers.

To actively grab the pointer, use .

int `XtGrabPointer` Widget widget Boolean owner_events unsigned int event_mask int pointer_mode int keyboard_mode Window confine_to Cursor cursor Time time

widget

  Specifies the widget for whose window the pointer is to be grabbed. Must be of class Core or any subclass thereof.

owner_events
event_mask
pointer_mode
keyboard_mode
confine_to
cursor
time

  Specify arguments to `XGrabPointer`; see Section 12.1 in Xlib — C Language X Interface.

If the specified widget is realized, calls `XGrabPointer`, specifying the widget's window as the grab window. The remaining arguments and return value are exactly as for `XGrabPointer`. If the widget is not realized, immediately returns `GrabNotViewable`. No future automatic ungrab is implied by .

To cancel an active pointer grab, use .

void `XtUngrabPointer` Widget widget Time time

widget

  Specifies the widget that has the active pointer grab.

time

  Specifies the time argument to `XUngrabPointer`; see Section 12.1 in Xlib — C Language X Interface.

calls `XUngrabPointer` with the specified time.

## Focusing Events on a Child

To redirect keyboard input to a normal descendant of a widget without calling `XSetInputFocus`, use .

void `XtSetKeyboardFocus` Widget subtree Widget descendent

subtree

  Specifies the subtree of the hierarchy for which the keyboard focus is to be set. Must be of class Core or any subclass thereof.

descendant

  Specifies either the normal (non-pop-up) descendant of subtree to which keyboard events are logically directed, or `None`. It is not an error to specify `None` when no input focus was previously set. Must be of class Object or any subclass thereof.

causes to remap keyboard events occurring within the specified subtree and dispatch them to the specified descendant widget or to an ancestor. If the descendant's class is not a subclass of Core, the descendant is replaced by its closest windowed ancestor.

When there is no modal cascade, keyboard events can be dispatched to a widget in one of five ways. Assume the server delivered the event to the window for widget E (because of X input focus, key or keyboard grabs, or pointer position).

- If neither E nor any of E's ancestors have redirected the keyboard focus, or if the event activated a grab for E as specified by a call to with any value of owner_events, or if the keyboard is actively grabbed by E with owner_events `False` via or on a previous key press, the event is dispatched to E.

- Beginning with the ancestor of E closest to the root that has redirected the keyboard focus or E if no such ancestor exists, if the target of that focus redirection has in turn redirected the keyboard focus, recursively follow this focus chain to find a widget F that has not redirected focus.

- - If E is the final focus target widget F or a descendant of F, the event is dispatched to E.

  - If E is not F, an ancestor of F, or a descendant of F, and the event activated a grab for E as specified by a call to for E, is called.

  - If E is an ancestor of F, and the event is a key press, and either

    - E has grabbed the key with and owner_events `False`, or

    - E has grabbed the key with and owner_events `True`, and the coordinates of the event are outside the rectangle specified by E's geometry, then the event is dispatched to E.

  - Otherwise, define A as the closest common ancestor of E and F:

    - If there is an active keyboard grab for any widget via either or on a previous key press, or if no widget between F and A (noninclusive) has grabbed the key and modifier combination with and any value of owner_events, the event is dispatched to F.

    - Else, the event is dispatched to the ancestor of F closest to A that has grabbed the key and modifier combination with .

When there is a modal cascade, if the final destination widget as identified above is in the active subset of the cascade, the event is dispatched; otherwise the event is remapped to a spring-loaded shell or discarded. Regardless of where it is dispatched, the Intrinsics do not modify the contents of the event.

When subtree or one of its descendants acquires the X input focus or the pointer moves into the subtree such that keyboard events would now be delivered to the subtree, a `FocusIn` event is generated for the descendant if `FocusChange` events have been selected by the descendant. Similarly, when subtree loses the X input focus or the keyboard focus for one of its ancestors, a `FocusOut` event is generated for descendant if `FocusChange` events have been selected by the descendant.

A widget tree may also actively manage the X server input focus. To do so, a widget class specifies an accept_focus procedure.

The accept_focus procedure pointer is of type .

typedef Boolean *`XtAcceptFocusProc` Widget w Time *time

w

  Specifies the widget.

time

  Specifies the X time of the event causing the accept focus.

Widgets that need the input focus can call `XSetInputFocus` explicitly, pursuant to the restrictions of the Inter-Client Communication Conventions Manual. To allow outside agents, such as the parent, to cause a widget to take the input focus, every widget exports an accept_focus procedure. The widget returns a value indicating whether it actually took the focus or not, so that the parent can give the focus to another widget. Widgets that need to know when they lose the input focus must use the Xlib focus notification mechanism explicitly (typically by specifying translations for `FocusIn` and `FocusOut` events). Widgets classes that never want the input focus should set the accept_focus field to NULL.

To call a widget's accept_focus procedure, use .

Boolean `XtCallAcceptFocus` Widget w Time *time

w

  Specifies the widget. Must be of class Core or any subclass thereof.

time

  Specifies the X time of the event that is causing the focus change.

The function calls the specified widget's accept_focus procedure, passing it the specified widget and time, and returns what the accept_focus procedure returns. If accept_focus is NULL, returns `False`.

### Events for Drawables That Are Not a Widget's Window

Sometimes an application must handle events for drawables that are not associated with widgets in its widget tree. Examples include handling `GraphicsExpose` and `NoExpose` events on Pixmaps, and handling `PropertyNotify` events on the root window.

To register a drawable with the Intrinsics event dispatching, use .

void `XtRegisterDrawable` Display *display Drawable drawable Widget widget

display

  Specifies the drawable's display.

drawable

  Specifies the drawable to register.

widget

  Specifies the widget to register the drawable for.

associates the specified drawable with the specified widget so that future calls to with the drawable will return the widget. The default event dispatcher will dispatch future events that arrive for the drawable to the widget in the same manner as events that contain the widget's window.

If the drawable is already registered with another widget, or if the drawable is the window of a widget in the client's widget tree, the results of calling are undefined.

To unregister a drawable with the Intrinsics event dispatching, use .

void `XtUnregisterDrawable` Display *display Drawable drawable

display

  Specifies the drawable's display.

drawable

  Specifies the drawable to unregister.

removes an association created with . If the drawable is the window of a widget in the client's widget tree the results of calling are undefined.

## Querying Event Sources

The event manager provides several functions to examine and read events (including file and timer events) that are in the queue. The next three functions are Intrinsics equivalents of the `XPending`, `XPeekEvent`, and `XNextEvent` Xlib calls.

To determine if there are any events on the input queue for a given application, use .

XtInputMask `XtAppPending` XtAppContext app_context

app_context

  Specifies the application context that identifies the application to check.

The function returns a nonzero value if there are events pending from the X server, timer pending, other input sources pending, or signal sources pending. The value returned is a bit mask that is the OR of `XtIMXEvent`, `XtIMTimer`, `XtIMAlternateInput`, and `XtIMSignal` (see `XtAppProcessEvent ).` If there are no events pending, flushes the output buffers of each Display in the application context and returns zero.

To return the event from the head of a given application's input queue without removing input from the queue, use .

Boolean `XtAppPeekEvent` XtAppContext app_context XEvent *event_return

app_context

  Specifies the application context that identifies the application.

event_return

  Returns the event information to the specified event structure.

If there is an X event in the queue, copies it into event_return and returns `True`. If no X input is on the queue, flushes the output buffers of each Display in the application context and blocks until some input is available (possibly calling some timeout callbacks in the interim). If the next available input is an X event, fills in event_return and returns `True`. Otherwise, the input is for an input source registered with , and returns `False`. The sample implementations provides XtAppPeekEvent as described. Timeout callbacks are called while blocking for input. If some input for an input source is available, will return `True` without returning an event.

To remove and return the event from the head of a given application's X event queue, use .

void `XtAppNextEvent` XtAppContext app_context XEvent *event_return

app_context

  Specifies the application context that identifies the application.

event_return

  Returns the event information to the specified event structure.

If the X event queue is empty, flushes the X output buffers of each Display in the application context and waits for an X event while looking at the other input sources and timeout values and calling any callback procedures triggered by them. This wait time can be used for background processing; see .

## Dispatching Events

The Intrinsics provide functions that dispatch events to widgets or other application code. Every client interested in X events on a widget uses to register which events it is interested in and a procedure (event handler) to be called when the event happens in that window. The translation manager automatically registers event handlers for widgets that use translation tables; see .

Applications that need direct control of the processing of different types of input should use .

void `XtAppProcessEvent` XtAppContext app_context XtInputMask mask

app_context

  Specifies the application context that identifies the application for which to process input.

mask

  Specifies what types of events to process. The mask is the bitwise inclusive OR of any combination of `XtIMXEvent`, `XtIMTimer`, `XtIMAlternateInput`, and `XtIMSignal`. As a convenience, `Intrinsic.h` defines the symbolic name `XtIMAll` to be the bitwise inclusive OR of these four event types.

The function processes one timer, input source, signal source, or X event. If there is no event or input of the appropriate type to process, then blocks until there is. If there is more than one type of input available to process, it is undefined which will get processed. Usually, this procedure is not called by client applications; see . processes timer events by calling any appropriate timer callbacks, input sources by calling any appropriate input callbacks, signal source by calling any appropriate signal callbacks, and X events by calling .

When an X event is received, it is passed to , which calls the appropriate event handlers and passes them the widget, the event, and client-specific data registered with each procedure. If no handlers for that event are registered, the event is ignored and the dispatcher simply returns.

To dispatch an event returned by , retrieved directly from the Xlib queue, or synthetically constructed, to any registered event filters or event handlers, call .

Boolean `XtDispatchEvent` XEvent *event

event

  Specifies a pointer to the event structure to be dispatched to the appropriate event handlers.

The function first calls `XFilterEvent` with the event and the window of the widget to which the Intrinsics intend to dispatch the event, or the event window if the Intrinsics would not dispatch the event to any handlers. If `XFilterEvent` returns `True` and the event activated a server grab as identified by a previous call to or , calls or with the timestamp from the event and immediately returns `True`. If `XFilterEvent` returns `True` and a grab was not activated, just immediately returns `True`. Otherwise, sends the event to the event handler functions that have been previously registered with the dispatch routine. returns `True` if `XFilterEvent` returned `True`, or if the event was dispatched to some handler, and `False` if it found no handler to which to dispatch the event. records the last timestamp in any event that contains a timestamp (see `XtLastTimestampProcessed`), regardless of whether it was filtered or dispatched. If a modal cascade is active with spring_loaded `True`, and if the event is a remap event as defined by , may dispatch the event a second time. If it does so, will call `XFilterEvent` again with the window of the spring-loaded widget prior to the second dispatch, and if `XFilterEvent` returns `True`, the second dispatch will not be performed.

## The Application Input Loop

To process all input from a given application in a continuous loop, use the convenience procedure .

void `XtAppMainLoop` XtAppContext app_context

app_context

  Specifies the application context that identifies the application.

The function processes events using , varying the mask parameter and using to ensure that it has a chance to handle events of all types, i.e., X events, timer events, input events and signal sources. This constitutes the main loop of X Toolkit applications. There is nothing special about ; it simply processes events in a conditional loop. At the bottom of the loop, it checks to see if the specified application context's destroy flag is set. If the flag is set, the loop breaks. The whole loop is enclosed between a matching and .

Applications can provide their own version of this loop, which tests some global termination flag or tests that the number of top-level widgets is larger than zero before circling back for the next event.

The design of has changed since Release 6. Originally it looped over calls to , and , but because the latter returns only after an X event (not for timers, signals, inputs), it was modified to allow any type of event to break out of the loop.

## Setting and Checking the Sensitivity State of a Widget

Many widgets have a mode in which they assume a different appearance (for example, are grayed out or stippled), do not respond to user events, and become dormant.

When dormant, a widget is considered to be insensitive. If a widget is insensitive, the event manager does not dispatch any events to the widget with an event type of `KeyPress`, `KeyRelease`, `ButtonPress`, `ButtonRelease`, `MotionNotify`, `EnterNotify`, `LeaveNotify`, `FocusIn`, or `FocusOut`.

A widget can be insensitive because its sensitive field is `False` or because one of its ancestors is insensitive and thus the widget's ancestor_sensitive field also is `False`. A widget can but does not need to distinguish these two cases visually.

Pop-up shells will have ancestor_sensitive `False` if the parent was insensitive when the shell was created. Since on the parent will not modify the resource of the pop-up child, clients are advised to include a resource specification of the form “*TransientShell.ancestorSensitive: True” in the application defaults resource file or to otherwise ensure that the parent is sensitive when creating pop-up shells.

To set the sensitivity state of a widget, use .

void `XtSetSensitive` Widget w Boolean sensitive

w

  Specifies the widget. Must be of class RectObj or any subclass thereof.

sensitive

  Specifies whether the widget should receive keyboard, pointer, and focus events.

The function first calls on the current widget with an argument list specifying the XtNsensitive resource and the new value. If sensitive is `False` and the widget's class is a subclass of Composite, recursively propagates the new value down the child tree by calling on each child to set ancestor_sensitive to `False`. If sensitive is `True` and the widget's class is a subclass of Composite and the widget's ancestor_sensitive field is `True`, sets the ancestor_sensitive of each child to `True` and then recursively calls on each normal descendant that is now sensitive to set ancestor_sensitive to `True`.

calls to change the sensitive and ancestor_sensitive fields of each affected widget. Therefore, when one of these changes, the widget's set_values procedure should take whatever display actions are needed (for example, graying out or stippling the widget).

maintains the invariant that, if the parent has either sensitive or ancestor_sensitive `False`, then all children have ancestor_sensitive `False`.

To check the current sensitivity state of a widget, use .

Boolean `XtIsSensitive` Widget w

w

  Specifies the object. Must be of class Object or any subclass thereof.

The function returns `True` or `False` to indicate whether user input events are being dispatched. If object's class is a subclass of RectObj and both sensitive and ancestor_sensitive are `True`, returns `True`; otherwise, it returns `False`.

## Adding Background Work Procedures

The Intrinsics have some limited support for background processing. Because most applications spend most of their time waiting for input, you can register an idle-time work procedure that is called when the toolkit would otherwise block in or . Work procedure pointers are of type .

typedef Boolean `(*XtWorkProc)` XtPointer client_data

client_data

  Passes the client data specified when the work procedure was registered.

This procedure should return `True` when it is done to indicate that it should be removed. If the procedure returns `False`, it will remain registered and called again when the application is next idle. Work procedures should be very judicious about how much they do. If they run for more than a small part of a second, interactive feel is likely to suffer.

To register a work procedure for a given application, use .

XtWorkProcId `XtAppAddWorkProc` XtAppContext app_context XtWorkProc proc XtPointer client_data

app_context

  Specifies the application context that identifies the application.

proc

  Specifies the procedure to be called when the application is idle.

client_data

  Specifies the argument passed to the specified procedure when it is called.

The function adds the specified work procedure for the application identified by app_context and returns an opaque unique identifier for this work procedure. Multiple work procedures can be registered, and the most recently added one is always the one that is called. However, if a work procedure adds another work procedure, the newly added one has lower priority than the current one.

To remove a work procedure, either return `True` from the procedure when it is called or use outside of the procedure.

void `XtRemoveWorkProc` XtWorkProcId id

id

  Specifies which work procedure to remove.

The function explicitly removes the specified background work procedure.

## X Event Filters

The event manager provides filters that can be applied to specific X events. The filters, which screen out events that are redundant or are temporarily unwanted, handle pointer motion compression, enter/leave compression, and exposure compression.

### Pointer Motion Compression

Widgets can have a hard time keeping up with a rapid stream of pointer motion events. Furthermore, they usually do not care about every motion event. To throw out redundant motion events, the widget class field compress_motion should be `True`. When a request for an event would return a motion event, the Intrinsics check if there are any other motion events for the same widget immediately following the current one and, if so, skip all but the last of them.

### Enter/Leave Compression

To throw out pairs of enter and leave events that have no intervening events, as can happen when the user moves the pointer across a widget without stopping in it, the widget class field compress_enterleave should be `True`. These enter and leave events are not delivered to the client if they are found together in the input queue.

### Exposure Compression

Many widgets prefer to process a series of exposure events as a single expose region rather than as individual rectangles. Widgets with complex displays might use the expose region as a clip list in a graphics context, and widgets with simple displays might ignore the region entirely and redisplay their whole window or might get the bounding box from the region and redisplay only that rectangle.

In either case, these widgets do not care about getting partial exposure events. The compress_exposure field in the widget class structure specifies the type and number of exposure events that are dispatched to the widget's expose procedure. This field must be initialized to one of the following values:

    #define XtExposeNoCompress              ((XtEnum)False)
    #define XtExposeCompressSeries          ((XtEnum)True)
    #define XtExposeCompressMultiple        <implementation-defined>
    #define XtExposeCompressMaximal         <implementation-defined>

optionally ORed with any combination of the following flags (all with implementation-defined values): `XtExposeGraphicsExpose`, `XtExposeGraphicsExposeMerged`, `XtExposeNoExpose`, and `XtExposeNoRegion`.

If the compress_exposure field in the widget class structure does not specify `XtExposeNoCompress`, the event manager calls the widget's expose procedure only once for a series of exposure events. In this case, all `Expose` or `GraphicsExpose` events are accumulated into a region. When the final event is received, the event manager replaces the rectangle in the event with the bounding box for the region and calls the widget's expose procedure, passing the modified exposure event and (unless `XtExposeNoRegion` is specified) the region. For more information on regions, see Section 16.5 in Xlib — C Language X Interface.

The values have the following interpretation:

`XtExposeNoCompress`

- No exposure compression is performed; every selected event is individually dispatched to the expose procedure with a region argument of NULL.

`XtExposeCompressSeries`

- Each series of exposure events is coalesced into a single event, which is dispatched when an exposure event with count equal to zero is reached.

`XtExposeCompressMultiple`

- Consecutive series of exposure events are coalesced into a single event, which is dispatched when an exposure event with count equal to zero is reached and either the event queue is empty or the next event is not an exposure event for the same widget.

`XtExposeCompressMaximal`

- All expose series currently in the queue for the widget are coalesced into a single event without regard to intervening nonexposure events. If a partial series is in the end of the queue, the Intrinsics will block until the end of the series is received.

The additional flags have the following meaning:

`XtExposeGraphicsExpose`

- Specifies that `GraphicsExpose` events are also to be dispatched to the expose procedure. `GraphicsExpose` events are compressed, if specified, in the same manner as `Expose` events.

`XtExposeGraphicsExposeMerged`

- Specifies in the case of `XtExposeCompressMultiple` and `XtExposeCompressMaximal` that series of `GraphicsExpose` and `Expose` events are to be compressed together, with the final event type determining the type of the event passed to the expose procedure. If this flag is not set, then only series of the same event type as the event at the head of the queue are coalesced. This flag also implies `XtExposeGraphicsExpose`.

`XtExposeNoExpose`

- Specifies that `NoExpose` events are also to be dispatched to the expose procedure. `NoExpose` events are never coalesced with other exposure events or with each other.

`XtExposeNoRegion`

- Specifies that the final region argument passed to the expose procedure is NULL. The rectangle in the event will still contain bounding box information for the entire series of compressed exposure events. This option saves processing time when the region is not needed by the widget.

## Widget Exposure and Visibility

Every primitive widget and some composite widgets display data on the screen by means of direct Xlib calls. Widgets cannot simply write to the screen and forget what they have done. They must keep enough state to redisplay the window or parts of it if a portion is obscured and then reexposed.

### Redisplay of a Widget: The expose Procedure

The expose procedure pointer in a widget class is of type .

typedef void `(*XtExposeProc)` Widget w XEvent *event Region region

w

  Specifies the widget instance requiring redisplay.

event

  Specifies the exposure event giving the rectangle requiring redisplay.

region

  Specifies the union of all rectangles in this exposure sequence.

The redisplay of a widget upon exposure is the responsibility of the expose procedure in the widget's class record. If a widget has no display semantics, it can specify NULL for the expose field. Many composite widgets serve only as containers for their children and have no expose procedure.

If the expose procedure is NULL, fills in a default bit gravity of `NorthWestGravity` before it calls the widget's realize procedure.

If the widget's compress_exposure class field specifies `XtExposeNoCompress` or `XtExposeNoRegion`, or if the event type is `NoExpose` (see ), region is NULL. If `XtExposeNoCompress` is not specified and the event type is not `NoExpose`, the event is the final event in the compressed series but x, y, width, and height contain the bounding box for all the compressed events. The region is created and destroyed by the Intrinsics, but the widget is permitted to modify the region contents.

A small simple widget (for example, Label) can ignore the bounding box information in the event and redisplay the entire window. A more complicated widget (for example, Text) can use the bounding box information to minimize the amount of calculation and redisplay it does. A very complex widget uses the region as a clip list in a GC and ignores the event information. The expose procedure is not chained and is therefore responsible for exposure of all superclass data as well as its own.

However, it often is possible to anticipate the display needs of several levels of subclassing. For example, rather than implement separate display procedures for the widgets Label, Pushbutton, and Toggle, you could write a single display routine in Label that uses display state fields like

    Boolean         invert;
    Boolean         highlight;
    Dimension       highlight_width;

Label would have invert and highlight always `False` and highlight_width zero. Pushbutton would dynamically set highlight and highlight_width, but it would leave invert always `False`. Finally, Toggle would dynamically set all three. In this case, the expose procedures for Pushbutton and Toggle inherit their superclass's expose procedure; see .

### Widget Visibility

Some widgets may use substantial computing resources to produce the data they will display. However, this effort is wasted if the widget is not actually visible on the screen, that is, if the widget is obscured by another application or is iconified.

The visible field in the core widget structure provides a hint to the widget that it need not compute display data. This field is guaranteed to be `True` by the time an exposure event is processed if any part of the widget is visible, but is `False` if the widget is fully obscured.

Widgets can use or ignore the visible hint. If they ignore it, they should have visible_interest in their widget class record set `False`. In such cases, the visible field is initialized `True` and never changes. If visible_interest is `True`, the event manager asks for `VisibilityNotify` events for the widget and sets visible to `True` on `VisibilityUnobscured` or `VisibilityPartiallyObscured` events and `False` on `VisibilityFullyObscured` events.

## X Event Handlers

Event handlers are procedures called when specified events occur in a widget. Most widgets need not use event handlers explicitly. Instead, they use the Intrinsics translation manager. Event handler procedure pointers are of the type .

typedef void `(*XtEventHandler)` Widget w XtPointer client_data XEvent *event Boolean *continue_to_dispatch

w

  Specifies the widget for which the event arrived.

client_data

  Specifies any client-specific information registered with the event handler.

event

  Specifies the triggering event.

continue_to_dispatch

  Specifies whether the remaining event handlers registered for the current event should be called.

After receiving an event and before calling any event handlers, the Boolean pointed to by continue_to_dispatch is initialized to `True`. When an event handler is called, it may decide that further processing of the event is not desirable and may store `False` in this Boolean, in which case any handlers remaining to be called for the event are ignored.

The circumstances under which the Intrinsics may add event handlers to a widget are currently implementation-dependent. Clients must therefore be aware that storing `False` into the continue_to_dispatch argument can lead to portability problems.

### Event Handlers That Select Events

To register an event handler procedure with the dispatch mechanism, use .

void `XtAddEventHandler` Widget w EventMask event_mask Boolean nonmaskable XtEventHandler proc XtPointer client_data

w

  Specifies the widget for which this event handler is being registered. Must be of class Core or any subclass thereof.

event_mask

  Specifies the event mask for which to call this procedure.

nonmaskable

  Specifies whether this procedure should be called on the nonmaskable events `( GraphicsExpose`, `NoExpose`, `SelectionClear`, `SelectionRequest`, `SelectionNotify`, `ClientMessage`, and `MappingNotify ).`

proc

  Specifies the procedure to be called.

client_data

  Specifies additional data to be passed to the event handler.

The function registers a procedure with the dispatch mechanism that is to be called when an event that matches the mask occurs on the specified widget. Each widget has a single registered event handler list, which will contain any procedure/client_data pair exactly once regardless of the manner in which it is registered. If the procedure is already registered with the same client_data value, the specified mask augments the existing mask. If the widget is realized, calls `XSelectInput`, if necessary. The order in which this procedure is called relative to other handlers registered for the same event is not defined.

To remove a previously registered event handler, use .

void `XtRemoveEventHandler` Widget w EventMask event_mask Boolean nonmaskable XtEventHandler proc XtPointer client_data

w

  Specifies the widget for which this procedure is registered. Must be of class Core or any subclass thereof.

event_mask

  Specifies the event mask for which to unregister this procedure.

nonmaskable

  Specifies whether this procedure should be removed on the nonmaskable events `( GraphicsExpose`, `NoExpose`, `SelectionClear`, `SelectionRequest`, `SelectionNotify`, `ClientMessage`, and `MappingNotify ).`

proc

  Specifies the procedure to be removed.

client_data

  Specifies the registered client data.

The function unregisters an event handler registered with or for the specified events. The request is ignored if client_data does not match the value given when the handler was registered. If the widget is realized and no other event handler requires the event, calls `XSelectInput`. If the specified procedure has not been registered or if it has been registered with a different value of client_data, returns without reporting an error.

To stop a procedure registered with or from receiving all selected events, call with an event_mask of `XtAllEvents` and nonmaskable `True`. The procedure will continue to receive any events that have been specified in calls to or .

To register an event handler procedure that receives events before or after all previously registered event handlers, use .

    typedef enum {XtListHead, XtListTail} XtListPosition;

void `XtInsertEventHandler` Widget w EventMask event_mask Boolean nonmaskable XtEventHandler proc XtPointer client_data XtListPosition position

w

  Specifies the widget for which this event handler is being registered. Must be of class Core or any subclass thereof.

event_mask

  Specifies the event mask for which to call this procedure.

nonmaskable

  Specifies whether this procedure should be called on the nonmaskable events `( GraphicsExpose`, `NoExpose`, `SelectionClear`, `SelectionRequest`, `SelectionNotify`, `ClientMessage`, and `MappingNotify ).`

proc

  Specifies the procedure to be called.

client_data

  Specifies additional data to be passed to the client's event handler.

position

  Specifies when the event handler is to be called relative to other previously registered handlers.

is identical to with the additional position argument. If position is `XtListHead`, the event handler is registered so that it is called before any event handlers that were previously registered for the same widget. If position is `XtListTail`, the event handler is registered to be called after any previously registered event handlers. If the procedure is already registered with the same client_data value, the specified mask augments the existing mask and the procedure is repositioned in the list.

### Event Handlers That Do Not Select Events

On occasion, clients need to register an event handler procedure with the dispatch mechanism without explicitly causing the X server to select for that event. To do this, use .

void `XtAddRawEventHandler` Widget w EventMask event_mask Boolean nonmaskable XtEventHandler proc XtPointer client_data

w

  Specifies the widget for which this event handler is being registered. Must be of class Core or any subclass thereof.

event_mask

  Specifies the event mask for which to call this procedure.

nonmaskable

  Specifies whether this procedure should be called on the nonmaskable events `( GraphicsExpose`, `NoExpose`, `SelectionClear`, `SelectionRequest`, `SelectionNotify`, `ClientMessage`, and `MappingNotify ).`

proc

  Specifies the procedure to be called.

client_data

  Specifies additional data to be passed to the client's event handler.

The function is similar to except that it does not affect the widget's event mask and never causes an `XSelectInput` for its events. Note that the widget might already have those mask bits set because of other nonraw event handlers registered on it. If the procedure is already registered with the same client_data, the specified mask augments the existing mask. The order in which this procedure is called relative to other handlers registered for the same event is not defined.

To remove a previously registered raw event handler, use .

void `XtRemoveRawEventHandler` Widget w EventMask event_mask Boolean nonmaskable XtEventHandler proc XtPointer client_data

w

  Specifies the widget for which this procedure is registered. Must be of class Core or any subclass thereof.

event_mask

  Specifies the event mask for which to unregister this procedure.

nonmaskable

  Specifies whether this procedure should be removed on the nonmaskable events `( GraphicsExpose`, `NoExpose`, `SelectionClear`, `SelectionRequest`, `SelectionNotify`, `ClientMessage`, and `MappingNotify ).`

proc

  Specifies the procedure to be registered.

client_data

  Specifies the registered client data.

The function unregisters an event handler registered with or for the specified events without changing the window event mask. The request is ignored if client_data does not match the value given when the handler was registered. If the specified procedure has not been registered or if it has been registered with a different value of client_data, returns without reporting an error.

To stop a procedure registered with or from receiving all nonselected events, call with an event_mask of `XtAllEvents` and nonmaskable `True`. The procedure will continue to receive any events that have been specified in calls to or .

To register an event handler procedure that receives events before or after all previously registered event handlers without selecting for the events, use .

void `XtInsertRawEventHandler` Widget w EventMask event_mask Boolean nonmaskable XtEventHandler proc XtPointer client_data XtListPosition position

w

  Specifies the widget for which this event handler is being registered. Must be of class Core or any subclass thereof.

event_mask

  Specifies the event mask for which to call this procedure.

nonmaskable

  Specifies whether this procedure should be called on the nonmaskable events `( GraphicsExpose`, `NoExpose`, `SelectionClear`, `SelectionRequest`, `SelectionNotify`, `ClientMessage`, and `MappingNotify ).`

proc

  Specifies the procedure to be registered.

client_data

  Specifies additional data to be passed to the client's event handler.

position

  Specifies when the event handler is to be called relative to other previously registered handlers.

The function is similar to except that it does not modify the widget's event mask and never causes an `XSelectInput` for the specified events. If the procedure is already registered with the same client_data value, the specified mask augments the existing mask and the procedure is repositioned in the list.

### Current Event Mask

To retrieve the event mask for a given widget, use .

EventMask `XtBuildEventMask` Widget w

w

  Specifies the widget. Must be of class Core or any subclass thereof.

The function returns the event mask representing the logical OR of all event masks for event handlers registered on the widget with and and all event translations, including accelerators, installed on the widget. This is the same event mask stored into the `XSetWindowAttributes` structure by and sent to the server when event handlers and translations are installed or removed on the realized widget.

### Event Handlers for X11 Protocol Extensions

To register an event handler procedure with the Intrinsics dispatch mechanism according to an event type, use .

void `XtInsertEventTypeHandler` Widget widget int event_type XtPointer select_data XtEventHandler proc XtPointer client_data XtListPosition position

widget

  Specifies the widget for which this event handler is being registered. Must be of class Core or any subclass thereof.

event_type

  Specifies the event type for which to call this event handler.

select_data

  Specifies data used to request events of the specified type from the server, or NULL.

proc

  Specifies the event handler to be called.

client_data

  Specifies additional data to be passed to the event handler.

position

  Specifies when the event handler is to be called relative to other previously registered handlers.

registers a procedure with the dispatch mechanism that is to be called when an event that matches the specified event_type is dispatched to the specified widget.

If event_type specifies one of the core X protocol events, then select_data must be a pointer to a value of type `EventMask`, indicating the event mask to be used to select for the desired event. This event mask is included in the value returned by . If the widget is realized, calls `XSelectInput` if necessary. Specifying NULL for select_data is equivalent to specifying a pointer to an event mask containing 0. This is similar to the function.

If event_type specifies an extension event type, then the semantics of the data pointed to by select_data are defined by the extension selector registered for the specified event type.

In either case the Intrinsics are not required to copy the data pointed to by select_data, so the caller must ensure that it remains valid as long as the event handler remains registered with this value of select_data.

The position argument allows the client to control the order of invocation of event handlers registered for the same event type. If the client does not care about the order, it should normally specify `XtListTail`, which registers this event handler after any previously registered handlers for this event type.

Each widget has a single registered event handler list, which will contain any procedure/client_data pair exactly once if it is registered with , regardless of the manner in which it is registered and regardless of the value(s) of select_data. If the procedure is already registered with the same client_data value, the specified mask augments the existing mask and the procedure is repositioned in the list.

To remove an event handler registered with , use .

void `XtRemoveEventTypeHandler` Widget widget int event_type XtPointer select_data XtEventHandler proc XtPointer client_data

widget

  Specifies the widget for which the event handler was registered. Must be of class Core or any subclass thereof.

event_type

  Specifies the event type for which the handler was registered.

select_data

  Specifies data used to deselect events of the specified type from the server, or NULL.

proc

  Specifies the event handler to be removed.

client_data

  Specifies the additional client data with which the procedure was registered.

The function unregisters an event handler registered with for the specified event type. The request is ignored if client_data does not match the value given when the handler was registered.

If event_type specifies one of the core X protocol events, select_data must be a pointer to a value of type `EventMask, indicating the event` mask to be used to deselect for the appropriate event. If the widget is realized, calls `XSelectInput` if necessary. Specifying NULL for select_data is equivalent to specifying a pointer to an event mask containing 0. This is similar to the function.

If event_type specifies an extension event type, then the semantics of the data pointed to by select_data are defined by the extension selector registered for the specified event type.

To register a procedure to select extension events for a widget, use .

void `XtRegisterExtensionSelector` Display *display int min_event_type int max_event_type XtExtensionSelectProc proc XtPointer client_data

display

  Specifies the display for which the extension selector is to be registered.

min_event_type

max_event_type

  Specifies the range of event types for the extension.

proc

  Specifies the extension selector procedure.

client_data

  Specifies additional data to be passed to the extension selector.

The function registers a procedure to arrange for the delivery of extension events to widgets.

If min_event_type and max_event_type match the parameters to a previous call to for the same display, then proc and client_data replace the previously registered values. If the range specified by min_event_type and max_event_type overlaps the range of the parameters to a previous call for the same display in any other way, an error results.

When a widget is realized, after the core.realize method is called, the Intrinsics check to see if any event handler specifies an event type within the range of a registered extension selector. If so, the Intrinsics call each such selector. If an event type handler is added or removed, the Intrinsics check to see if the event type falls within the range of a registered extension selector, and if it does, calls the selector. In either case the Intrinsics pass a list of all the widget's event types that are within the selector's range. The corresponding select data are also passed. The selector is responsible for enabling the delivery of extension events required by the widget.

An extension selector is of type .

typedef void `(*XtExtensionSelectProc)` Widget widget int *event_types XtPointer *select_data int count XtPointer client_data

widget

  Specifies the widget that is being realized or is having an event handler added or removed.

event_types

  Specifies a list of event types that the widget has registered event handlers for.

select_data

  Specifies a list of the select_data parameters specified in .

count

  Specifies the number of entries in the event_types and select_data lists.

client_data

  Specifies the additional client data with which the procedure was registered.

The event_types and select_data lists will always have the same number of elements, specified by count. Each event type/select data pair represents one call to .

To register a procedure to dispatch events of a specific type within , use .

XtEventDispatchProc `XtSetEventDispatcher` Display *display int event_type XtEventDispatchProc proc

display

  Specifies the display for which the event dispatcher is to be registered.

event_type

  Specifies the event type for which the dispatcher should be invoked.

proc

  Specifies the event dispatcher procedure.

The function registers the event dispatcher procedure specified by proc for events with the type event_type. The previously registered dispatcher (or the default dispatcher if there was no previously registered dispatcher) is returned. If proc is NULL, the default procedure is restored for the specified type.

In the future, when is called with an event type of event_type, the specified proc (or the default dispatcher) is invoked to determine a widget to which to dispatch the event.

The default dispatcher handles the Intrinsics modal cascade and keyboard focus mechanisms, handles the semantics of compress_enterleave and compress_motion, and discards all extension events.

An event dispatcher procedure pointer is of type .

typedef Boolean `(*XtEventDispatchProc)` XEvent *event

event

  Passes the event to be dispatched.

The event dispatcher procedure should determine whether this event is of a type that should be dispatched to a widget.

If the event should be dispatched to a widget, the event dispatcher procedure should determine the appropriate widget to receive the event, call `XFilterEvent` with the window of this widget, or `None` if the event is to be discarded, and if `XFilterEvent` returns `False`, dispatch the event to the widget using . The procedure should return `True` if either `XFilterEvent` or returned `True` and `False` otherwise.

If the event should not be dispatched to a widget, the event dispatcher procedure should attempt to dispatch the event elsewhere as appropriate and return `True` if it successfully dispatched the event and `False` otherwise.

Some dispatchers for extension events may wish to forward events according to the Intrinsics' keyboard focus mechanism. To determine which widget is the end result of keyboard event forwarding, use .

Widget `XtGetKeyboardFocusWidget` Widget widget

widget

  Specifies the widget to get forwarding information for.

The function returns the widget that would be the end result of keyboard event forwarding for a keyboard event for the specified widget.

To dispatch an event to a specified widget, use .

Boolean `XtDispatchEventToWidget` Widget widget XEvent *event

widget

  Specifies the widget to which to dispatch the event.

event

  Specifies a pointer to the event to be dispatched.

The function scans the list of registered event handlers for the specified widget and calls each handler that has been registered for the specified event type, subject to the continue_to_dispatch value returned by each handler. The Intrinsics behave as if event handlers were registered at the head of the list for `Expose`, `NoExpose`, `GraphicsExpose`, and `VisibilityNotify` events to invoke the widget's expose procedure according to the exposure compression rules and to update the widget's visible field if visible_interest is `True`. These internal event handlers never set continue_to_dispatch to `False`.

returns `True` if any event handler was called and `False` otherwise.

## Using the Intrinsics in a Multi-Threaded Environment

The Intrinsics may be used in environments that offer multiple threads of execution within the context of a single process. A multi-threaded application using the Intrinsics must explicitly initialize the toolkit for mutually exclusive access by calling .

### Initializing a Multi-Threaded Intrinsics Application

To test and initialize Intrinsics support for mutually exclusive thread access, call .

Boolean `XtToolkitThreadInitialize` void

returns `True` if the Intrinsics support mutually exclusive thread access, otherwise it returns `False`. must be called before , , , or `XtSetLanguageProc` is called. may be called more than once; however, the application writer must ensure that it is not called simultaneously by two or more threads.

### Locking X Toolkit Data Structures

The Intrinsics employs two levels of locking: application context and process. Locking an application context ensures mutually exclusive access by a thread to the state associated with the application context, including all displays and widgets associated with it. Locking a process ensures mutually exclusive access by a thread to Intrinsics process global data.

A client may acquire a lock multiple times and the effect is cumulative. The client must ensure that the lock is released an equal number of times in order for the lock to be acquired by another thread.

Most application writers will have little need to use locking as the Intrinsics performs the necessary locking internally. Resource converters are an exception. They require the application context or process to be locked before the application can safely call them directly, for example:

            ...
            XtAppLock(app_context);
            XtCvtStringToPixel(dpy, args, num_args, fromVal, toVal, closure_ret);
            XtAppUnlock(app_context);
            ...

When the application relies upon or a converter to provide the storage for the results of a conversion, the application should acquire the process lock before calling out and hold the lock until the results have been copied.

Application writers who write their own utility functions, such as one which retrieves the being_destroyed field from a widget instance, must lock the application context before accessing widget internal data. For example:

    #include <X11/CoreP.h>
    Boolean BeingDestroyed (Widget widget)
    {
            Boolean ret;
            XtAppLock(XtWidgetToApplicationContext(widget));
            ret = widget->core.being_destroyed;
            XtAppUnlock(XtWidgetToApplicationContext(widget));
            return ret;
    }

A client that wishes to atomically call two or more Intrinsics functions must lock the application context. For example:

            ...
            XtAppLock(XtWidgetToApplicationContext(widget));
            XtUnmanageChild (widget1);
            XtManageChild (widget2);
            XtAppUnlock(XtWidgetToApplicationContext(widget));
            ...

#### Locking the Application Context

To ensure mutual exclusion of application context, display, or widget internal state, use `XtAppLock.`

void `XtAppLock` XtAppContext app_context

app_context

  Specifies the application context to lock.

blocks until it is able to acquire the lock. Locking the application context also ensures that only the thread holding the lock makes Xlib calls from within Xt. An application that makes its own direct Xlib calls must either lock the application context around every call or enable thread locking in Xlib.

To unlock a locked application context, use `XtAppUnlock.`

void `XtAppUnlock` XtAppContext app_context

app_context

  Specifies the application context that was previously locked.

#### Locking the Process

To ensure mutual exclusion of X Toolkit process global data, a widget writer must use `XtProcessLock.`

void `XtProcessLock` void

blocks until it is able to acquire the lock. Widget writers may use XtProcessLock to guarantee mutually exclusive access to widget static data.

To unlock a locked process, use .

void `XtProcessUnlock` void

To lock both an application context and the process at the same time, call first and then . To release both locks, call first and then . The order is important to avoid deadlock.

### Event Management in a Multi-Threaded Environment

In a nonthreaded environment an application writer could reasonably assume that it is safe to exit the application from a quit callback. This assumption may no longer hold true in a multi-threaded environment; therefore it is desirable to provide a mechanism to terminate an event-processing loop without necessarily terminating its thread.

To indicate that the event loop should terminate after the current event dispatch has completed, use .

void `XtAppSetExitFlag` XtAppContext app_context

app_context

  Specifies the application context.

tests the value of the flag and will return if the flag is `True`.

Application writers who implement their own main loop may test the value of the exit flag with .

Boolean `XtAppGetExitFlag` XtAppContext app_context

app_context

  Specifies the application context.

will normally return `False`, indicating that event processing may continue. When returns `True`, the loop must terminate and return to the caller, which might then destroy the application context.

Application writers should be aware that, if a thread is blocked in , , or and another thread in the same application context opens a new display, adds an alternate input, or a timeout, any new source(s) will not normally be “noticed” by the blocked thread. Any new sources are “noticed” the next time one of these functions is called.

The Intrinsics manage access to events on a last-in, first-out basis. If multiple threads in the same application context block in , , or , the last thread to call one of these functions is the first thread to return.