=head1 NAME

Prima::Clipboard - GUI interprocess data exchange

=head1 DESCRIPTION

Prima::Clipboard is an interface to system clipboards. Depending on the OS,
there can be only one clipboard (Win32), or three (X11). The class is also used
for data exchange in drag-and-drop interactions.

=head1 SYNOPSIS

   my $c = $::application-> Clipboard;

   # paste data
   my $string = $c-> text;
   my $image  = $c-> image;
   my $other  = $c-> fetch('Other type');

   # copy datum
   $c-> text( $string);

   # copy data
   $c-> open;
   $c-> text( $string);
   $c-> image( $image);
   $c-> store( $image);
   $c-> close;

   # clear
   $c-> clear;

=head1 USAGE

Prima::Clipboard provides access to the system clipboard data storage. For the
easier communication, the system clipboard has one 'format' field, that is
stored along with the data.  This field is used to distinguish between data
formats.  Moreover, a clipboard can hold simultaneously several data instances,
of different data formats. Since the primary usage of a clipboard is 'copying'
and 'pasting', an application can store copied information in several formats,
increasing possibility that the receiving application can recognize the data.

Different systems provide spectrum of predefined data types, but the toolkit
uses only three of these out of the box - ascii text, utf8 text, and image. It does not limit,
however, the data format being one of these three types - an application is
free to register its own formats. Both predefined and newly defined data
formats are described by a string, and the three predefined formats are
represented by C<'Text'>, C<'UTF8'>, and C<'Image'> string constants.

The most frequent usage of Prima::Clipboard is to preform two tasks - copying
and pasting. Both can be exemplified by the following:

   my $c = $::application-> Clipboard;

   # paste
   my $string = $c-> text;

   # copy
   $c-> text( $string);

Here is what happens under the hood:

First, the default clipboard is accessible by an implicit name call, as an
object named 'Clipboard'. This scheme makes it easily overridable.  A more
important point is, that the default clipboard object might be accompanied by
other clipboard objects. This is the case with X11 environment, which defines
also 'Primary' and 'Secondary' system clipboards. Their functionality is
identical to the default clipboard, however. C<get_standard_clipboards()>
method returns strings for the clipboards, provided by the system.

Second, code for fetching and/or storing multi-format data is somewhat different.
Clipboard is viewed as a shared system resource, and has to be 'opened',
before a process can grab it, so other processes can access the clipboard data
only after the clipboard is 'closed' ( note: It is not so under X11, where
there is no such thing as clipboard locking, -- but the toolkit imposes this model
for the cosistency sake).

C<fetch()> and C<store()> implicitly call C<open()> and C<close()>, but these
functions must be called explicitly for the multi-format data handling. The
code below illustrates the said:

    # copy text and image
    if ( $c-> open) {
       $c-> clear;
       $c-> store('Text', $string);
       $c-> store('Image', $image);
       $c-> close;
    }

    # check present formats and paste
   if ( $c-> open) {
      if ( $c-> format_exists('Text')) {
         $string = $c-> fetch('Text');
      }
      # or, check the desired format alternatively
      my %formats = map { $_ => 1 } $c-> get_formats;
      if ( $formats{'Image'}) {
         $image = $c-> fetch('Image');
      }

      $c-> close;
   }


The clear() call in the copying code is necessary so
the newly written data will not mix with the old.

At last, the newly registered formats can be accessed
by a program:

   my $myformat = 'Very Special Old Pale Data Format';
   if ( $c-> register_format($myformat)) {
      $c-> open;
      $c-> clear;
      $c-> store('Text', 'sample text');
      $c-> store($myformat', 'sample ## text');
      $c-> close;
   }

=head2 On-demand storage

Under X11 it is possible to skip the generation of data in all possible
clipboard format when when copying. The native X11 mechanism allows to ask the
source application for the exact data format needed by the target application,
and the toolkit uses special event C<onClipboard> triggered on the application
whenever necessary.

By default this event handler responds to querying image in file encoded
formats (gif,jpg) under X11 on the fly. It can be extended to generate other
formats as well. See L<Prima::Application/Events> Clipboard for the details.

=head2 Custom formats

Once registered, all processes in a GUI space can access the data by this
format. The registration must take place also if a Prima-driven program needs
to read data in a format, defined by an another program. In either case, the
duplicate registration is a valid case.  When no longer needed, a format can be
de-registered. It is not a mandatory action, however - the toolkit cleans up
before exit. Moreover, the system maintains a reference counter on the
custom-registered formats; de-registering thus does not mean deletion. If two
processes use a custom format, and one exits and re-starts, the other still can
access the data in the same format, registered by its previous incarnation.

=head2 Unicode

Applications can interchange text in both ascii and utf8, leaving the selection
choice to reader programs. While it is possible to access both at the same
time, by C<fetch>'ing content of C<Text> and C<UTF8> clipboard slots, the wiget
proposes its own pasting scheme, where the mechanics are hidden under the
C<text> property call. The property is advised to be used instead of
individual C<'Text'> and C<'UTF8'> formats. This method is used in all the
standard widgets, and is implemented so the programmer can reprogram its
default action by overloading C<PasteText> notification of
C<Prima::Application> ( see L<Prima::Application/PasteText> ).

The default action of C<PasteText> is to query first if C<'Text'> format is
available, and if so, return the ascii text scalar. If
C<Prima::Application::wantUnicodeInput> is set (default), C<'UTF8'> format is
checked before resorting to C<'Text'>. It is clear that this scheme is not the
only possibly needed, for example, an application may want to ignore ASCII
text, or, ignore UTF8 text but have C<Prima::Application::wantUnicodeInput>
set, etc.

The symmetric action is C<CopyText>, that allows for a custom text conversion
code to be installed.

=head2 Images

Image data can be transferred in different formats in different OSes. The
lowest level is raw pixel data in display-based format, whereas GTK-based
applications can also exchange images in file-based formats, such as bmp, png
etc. To avoid further complications in the implementations, C<PasteImage>
action was introduced to handle these cases, together with a symmetrical
C<CopyImage>.

The default action of C<PasteImage> is to check whether lossless encoded image data
is present, and if so, load a new image from this data, before falling back to
OS-dependent image storage.

When storing the image on the clipboard, only the default format, raw pixel data
is used. Under X11 the toolkit can also serve images encoded as file formats.

=head2 Exact and meta formats

Prima registers two special I<meta formats>, C<Image> and C<Text>, that
interoperate with the system clipboard, storing data in the format that matches
best with system convention when copying and pasting images and text,
correspondinly. It is recommended to use meta-format calls (has_format, text,
image, copy, paste) rather than exact format calls (format_exists, store,
fetch) when possible.

Where the exact format method operate on a single format data storage, meta
format calls may operate on several exact formats. F.ex. C<text> can check
whether there exists a UTF-8 text storage, before resorting to 8-bit text.
C<image> on X11 is even more complicated, and may use image codecs to transfer
encoded PNG streams, for example.

=head1 API

=head2 Properties

=over

=item image OBJECT, [KEEP]

Provides access to an image, stored in the system clipboard. In get-mode call
return C<undef> if no image is stored.  In set-mode clears the clipboard unless
KEEP is set.

=item text STRING, [KEEP]

Provides access to the text stored in the system clipboard. In get-mode call
return C<undef> if no text information is present.  In set-mode clears the
clipboard unless KEEP is set.

=back

=head2 Methods

=over

=item clear

Deletes all data from clipboard.

=item close

Closes the open/close brackets. open() and close() can
be called recursively; only the last close() removes the
actual clipboard locking, so other processes can use it as well.

=item copy FORMAT, DATA, KEEP

Sets DATA in FORMAT. Clears the clipboard before unless KEEP is set.

=item deregister_format FORMAT_STRING

De-registers a previously registered data format.  Called implicitly for all
not de-registered format before a clipboard object is destroyed.

=item fetch FORMAT_STRING

Returns the data of exact FORMAT_STRING data format, if present in the
clipboard.  Depending on FORMAT_STRING, data is either text string for
C<'Text'> format, Prima::Image object for C<'Image'> format and a binary scalar
value for all custom formats.

=item format_exists FORMAT_STRING

Returns a boolean flag, showing whether FORMAT_STRING exact format data is
present in the clipboard or not.

=item has_format FORMAT_STRING

Returns a boolean flag, showing whether FORMAT_STRING meta format data is
present in the clipboard or not.

=item get_handle

Returns the system handle for the clipboard object.

=item get_formats INCLUDE_UNREGISTERED = 0

Returns an array of strings, where each is a format ID, reflecting the formats
present in the clipboard.

Only the predefined formats, and the formats registered via
C<register_format()> are returned if C<INCLUDE_UNREGISTERED> is unset.  If the
flag is set, then all existing formats returned, however their names are not
necessarily are the same as registered with Prima.

=item get_registered_formats

Returns an array of strings, each representing a registered format. C<Text> and
C<Image> are returned also.

=item get_standard_clipboards

Returns array of strings, each representing a system clipboard. The default
C<Clipboard> is always present. Other clipboards are optional. As an example,
this function returns only C<Clipboard> under win32, but also C<Primary> and
C<Secondary> under X11. The code, specific to these clipboards must refer to
this function first.

=item is_dnd

Returns 1 if the clipboard is the special clipboard used as
a proxy for drag and drop interactions.

See also: C<Widget/Drag and drop>, C<Application/get_dnd_clipboard>.

=item open

Opens a system clipboard and locks it for the process single use; returns a
success flag. Subsequent C<open> calls are possible, and always return 1. Each
C<open()> must correspond to C<close()>, otherwise the clipboard will stay
locked until the blocking process is finished.

=item paste FORMAT_STRING

Returns data of meta format FORMAT_STRING if found in the clipboard, or undef otherwise.

=item register_format FORMAT_STRING

Registers a data format under FORMAT_STRING string ID, returns a success flag.
If a format is already registered, 1 is returned. All formats, registered via
C<register_format()> are de-registered with C<deregister_format()> when a
program is finished.

=item store FORMAT_STRING, SCALAR

Stores SCALAR value into the clipboard in FORMAT_STRING exact data format.
Depending of FORMAT_STRING, SCALAR is treated as follows:

   FORMAT_STRING     SCALAR
   ------------------------------------
   Text              text string in ASCII
   UTF8              text string in UTF8
   Image             Prima::Image object
   other formats     binary scalar value


NB. All custom formats treated as a binary data. In case when the data are
transferred between hosts with different byte orders no implicit conversions
are made. It is up to the programmer whether to convert the data in a portable
format, or leave it as is. The former option is of course preferable. As far as
the author knows, the I<Storable> module from I<CPAN> collection provides the
system-independent conversion routines.

=back

=head1 AUTHOR

Dmitry Karasik, E<lt>dmitry@karasik.eu.orgE<gt>.

=head1 SEE ALSO

L<Prima>, L<Prima::Component>, L<Prima::Application>

