# U

`+UndoButton`
  An 'Undo' button. See change, +RedoButton.

`+UpField`
  A file upload field. Example:

          (gui '(+UpField) 30)

  Often used together with +Drop. See also +TextField.

`+UpButton`
  A button used scroll a chart one row up. See also chart, +Tiny, +Rid, +JS, +Able +GoButton, +Button and scrolling.

`+Upd`
  +Upd is a prefix class that can be used to override the `upd>` method in existing classes. The following example uses `+Upd` to print updated values to standard error:

          (app)
          (default *Number 0)

          (action
             (html 0 "+Upd" "@lib.css" NIL
                (form NIL
                   (gui '(+Upd +Var +NumField)
                      '(prog (extra) (msg *Number))
                      '*Number 8 )
                   (gui '(+JS +Button) "Increment"
                      '(inc '*Number) ) ) ) )

`+Url`
  Constructs a URL, building it up with session id and an icon. May be used as a prefix class in several, "clickable", classes but is useful by itself too. The following example makes a context clickable:

         (gui '(+View +Url +TextField)
            '(and (port> (: home obj)) (pack "https://foo.net/" @))
            '((Url) (pack Url "/umc/adm.l?*PW=" (pw))) )

  See also +HttpField, +MailField, +TelField and +TextField.

`(url URL . @)`
  Can be called in a button action to jump to an arbitrary URL instead of the normal re-display of the current form.

          (gui '(+Rid +Button) "Contemporaries"
             '(url "!contemporaries" (: home obj)) )

  See also +Button, +Rid.

`+Uppc`
  Prefix class, converts to upper case.

        "Upper case" (gui '(+Uppc +TextField) 30)

  See also +Lowc.