NAME

       Prima::Clipboard - GUI interprocess data exchange

DESCRIPTION

       Prima::Clipboard class is a descendant of Prima::Component.  It serves as an interface to
       the specific data storage, called clipboard, visible to all clients of one GUI space. The
       system clipboard is intended for the exchange of information of an arbitrary type between
       graphic applications.

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;

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
       recognizes the data.

       Different systems provide spectrum of predefined data types, but the toolkit uses only
       three of these - 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 'Text', 'UTF8', and '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);

       This simplistic code hides other aspects of Prima::Clipboard class.

       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.
       "get_standard_clipboards()" method returns strings for the clipboards, provided by the
       system.

       Second, code for fetching and storing multi-format data is somewhat different. Clipboard
       is viewed as a shared system resource, and have 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 the clipboard locking is advisory, and any
       process can grab clipboard at any time) .

       "fetch()" and "store()" implicitly call "open()" and "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;
          }

   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 event.  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 does not mean deletion,
       thus. If two processes use a custom format, and one exits and re-starts, it still can
       access the data in the same format, registered by its previous incarnation.

   Unicode
       In real life, application often interchange text in both ascii and utf8, leaving the
       choice to reader programs.  While it is possible to access both at the same time, by
       "fetch"'ing content of "Text" and "UTF8" clipboard slots, widgets implement their own
       pasting scheme. To avoid hacking widget code, usage of "text" property is advised instead
       of indicating 'Text' and 'UTF8' constants. This method is used in standard widgets, and is
       implemented so the programmer can reprogram its default action by overloading "PasteText"
       notification of "Prima::Application" ( see "PasteText" in Prima::Application ).

       The default action of "PasteText" is to query first if 'Text' format is available, and if
       so, return the ascii text scalar. If "Prima::Application::wantUnicodeInput" is set, 'UTF8'
       format is checked before resorting to '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 "Prima::Application::wantUnicodeInput" set, etc.

API

   Properties
       image OBJECT
           Provides access to an image, stored in the system clipboard.  In get-mode call, return
           "undef" if no image is stored.

       text STRING
           Provides access to text stored in the system clipboard.  In get-mode call, return
           "undef" if no text information is present.

   Methods
       clear
           Deletes all data from clipboard.

       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.

       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.

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

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

       get_handle
           Returns a system handle for a clipboard object.

       get_formats
           Returns 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 "register_format()" are
           returned. There is no way to see if a format, not registered before, is present.

       get_registered_formats
           Returns array of strings, each representing a registered format. "Text" and "Image"
           are returned also.

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

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

       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
           "register_format()" are de-registered with "deregister_format()" when a program is
           finished.

       store FORMAT_STRING, SCALAR
           Stores SCALAR value into the clipboard in FORMAT_STRING 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 Storable
           module from CPAN collection provides the system-independent conversion routines.

AUTHOR

       Dmitry Karasik, <dmitry@karasik.eu.org>.

SEE ALSO

       Prima, Prima::Component, Prima::Application