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Prima::Clipboard

GUI interprocess data exchange

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.

   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;

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;
   }

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.

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.

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

Deletes all data from clipboard.
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.
De-registers a previously registered data format. Called implicitly for all not de-registered format before a clipboard object is destroyed.
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.
Returns a boolean flag, showing whether FORMAT_STRING format data is present in the clipboard or not.
Returns a system handle for a clipboard object.
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.

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

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

Prima, Prima::Component, Prima::Application