Prima::Clipboard - GUI interprocess data exchange


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.


   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 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 '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);

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. 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).

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;

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 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 "Events" in Prima::Application Clipboard for the details.

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.


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 fetch'ing content of Text and UTF8 clipboard slots, the wiget proposes its own pasting scheme, where the mechanics are hidden under the text property call. The property is advised to be used instead of individual 'Text' and 'UTF8' formats. This method is used in all the 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 (default), '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.

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


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, PasteImage action was introduced to handle these cases, together with a symmetrical CopyImage.

The default action of 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.

Note: Under X11 you'll need to keep the image alive during the whole time it might get copied from the application - Prima doesn't keep a copy of the image, only the reference. Changing the image after it was stored in the clipboard will affect the clipboard content.

Exact and meta formats

Prima registers two special meta formats, Image and 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. text can check whether there exists a UTF-8 text storage, before resorting to 8-bit text. image on X11 is even more complicated, and may use image codecs to transfer encoded PNG streams, for example.



image OBJECT, [KEEP]

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


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



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.


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

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.


Returns the data of exact 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 exact format data is present in the clipboard or not.

has_format FORMAT_STRING

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


Returns the system handle for the clipboard object.


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 register_format() are returned if 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.


Returns an 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.


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

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


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.


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

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.


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

   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, <>.


Prima, Prima::Component, Prima::Application