Using BClipboard

A BClipboard object is an interface to a clipboard, a resource that provides system-wide, temporary data storage. Clipboards are identified by name; if two apps want to refer to the same clipboard, they simply create respective BClipboard objects with the same name:

 /* App A: This creates a clipboard named "MyClipboard". */
 BClipboard *appAclipboard = new BClipboard("MyClipboard");

 /* App B: This object refers to the clipboard already created by App A. */
 BClipboard *appBclipboard = new BClipboard("MyClipboard");

The System Clipboard

: In practice, you rarely need to construct your own BClipboard object instead, you use the BClipboard that's created for you by your BApplication object. This object, which you refer to through the global be_clipboard variable, accesses the default system clipboard. Data that you write to your be_clipboard object can be read from any other app's be_clipboard. For example, the cut/copy/paste operations defined by BTextView transfer data through the system clipboard.

Note:: To access the system clipboard without creating a BApplication object, construct a BClipboard object with the name "system". The system clipboard is under the control of the user - you should only read or write the systemclipboard as a direct result of the user's actions. If you create your own clipboards don't name them "system".

The Clipboard Message

: To access a clipboard's data, you call functions on a BMessage that the BClipboard object hands you (through its Data() function). The BMessage follows these conventions:

The what value is unused.

The data is stored in a message field. The field should be typed as B_MIME_TYPE, the MIME type that describes the data should be used as the name of the field that holds the data (see "Writing to the Clipboard" for an example).

If the BMessage contains more than one field, each field should present the same data in a different format. For example, the StyledEdit app writes text data in its own format (in order to encode the fonts, colors, etc.) and also writes the data as plain ASCII text (MIME type "text/plain").

Writing to the Clipboard

: The following annotated example shows how to write to the clipboard.

 BMessage *clip = (BMessage *)NULL;
 
 if (be_clipboard->Lock() { // 1
    be_clipboard->Clear(); // 2
    if ((clip = be_clipboard->Data() { // 3
        clip->AddData("text/MyFormat", B_MIME_TYPE, myText, myLength); // 4
        clip->AddData("text/plain", B_MIME_TYPE, asciiText, asciiLength); // 4
        be_clipboard->Commit(); // 5
    }
    be_clipboard->Unlock(); // 6
 }

: 1 Lock() your BClipboard object. This uploads data from the clipboard into your BClipboard's local BMessage object, and prevents other threads in your application from accessing the BClipboard's data. Note that locking does not lock the underlying clipboard data - other applications can change the clipboard while you have your object locked.

: 2 Prepare the BClipboard for writing by calling Clear(). This erases the data that was uploaded from the clipboard.

: 3 Call Data() to get a pointer to the BClipboard's BMessage object.

: 4 Write the data by invoking AddData() directly on the BMessage. In the example, we write the data in two different formats.

: 5 Call Commit() to copy your BMessage back to the clipboard. As soon as you call Commit(), the data that you added is visible to other clipboard clients.

: 6 Unlock() balances the Lock(). The BClipboard object can now be accessed by other threads in your application.

: If you decide that you don't want to commit your changes, you should call Revert() before you unlock.

Reading from the Clipboard

: Here we show how to read a simple string from the clipboard.

 const char *text;
 int32 textLen;
 BMessage *clip = (BMessage *)NULL;

 if (be_clipboard->Lock() { // 1
    if ((clip = be_clipboard->Data();
        clip->FindData("text/plain", B_MIME_TYPE,
            (const void **)&text, &textlen); // 2
    be_clipboard->Unlock(); // 3
 }

: 1 As in writing, we bracket the operation with Lock() and Unlock(). Keep in mind that Lock() uploads data from the clipboard into our object. Any changes that are made to the clipboard (by some other application) after Lock() is called won't be seen here.

: 2 In this example, we only look for one hard-coded format. In a real application, you may have a list of formats that you can look for.

: 3 It isn't necessary to examine the clipboard data before you unlock it. The FindData() call could just as well have been performed after the Unlock() call.

Persistence

: Inter-boot persistence: Clipboard data does not persist between boots - the constructor provides a persistence flag, but it's currently unused.

: Intra-boot persistence: Once you've created a clipboard, that clipboard will exist until you reboot your computer. For example, let's say you design an app that creates a clipboard called "MyClip": You launch the app, write something to "MyClip", and then quit the app. The clipboard - and the data that you wrote to it - will still exist: If you relaunch your app (or any app that knows about "MyClip"), you can pick up the data by reading from the "MyClip" clipboard.