X Window Selection - Selections

Selections

The X Window System supports an arbitrary number of selections; every selection is identified by a string (more precisely, an atom). The most used selection is the PRIMARY selection.

The following requests are specific to selection transfer, although transfer also involves other requests:

1. request to know which window owns the selection
2. request to set the window that owns the selection
3. request to convert the selection

The owner of the selection is typically the window in which the selected text is located, if any. When the user selects some text in a window, the client handling the window must tell the server that the window is the owner of the selection.

When the user tries to paste the selection into another window, that window's handler initiates a protocol for getting the selected text from the other client. This protocol involves the second and third requests of the list above, and is not specified by the X protocol but as a convention in the Inter-Client Communication Convention Manual (ICCCM).

In particular, the destination client begins by asking the server which window owns the selection. Then the two clients transfer the selection via the server. This exchange involves a property of a window, an arbitrary piece of data attached to the window. If the content of the selection is considered small enough to be transferred all at once, the steps that take place are:

1. the recipient of the selection requests the selection to be converted, specifying a property of a window (this may be the window where the text has to be pasted)
2. in response, the server sends to the current owner of the selection a SelectionRequest event;
3. the owner places the selected text in the property of the window that the requestor has specified by sending a ChangeProperty; request to the server
4. the owner sends a request to the server to send the requester a SelectionNotify to notify that the selection has been transferred
5. the requester can now read the selection in the property of the window by sending one or more GetProperty requests to the server;
6. the requester destroys the property; if the owner has requested to be informed of this, it is sent a PropertyNotify event.

If the content is large, it should be transferred in chunks. In this case, both clients express interest in PropertyNotify events: this way, the selection owner knows when the selection has been read, and the requester knows when another chunk has been placed in the property.