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The following text is lifted verbatim from Ben Wing's comments in `ExternalShell.c'.
This is a special Shell that is designed to use an externally- provided window created by someone else (possibly another process). That other window should have an associated widget of class ExternalClient. The two widgets communicate with each other using ClientMessage events and properties on the external window.
Ideally this feature should be independent of Emacs. Unfortunately there are lots and lots of specifics that need to be dealt with for this to work properly, and some of them can't conveniently be handled within the widget's methods. Some day the code may be rewritten so that the embedded-widget feature can be used by any application, with appropriate entry points that are called at specific points within the application.
This feature is similar to the OLE (Object Linking & Embedding) feature provided by MS Windows.
Communication between this shell and the client widget:
Communication is through ClientMessage events with message_type EXTW_NOTIFY and format 32. Both the shell and the client widget communicate with each other by sending the message to the same window (the "external window" below), and the data.l value is used to determine who sent the message.
The data is formatted as follows:
data.l = who sent this message: external_shell_send (0) or external_client_send (1) data.l = message type (see enum en_extw_notify below) data.l[2-4] = data associated with this message
EventHandler() handles messages from the other side.
extw_send_notify_3() sends a message to the other side.
extw_send_geometry_value() is used when an XtWidgetGeometry structure needs to be sent. This is too much data to fit into a ClientMessage, so the data is stored in a property and then extw_send_notify_3() is called.
extw_get_geometry_value() receives an XtWidgetGeometry structure from a property.
extw_wait_for_response() is used when a response to a sent message is expected. It looks for a matching event within a particular timeout.
The particular message types are as follows:
1) extw_notify_init (event_window, event_mask)
This is sent from the shell to the client after the shell realizes its EmacsFrame widget on the client's "external window". This tells the client that it should start passing along events of the types specified in event_mask. event_window specifies the window of the EmacsFrame widget, which is a child of the client's external window.
When the client receives an extw_notify_init message from the shell, it sends back a message of the same sort specifying the type of the toolkit used by the client (Motif, generic Xt, or Xlib).
2) extw_notify_end ()
This is sent from the shell to the client when the shell's EmacsFrame widget is destroyed, and tells the client to stop passing events along.
3) extw_notify_qg (result)
This is sent from the client to the shell when a QueryGeometry request is received on the client. The XtWidgetGeometry structure specified in the QueryGeometry request is passed on in the EXTW_QUERY_GEOMETRY property (of type EXTW_WIDGET_GEOMETRY) on the external window. result is unused.
In response, the shell passes the QueryGeometry request down the widget tree, and when a response is received, sends a message of type extw_notify_qg back to the client, with result specifying the GeometryResult value. If this value is XtGeometryAlmost, the returned XtWidgetGeometry structure is stored into the same property as above. [BPW is there a possible race condition here?]
4) extw_notify_gm (result)
A very similar procedure to that for extw_notify_qg is followed when the shell's RootGeometryManager method is called, indicating that a child widget wishes to change the shell's geometry. The XtWidgetGeometry structure is stored in the EXTW_GEOMETRY_MANAGER property.
5) extw_notify_focus_in (), extw_notify_focus_out ()
These are sent from the client to the shell when the client gains or loses the keyboard focus. It is done this way because Xt maintains its own concept of keyboard focus and only the client knows this information.
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