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Swallowing X apps

To Swallow an X app, you basically reparent the toplevel window of the desired app, to find the apps window the method is to continually XQueryTree on the root window, and XFetchName on each window until the name of the app you want to swallow appears. Then you just reparent that window. A number of issues arise here.

The window manager desires to reparent that window itself and put its decoration windows around it. So by the time your program gets around to looking for the window it could be the child or the child of a child of a window manager window. Thus you have to XQueryTree to 3 levels down. Window managers such as twm and afterstep only put one window between the app window and the root window, whilst the CDE and fvwm etc place two. This appears to be the max in practice, but i havent checked Enlightment which id guess might even do more. Once the window has been found, the window manager will probably be managing it, so you must inform it to no longer worry about that window, which is done via XWithdrawWindow.
When the window is withdrawn prior to being reparented then according to ICCCM Version 2.0, Section 4.1.4 when a client withdraws a window it should wait until the WM_STATE property is updated or removed by the windowmanager before reparenting the window, the code (at the end) i use doesn't do this yet, so i moronically reparented loads of times and eventually it gets reparented. I figured it out, but the code here does the wrong thing. The full XSwallow (http://www.csn.ul.ie/~caolan/docs/XSwallow.html) netscape plugin which prompted this article now does the right thing.
The program to be swallowed must appear on the screen before being swallowed, so theres a brief time before the window appears and when its reparanted to the swallowing app. To solve this irritation you could run the program to be swallowed as an icon and/or off screen, something like

 xload -iconic -geometry +9000+9000

Another issue is that a window manager that requires you to place a programs window manually when its run will be awkward to use, as you'll have to manually place the program thats going to be swallowed, then have it dissappear and reappear. Running it iconic though appears to solve this little annoyance for all the window managers that i used that have this behaviour (twm/afterstep). On the other hand fvwm95 wont get rid of the window if it is run iconic, for this window manager you have to run the app non-iconic or the window wont go away.
Now yet another issue is the kind of behaviour that the window manager WindowMaker exhibits in which an application that is run under it will be assigned a slot on its dock, even after the application is reparented that icon remains, this can be over come by telling WindowMaker that it is to no longer care about this app by setting the WMHints of the swallowed app so that the window_group member is changed to the root window of the screen, doing that will get WindowMaker to ignore the app

Seeing as the swallowing app has to look for the windows name it makes sense to make that name unique. So you could run the app to be swallow with a unique name (done usually via a -name commandline option) to make the chance of the swallowing program swallowing the wrong program minimal.

The source of a small program that swallows any X app that calls itself "barf" is available (http://www.csn.ul.ie/~caolan/pub/X/xswallow.c)

compile it like this

 gcc -o xswallow xswallow.c -lXaw -lXt -lX11 -L/usr/X11R6/lib

run thus

 ./xswallow &

and get it to swallow for example xload like this

 ./xload -name barf &

apps window, it really dumb but shows the swallowing clearly. Once the app is swallowed, its just another window and the managing of child windows is outside the scope of this likkle hack.

All told this looks like a very handy way to include X programs within others, without pain.

Swallowing X apps

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