[Xevie] Question about synchronous grabs

[Jim Gettys]

Yes, you are onto why there are passive grabs in X...

Here's an example scenario for passive grabs:

You want to click on a window to set the focus, pop the window to the
top, and also want the event to set the input point in the input window;
you want subsequent typing to go to the "right place".

So the window manager might set a passive grab on a window, get the
event, set the input focus and restack the windows, and then AllowEvents
and replay the event to set the input focus to the "right place", and
the user keyboard events will go to the "right place" in the right
order.

This allows X to do various operations reliably even if applications are
not immediately responsive, and avoid a ton of race conditions.

You can think of passive grabs as "traps" waiting to be sprung, so that
events can always get delivered in the right order, despite the need for
other processing.

Unfortunately, many of the current toolkits do not exploit this
capability in X as much as they might/should.  Machines are no longer
so slow, but if you are paged out, they can still take measurable
amounts of time before applications are able to respond....
				- Jim

On Thu, 2004-11-11 at 11:58 -0800, Deron Johnson wrote:
> Does anybody know what is the purpose of a synchronous grab in X?
> Basically, a synchronous grab records incoming events while it is
> active instead of sending them down the event pipeline. Only when
> a client actively releases the synchronous grab does the event
> pipeline start flowing again. And the recorded events are first played
> back. So I know how synchronous grabs behave, but I'm not entirely
> sure that I know what purpose they serve.
> 
> One purpose I can think of is to implement type-ahead (or mouse-ahead).
> The user clicks a button which brings up a dialog. The button click
> triggers a passive grab which is synchronous. The app then maps a
> dialog window. After the window is up the app releases the synchronous
> grab and any key events which the user has typed while the dialog
> window was coming up will be sent to the dialog window.
> 
> Are there any other uses of synchronous grabs? How prevalent are
> apps that use synchronous grabs? Does anyone know whether GNOME or
> KDE use them?
> 
> The reason I ask is because synchronous grabs account of a significant
> amount of complexity in the X server event code. I am questioning
> whether it is worth trying to support these in my Display Server event
> subsystem. The code is so complicated and involved that there are bound
> to be transcription errors during any port.
> 
> 
> 
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