Migration of windows between displays
perry at coders.net
Wed Nov 10 11:50:29 EET 2004
Havoc Pennington wrote:
>>The bugzilla entry is available here:
>>What are peoples comments on this idea?
> There's been some previous hacking on it, though I don't think anyone
> finished; see for example this thread:
> Google or bugzilla search may turn up some more history.
I've not found any other discussion in bugzilla or google despite a long
Encouragingly the wm-spec discussion you linked to above basically did
exactly what I did. The specification as presented there covers a few
more corner cases I did not consider (such as the property not being
cleared on failure).
Their specification also had a list of key/value pairs, for
extensibility. What I implemented only used the raw name of the
$DISPLAY you wanted the window moved to which I think is conceptually
simple and easy to follow. Future expansion can be achieved by adding
key/value pairs after the display separated by a space if absolutely
The older spec has the display broken apart into key/value pairs which
(to me) seems excessively complicated and requires complicated parsing
for no good reason, on the flip side this seems to be inherited from the
startup-notification spec so presumably applications will already be
able to parse these. The GPE project use a similar protocol to me,
however they add an authentication type and key after the display (space
The specification you referred to used _NET_SCREEN_CHANGE, which to me
implies that it's only used for changing between two screens within a
display, where the protocol should be able to be used for changing
between displays too. Hence why I called the atom in my original patch
Matthias Clasen suggests making this a ClientMessage instead of an X
Property. I don't really understand the pro's and con's of these two
approaches. ClientMessage does sound like a more elegant system, but
from what I've seen of the X11 protocol, property notification seems to
be rather prevalent for initiating actions. Is there any major reason
to choose one over the other?
Someone suggested adding a timestamp, I'm not entirely sure why you'd
want a timestamp on the message? If it's necessary (or even likely to
be useful) it could be included after the display separated by a space.
My current thoughts are:
* _NET_DISPLAY_CHANGE is advertised in WM_PROTOCOLS. this is the same
name as the property used to initiate the action to reduce the number of
atoms needing to be created avoiding unnecessary round trips to the server.
* Someone sets the property _NET_DISPLAY_CHANGE on a window that
window then tries to move itself to the destination display immediately.
* The format of the data part of the property is the new $DISPLAY value
optionally followed by a space and some data which is currently ignored
(used for future expansion if necessary).
* If the move fails then the property is removed.
* No extra authentication is done, if you have permission to write
properties to a window then you have permission to migrate the window to
* Transients associated with the window should be moved too.
* Error handling is undefined. Perhaps the sending application can poll
infrequently to see if the window still exists with a now empty
_NET_DISPLAY_CHANGE as indication of failure (although no reason can be
given). This implies that you should not remove the property from the
window before removing the window if the change may be successful (as
this would appear as a failure). Is this prone to race conditions? Are
window ID's reused frequently? Is there a better mechanisms for
notifying an application of failure without needlessly complicating the
* No provision for moving the entire application other than the
initiating application doing it's own iterations over everything within
a class or group, ignoring transients (which are moved as above). An
application is an implementation detail that doesn't need to be
presented to the user (is one application that supports two documents in
separate windows different from two applications with one document open
* There is no _NET_DISPLAY_CHANGE_BEGIN property, I'm not sure why you'd
: Is there some jargon for the format string that the $DISPLAY
variable holds? "display string"?
: People seem to use _NET_DISPLAY_CHANGE rather than
_NET_CHANGE_DISPLAY like I used in my original patch. is there any
convention here I should be aware of?
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