On Mon, 11 Sep 2023 12:53:15 +0100 John Cox <j...@kynesim.co.uk> wrote:
> Hi > I am clearly deeply confused by event queues and proxies - I have this > snippet of code (from some VLC work I'm doing): > > sys->eventq = wl_display_create_queue(video_display(sys)); > if (sys->eventq == NULL) { > msg_Err(vd, "Failed to create event Q"); > goto error; > } > if ((sys->wrapped_display = > wl_proxy_create_wrapper(video_display(sys))) == NULL) > { > msg_Err(vd, "Failed to create wrapper"); > goto error; > } > wl_proxy_set_queue((struct wl_proxy *)sys->wrapped_display, sys->eventq); > > struct wl_callback * cb; > struct wl_registry *const registry = > wl_display_get_registry(sys->wrapped_display); > if (registry == NULL) { > msg_Err(vd, "Cannot get registry for display"); > goto error; > } > > wl_registry_add_listener(registry, ®istry_cbs, vd); > cb = wl_display_sync(sys->wrapped_display); > wl_callback_add_listener(cb, ®_done_sync_listener, vd); > > msg_Info(vd, "Roundtrip start"); > wl_display_roundtrip_queue(sys->wrapped_display, sys->eventq); The problem is probably here. wrapped_display is just a shell of a wl_proxy, but wl_display_roundtrip_queue() needs the original wl_display, just like wl_display_dispatch_queue() it calls. Thanks to elinor in IRC for pointing this out, when I first went on a complete wild goose chase wondering if a wl_display can even be wrapped, but of course it must be wrappable. > msg_Info(vd, "Roundtrip done"); > wl_registry_destroy(registry); > > video_display() returns the display we are rendering to. My > expectation was that the registry & sync listeners would be called > somewhere between the roundtrip start/end debug lines, but what seems > to happen is that the roundtrip just returns immediately. > > I have clearly misunderstood something at a fundamental level. I > thought that the wrapped display would set the Q that the listeners > would send events on and th eroundtrip would wait for that. The > wl_proxy_create_wrapper docn on > https://wayland.freedesktop.org/docs/html/apb.html#Client-classwl__proxy > also seems to imply this. > > Any hints would be gratefully received - thanks Yeah, it's a bit awkward to have wl_display both as a proxy and a Wayland connection. Some APIs need only a wl_proxy, others need the connection. That's something to add in the docs somehow. As a rule of thumb, I'd say that anything that explicitly takes a 'struct wl_display *' as an argument in wayland-client-core.h needs the original wl_display and does not work with a proxy-wrapper. Another exception is that you cannot change the queue of the original wl_display (or at least it's buggy), while a proxy-wrapped wl_display you can. Thanks, pq
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