[gst-devel] how to make gstreamer run in separate thread

[Gregory Petrosyan]

On Mon, May 3, 2010 at 12:21 PM, Gregory Petrosyan
<gregory.petrosyan at gmail.com> wrote:
> On Mon, May 3, 2010 at 11:32 AM, Edward Hervey <bilboed at gmail.com> wrote:
>> On Sun, 2010-05-02 at 17:20 +0400, Gregory Petrosyan wrote:
>>> On Sun, Apr 25, 2010 at 10:35 PM, Alexey Chernov <4ernov at gmail.com> wrote:
>>> > Thank you guys for support, I finally managed to make gstreamer process
>>> > callbacks in separate thread. The solution was to set bus handler with
>>> > gst_bus_set_sync_handler() instead of gst_bus_add_watch() as wl2776 proposed,
>>> > special thanks for him. So here're some things which I found out from this
>>> > case:
>>> > 1. Qt actually dispatches glib events and no additional g_main_loop_run() is
>>> > necessary for Qt applications.
>>> > 2. Qt seems to create separate glib context for glib objects which are created
>>> > in separate QThread objects.
>>> > 3. To make callbacks be called in separate thread where the certain gst
>>> > objects were created you need to set them as a handler with
>>> > gst_bus_set_sync_handler() to make them processed in the context of certain
>>> > objects and not in the main context.
>>>
>>> Hi!
>>>
>>> I'm also trying to use GStreamer with Qt, and I'm experiencing some
>>> weird problems.
>>>
>>> 1) I'm not sure if Qt dispatches glib events (on Windows, at least):
>>> gst_bus_add_watch()'ed callback is never called
>>
>>  Use gst_bus_add_signal_watch(bus) and then connect to the 'message'
>> event of the bus. If Qt is built with glib and uses the GMainLoop
>> internally, the message signal will be emitted in the main thread.
>
> Thanks,
>
> If you mean that I should use g_signal_connect(bus, "message", ...)
> after gst_bus_add_signal_watch(bus), then no, unfortunately that does
> not work. Looks like default windows Qt version does not dispatch glib
> events.
>
>>> 2) gst_bus_set_sync_handler() works, but events that I send from it
>>> (using Qt::QueuedConnection) seem to never reach the main loop, and I
>>> don't even know how to debug this
>>
>>  It's a *synchronous* handler, therefore the callback you set will be
>> called in the thread context of whoever emitted that message.
>
> Sure, I know this :-)
>
> I was trying to post Qt events from this synchronous handler to the
> main thread (using QMetaObject::invokeMethod(&obj, "method",
> Qt::QueuedConnection) to be precise).

OK, I've nailed down the problem. It is in no way GStreamer-related:
I've reimplemented QObject::event() in my widget, and forgot to call
the base method. Because of this, events were not dispatching
correctly. Hope this message can save hours of debugging for someone
:-)

                Gregory