[gst-devel] bus-sync-handler vs bus-watch vs both.

[wl2776]

wl2776 wrote:
> 
> 
> wl2776 wrote:
>> 
>> My player sets both the bus-sync-signal-handler and the bus-watch.
>> And, it seems, bus-watch is not called at all.
>> And the code, processing the messages, is identical in these handlers.
>> The difference is only in the return values.
>> So, is it necessary to set up both handlers?
>> If no, what should I choose?
>> 
> Found. The documentation says, the bus-sync-signal-handler "is usually
> only called by the creator of the bus. Applications should handle messages
> asynchronously using the gst_bus watch and poll functions."
> However, my player has to pause the pipeline after the first frame is
> displayed, I can only do this in the bus-sync-handler.
> 
Finally, after some research, I've come to an answer. 
I can also answer the question, which was here, about the running
g_main_loop in a separate thread - that's possible, I've performed this
using GThreads.
I am writing an embedded player, which is an ActiveX component, and it
seems, there is no main loop by default, where I could insert gst_bus_poll.

When the message is posted, the bus-sync-handler is called immediately, and
there are a lot of functions like gst_element_post_message in the call stack
- this handler is called from elements' threads.
A bit later, a bus-watch is called, the call stack is much less, and it
contains g_main_loop function.

Since that, bus-sync-handler should process a minimum messages, which really
must be processed very quickly. All other processing should be in the
bus-watch. 

For example, my handler processes GST_MESSAGE_ELEMENT, if it contains the
"prepare-xwindow-id" bit, to set the HWND, and GST_MESSAGE_STATE_CHANGED in
the very beginning, if the message source is the playbin2 - my player has to
display the first video frame and go to the paused state. 
The sync-handler returns GST_BUS_DROP on these messages and unrefs them and
GST_BUS_PASS on all others.
All other messages (EOS, ERROR, BUFFERING, TAG, etc) are processed in the
bus-watch.

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