Asynchronous state change

[Carlos Rafael Giani]

On 2015-02-19 11:40, Alicia Romero wrote:
> Hi List,
> I want to change the state of a pipeline and wait till the pipeline 
> has the wanted state.
> Is there a way to know when the pipeline has change to the state we 
> set before?
>
> Thank!!
>
>
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You need to watch out in the bus watch or sync handler for the 
GST_MESSAGE_STATE_CHANGED message. This is the recommended way to do this.
Note that you will get statechange messages from ALL elements, which is 
probably not what you want. GST_MESSAGE_SRC(msg) tells you what element 
sent the msg message. I use this to check if the message was sent by the 
pipeline element. If so, I handle it, otherwise I ignore it. For 
debugging purposes, I also call GST_DEBUG_BIN_TO_DOT_FILE_WITH_TS() in 
the statechange message handler code.

Also keep in mind that the READY->NULL state change is not announced 
anywhere, since the bus is put in a flushing state then.

In the READY->NULL change , I think you can safely assume the change 
happened after the set_state call. I have never seen this state change 
happen asynchronously. However, when I used a sync handler instead of a 
bus watch, I sometimes got statechange messages even after calling 
gst_element_set_state(pipeline, GST_STATE_NULL). I added a check to 
circumvent this strange issue; if the pending state change 
gst_message_parse_state_changed() gives me is set to GST_STATE_NULL 
(note: not the same as GST_STATE_VOID_PENDING), I ignore the 
statechange. This trick is not necessary if you are using a bus watch.
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