Upstream events are generated by an element somewhere downstream in the pipeline (example: a video sink may generate navigation events that informs upstream elements about the current position of the mouse pointer). This may also happen indirectly on request of the application, for example when the application executes a seek on a pipeline this seek request will be passed on to a sink element which will then in turn generate an upstream seek event.
The most common upstream events are seek events, Quality-of-Service (QoS) and reconfigure events.
An upstream event can be sent using the
gst_pad_send_event function. This
function simply call the default event handler of that pad. The default
event handler of pads is
it basically sends the event to the peer of the internally linked pad.
So upstream events always arrive on the src pad of your element and are
handled by the default event handler except if you override that handler
to handle it yourself. There are some specific cases where you have to
do that :
If you have multiple sink pads in your element. In that case you will have to decide which one of the sink pads you will send the event to (if not all of them).
If you need to handle that event locally. For example a navigation event that you will want to convert before sending it upstream, or a QoS event that you want to handle.
The processing you will do in that event handler does not really matter but there are important rules you have to absolutely respect because one broken element event handler is breaking the whole pipeline event handling. Here they are :
Always handle events you won't handle using the default
gst_pad_event_default method. This method will
depending on the event, forward the event or drop it.
If you are generating some new event based on the one you received don't forget to gst_event_unref the event you received.
Event handler function are supposed to return TRUE or FALSE indicating if the event has been handled or not. Never simply return TRUE/FALSE in that handler except if you really know that you have handled that event.
Remember that the event handler might be called from a different thread than the streaming thread, so make sure you use appropriate locking everywhere.