public delegate BusSyncReply BusSyncHandler
Handler will be invoked synchronously, when a new message has been injected into the bus.
This function is mostly used internally. Only one sync handler can be attached to a given bus.
If the handler returns GST_BUS_DROP, it should unref the message, else the message should not be unreffed by the sync handler.
||the Bus that sent the message
||user data that has been given, when registering the handler
|BusSyncReply stating what to do with the message