Sets the synchronous handler on the bus.
The function will be called every time a new message is posted on the bus. Note that the function will be called in the same thread context as the posting object. This function is usually only called by the creator of the bus. Applications should handle messages asynchronously using the gst_bus watch and poll functions.
Before 1.16.3 it was not possible to replace an existing handler and clearing an existing handler with
null was not thread-safe.
a Bus to install the handler on
The handler function to install
User data that will be sent to the handler function.