Set the callback function that is called when the operation state
* changes. Usually this is not necessary, since the functions that
* create pa_operation objects already take a callback that is called
* when the operation finishes. Registering a state change callback is
* mainly useful, if you want to get called back also if the operation
* gets cancelled. \since 4.0
Set the callback function that is called when the operation state * changes. Usually this is not necessary, since the functions that * create pa_operation objects already take a callback that is called * when the operation finishes. Registering a state change callback is * mainly useful, if you want to get called back also if the operation * gets cancelled. \since 4.0