Adds an observer for a single parameter or all parameters in a group.
- iOS 10.0+
- macOS 10.12+
- tvOS 10.0+
A block called after the value of a parameter has changed.
An audio unit view can use an observer to be notified of changes to a single parameter or all parameters in a group. These callbacks are throttled so as to limit the rate of redundant notifications in the case of frequent changes to a single parameter.
This block is called in an arbitrary thread context and it is responsible for thread-safety. It must not make any calls to add or remove other observers, including itself, as this will deadlock.