Callback function called zero or more times with a sequence of values.
may be called back synchronously, immediately when selector function is called.
may also be called back later asynchronously any number of times.
may be called multiple sequential times with the same value.
Callback function called zero or more times with a sequence of values. may be called back synchronously, immediately when selector function is called. may also be called back later asynchronously any number of times. may be called multiple sequential times with the same value.