Whether this Ticker has scheduled a call to call its callback on the next frame.
A ticker that is muted can be active (see isActive) yet not be ticking. In that case, the ticker will not call its callback, and isTicking will be false, but time will still be progressing.
Source
// TODO(ianh): we should teach the scheduler binding about the lifecycle events // and then this could return an accurate view of the actual scheduler. bool get isTicking => _completer != null && !muted;