onFinish method
This method is called once when the effect is about to finish, but before
it is removed from its parent. The notion of "about to finish" is defined
by the controller: this method is called when controller.completed
property first becomes true.
If the effect is reset, its onFinish method will be called again after the effect has finished again.
Implementation
@mustCallSuper
void onFinish() {
onComplete?.call();
_completer?.complete();
_completer = null;
}