onStateChange method
Implementation
@override
void onStateChange(LifecycleOwner owner, LifecycleState state) {
if (state == LifecycleState.destroyed) {
clear();
} else if (_weakReference.target == null) {
// 如果发现已经被销毁了 则将当前的Observer也注销掉
owner.removeLifecycleObserver(this, fullCycle: false);
}
}