Lifecycle 使用及原理解析 一文搞懂Lifecycle是Android Architecture Compone - 掘金 (juejin.cn)
Lifecycle原理解析，人人都能看得懂 - 知乎
1.创建生命周期观察者
- 实现 LifecycleObserver接口，并搭配使用@OnLifecycleEvent注解的方式
- 如果使用注解，ObserverWithState会调用Lifecycling.lifecycleEventObserver(observer)生成对应的ReflectiveGenericLifecycleObserver，采用反射的方式解析成HashMap<Event, Method>集合，然后遍历HashMap，通过event拿到对应的method进行会调。
public class MyObserver implements LifecycleObserver {
private static final String TAG = "MyObserver";
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
public void onCreate() {
Log.w(TAG, "onCreate: ");
}
@OnLifecycleEvent(Lifecycle.Event.ON_START)
public void onStart() {
Log.w(TAG, "onStart: ");
}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
public void onResume() {
Log.w(TAG, "onResume: ");
}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
public void onPause() {
Log.w(TAG, "onPause: ");
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
public void onStop() {
Log.w(TAG, "onStop: ");
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
public void onDestroy() {
Log.w(TAG, "onDestroy: ");
}
}
- 实现 LifecycleEventObserver 接口，实现onStateChanged接口，不使用@OnLifecycleEvent注解
public interface LifecycleEventObserver extends LifecycleObserver {
void onStateChanged(LifecycleOwner source, Lifecycle.Event event);
}
public interface LifecycleOwner {
Lifecycle getLifecycle();
}
public class MyObserver2 implements LifecycleEventObserver{
@Override
public void onStateChanged(LifecycleOwner source,Lifecycle.Event event) {
if (event == Lifecycle.Event.ON_DESTROY) {
if (!isChangingConfigurations()) {
getViewModelStore().clear();
}
}
}
}
2.观察生命周期
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//添加一个生命周期观察者 getLifecycle()是FragmentActivity中的方法
MyObserver observer = new MyObserver();
getLifecycle().addObserver(observer);
}
}
3.LifeCycle原理解析
- Activity的父类 ComponentActivity
public class ComponentActivity extends Activity implements LifecycleOwner{
//实例化了一个被观察者
private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
protected void onCreate(@Nullable Bundle savedInstanceState) {
//注入了一个ReportFragment用来感知Activity的生命周期
ReportFragment.injectIfNeededIn(this);
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
}
//接口，用来获取LicycleRegister 实例
public interface LifecycleOwner {
Lifecycle getLifecycle();
}
//抽象类，LifecycleRegistry继承了Lifecycle
public abstract class Lifecycle {
@MainThread
public abstract void addObserver(@NonNull LifecycleObserver observer);
@MainThread
public abstract void removeObserver(@NonNull LifecycleObserver observer);
@MainThread
@NonNull
public abstract State getCurrentState();
@SuppressWarnings("WeakerAccess")
public enum Event {
/**
* Constant for onCreate event of the {@link LifecycleOwner}.
*/
ON_CREATE,
/**
* Constant for onStart event of the {@link LifecycleOwner}.
*/
ON_START,
/**
* Constant for onResume event of the {@link LifecycleOwner}.
*/
ON_RESUME,
/**
* Constant for onPause event of the {@link LifecycleOwner}.
*/
ON_PAUSE,
/**
* Constant for onStop event of the {@link LifecycleOwner}.
*/
ON_STOP,
/**
* Constant for onDestroy event of the {@link LifecycleOwner}.
*/
ON_DESTROY,
/**
* An {@link Event Event} constant that can be used to match all events.
*/
ON_ANY
}
@SuppressWarnings("WeakerAccess")
public enum State {
DESTROYED,
INITIALIZED,
CREATED,
STARTED,
RESUMED;
public boolean isAtLeast(@NonNull State state) {
return compareTo(state) >= 0;
}
}
}
//使用ReportFragment的生命周期来感知Activity的生命周期
public class ReportFragment extends Fragment {
public void onActivityCreated(Bundle savedInstanceState) {
getActivity().getLifecycle().handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
}
public void onStart() {
getActivity().getLifecycle().handleLifecycleEvent(Lifecycle.Event.ON_START);
}
public void onResume() {
getActivity().getLifecycle().handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
}
...
}
image.png
- Lifecycle实现类LifecycleRegistry 被观察者类（添加观察者，删除观察者，通知观察者）
public class LifecycleRegistry extends Lifecycle {
public LifecycleRegistry(@NonNull LifecycleOwner provider) {
mLifecycleOwner = new WeakReference<>(provider);//弱引用
mState = INITIALIZED;//初始state为Initialized 枚举序值为1
}
public void addObserver(@NonNull LifecycleObserver observer) {
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
//把observer封装成ObserverWithState类作为value值
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
// 使用map集合存储观察者
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
}
public void removeObserver(@NonNull LifecycleObserver observer) {
mObserverMap.remove(observer);
}
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);//把event转换成对应的state
mState=next；//把当前event对应的状态赋值给mState
sync();
}
private void sync() {
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
/*当 mObserverMap 中存储的ObserverWhitState最新的状态(刚插入的) 与
最老的状态(最先插入的) 一致，并且当前状态等于 最新的状态，停止同步。 */
while (!isSynced()) {
mNewEventOccurred = false;
//生命周期后退
if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
backwardPass(lifecycleOwner);
}
//生命周期前进
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner);
}
}
}
//生命周期后退
private void backwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
mObserverMap.descendingIterator();
while (descendingIterator.hasNext() && !mNewEventOccurred) {
Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
Event event = downEvent(observer.mState);
pushParentState(getStateAfter(event));
//分发事件
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
//生命周期前进
private void forwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
mObserverMap.iteratorWithAdditions();
while (ascendingIterator.hasNext() && !mNewEventOccurred) {
Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
pushParentState(observer.mState);
//分发事件
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
popParentState();
}
}
}
}
这里会通过LifeRegistry中当前状态mState和存储的状态进行比较操作，判断当然流程是向正在可见发展 还是正在向不可见发展，例如当前执行的状态是mState=START，与上个状态相比，如果上个状态是CREATE，相比结果就是>0 ，说明是正在可见，调用forwardPass()；如果上个状态是RESUME ，相比结果<0 ，说明是正在不可见,backwardPass()
- backwardPass 和 forwardPass 主要有两点不同
1.在对mObserverMap 存储的状态进行遍历时，backwardPass 是以 栈的形式遍历，forwardPass是以队列的形式遍历。
2.对状态还原的时候，backwardPass 是不可见方向还原，也就是上图的粉色箭头方向，forwardPass 是以可见方向还原，也就是上图的 青绿色箭头方向。
- ObserverWithState是对观察者对象和状态进行了一个包装，在这里我们来详细看一下。
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
//如果观察是实现了onStateChange()或者使用@OnLifecycleEvent注解的方式，就会生成对应的Observer
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = event.getTargetState();
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
总结
- 首先我们定义观察者，实现LifecycleEventObserver接口，实现了onStateChanged(event)方法.
- Activity的父类ComponentActivity实现了LifecycleOwner，通过getLifecycle可以获取被观察者对象lifecycleRegistry实例，可以添加删除观察者，遍历观察者, getLifecycle().addObserver()添加观察者的时候，会通过一个map把observer作为key，然后把observer 和状态state【正在可见就是，INITIALIZED，正在消失DESTROY】封装成observerWithState类，作为值存储到一个hashMap里面。
- Activity在创建的时候，调用父类ComponentActivity的onCreate()方法，注入了一个ReportFragment方法用来感知自身的生命周期，所以ReportFragment对应生命周期方法中，onActivityCreate,onResume方法
getActivity.getLifecycle.handleLifecycleEvent(event)对事件进行分发，然后遍历map观察者集合，拿到每个ObserverWithState对象，调用dispatchEvent()分发时间，最终调用observer的onStateChanged方法会调.【在分发过程中，Lifecycle 通过内部维护的状态机 将生命周期事件转化为State状态，并进行存储，在 分发过程中，通过状态比较来判断 当前过程是正在可见还是正在不可见，不同过程采用不同策略】
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