产生的背景
先看个代码
class LifeCycleOldActivity : AppCompatActivity(){ lateinit var presenter: Presenter override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) presenter = Presenter() presenter.onCreate() } override fun onDestroy() { super.onDestroy() presenter.onDestroy() } } class Presenter{ fun onCreate(){} fun onDestroy(){} }乍一看，很普通的逻辑，onCreate创建，onDestroy销毁，规范合理严谨。这是咱们祖辈总结出的模板性代码。但是这个有以下几个问题：
- 页面多了这种代码将到处都是
- Activity里太多P的代码，依赖度太高
- 执行顺序上不一定有保证，presenter.onDestroy不是一定会在presenter.onCreate后执行的，有可能出现onDestroy都走到了，onCreate后的一些方法还在执行
- 感知其它方法又要搞一大堆代码，乏味还不一定对
主要解决的痛点
核心还是优雅的去感知生命周期组件:
- 将声明周期感知的逻辑从生命周期组件中分离出来，将感知对象作为生命周期组件的一个观察者就好
- 感知自由度高且渐变，想感知啥的话注解加一下就好
- LifeCycle内部处理严格，基于状态去处理，可以自由的根据状态去执行相应逻辑，确保生命周期执行的合理和安全性
class LifecycleNewActivity : AppCompatActivity(){ override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) //LifecycleNewActivity 实现了LifecycleOwner 是一个生命周期组件 //添加对应的观察者 lifecycle.addObserver(NewPresenter()) } fun requestSuccess(){ //可以确保此处执行的安全性 if(lifecycle.currentState.isAtLeast(Lifecycle.State.STARTED)){ //refresh } } } /** * 生命周期组件的观察者，在这里感应生命周期变化就好 */ class NewPresenter : LifecycleObserver{ //通过注解来决定感知啥事件 @OnLifecycleEvent(Lifecycle.Event.ON_CREATE) fun onCreate(){ } }
核心类说明
LifeCycle
- 三个核心方法
- addObserver：添加观察者
- removeObserver：移除观察者
- getCurrentState：获取当前状态
- 两个枚举对象
- Event 事件
ON_CREATE,
ON_START,
ON_RESUME,
ON_PAUSE,
ON_STOP,
ON_DESTROY,
ON_ANY- State 状态
- INITLALIZED 初始态：这是他被构造但还没收完成onCreate方法时的状态-->onCreate方法体状态
- CREATED 创建态：在两种情况下会达到此状态:在onCreate调用之后;就在onStop调用之前-->onStart onStop 方法体状态
- STARTED 启动态：在两种情况下会达到此状态:在onStart调用之后;就在onPause调用之前-->onResume onPause 方法体状态
- RESUMED 可见态：调用onResume后达到此状态-->页面可见后onResume最后随便一个异步后就是此状态了
- DESTORYED 销毁态：此状态后，将不再分发其它事件；在调用onDestroy方法前就会到达这个状态
LifecycleOwner
生命周期的拥有者，只有一个接口方法Lifecycle getLifecycle()；实现了该接口的类，被定义为生命周期组件。
LifecycleObserver
生命周期组件的观察者，这是一个没有接口方法的接口，标识性接口，实现该接口的类在运行时通过解析OnLifecycleEvent注解达到指定生命周期事件的观察者的回调
LifecycleRegistry
系统提供的Lifecycle的实现，提供了如何感应生命周期变化的标准实践
源码段解析
以Activity为例解析生命周期感知的源码流程
//实现了LifecycleOwner public class ComponentActivity ... implements LifecycleOwner { protected void onCreate(@Nullable Bundle savedInstanceState) { ... // 转接给ReportFragment去影响生命周期，这也是Glide影响生命周期的方案 ReportFragment.injectIfNeededIn(this); ... } } public class ReportFragment extends android.app.Fragment { ... public static void injectIfNeededIn(Activity activity) { if (Build.VERSION.SDK_INT >= 29) { // 29+版本 通过LifecycleCallbacks感知生命周期 LifecycleCallbacks.registerIn(activity); } //通过添加个无页面Fragment感知生命周期 android.app.FragmentManager manager = activity.getFragmentManager(); if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) { manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit(); manager.executePendingTransactions(); } } @Override public void onActivityCreated(Bundle savedInstanceState) { ... //分发生命周期事件 dispatch(Lifecycle.Event.ON_CREATE); } static class LifecycleCallbacks implements Application.ActivityLifecycleCallbacks { //onCreate执行后才会发送ON_CREATE事件 @Override public void onActivityPostCreated(@NonNull Activity activity, @Nullable Bundle savedInstanceState) { dispatch(activity, Lifecycle.Event.ON_CREATE); } } /** * 最终交由LifecycleRegistry的handleLifecycleEvent方法处理 **/ static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) { if (activity instanceof LifecycleRegistryOwner) { ((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event); return; } if (activity instanceof LifecycleOwner) { Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle(); if (lifecycle instanceof LifecycleRegistry) { ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event); } } } } public class LifecycleRegistry extends Lifecycle { public void handleLifecycleEvent(@NonNull Lifecycle.Event event) { //根据当前事件获取下一个目标状态 State next = getStateAfter(event); //根据当前事件将状态移动到对应状态 moveToState(next); } public void addObserver(@NonNull LifecycleObserver observer) { ... //粘性事件的源头 statefulObserver.dispatchEvent(lifecycleOwner, event); ... } //移动状态 private void moveToState(State next) { //当前状态和目标状态一直，不继续了 if (mState == next) { return; } //修改当前状态为目标状态 mState = next; //正在处理事件，或者正在添加观察者暂不继续 if (mHandlingEvent || mAddingObserverCounter != 0) { mNewEventOccurred = true; // we will figure out what to do on upper level. return; } //修改处理事件中标注为true mHandlingEvent = true; //同步去处理状态 sync(); //处理完后将标志改为false mHandlingEvent = false; } //同步生命周期状态 private void sync() { LifecycleOwner lifecycleOwner = mLifecycleOwner.get(); //生命周期拥有者为空了，直接抛异常，一般出现在被系统回收后，异常的意思是，拥有者被回收了，改状态太晚了！ if (lifecycleOwner == null) { throw new IllegalStateException("LifecycleOwner of this LifecycleRegistry is already" + "garbage collected. It is too late to change lifecycle state."); } //还有未同步的观察者的话会无限循环去进行状态同步 while (!isSynced()) { mNewEventOccurred = false; // no need to check eldest for nullability, because isSynced does it for us. if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) { //向后同步 backwardPass(lifecycleOwner); } Map.Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest(); if (!mNewEventOccurred && newest != null && mState.compareTo(newest.getValue().mState) > 0) { //向前同步 forwardPass(lifecycleOwner); } } mNewEventOccurred = false; } //true 都同步完了 false 还有未同步的 private boolean isSynced() { if (mObserverMap.size() == 0) { return true;//木有观察者，按都是已经同步过的处理 } State eldestObserverState = mObserverMap.eldest().getValue().mState; State newestObserverState = mObserverMap.newest().getValue().mState; return eldestObserverState == newestObserverState && mState == newestObserverState; } private void backwardPass(LifecycleOwner lifecycleOwner) { ... //根据状态映射回事件 Event event = Event.downFrom(observer.mState); ... //想观察者发送事件 observer.dispatchEvent(lifecycleOwner, event); ... } } static class ObserverWithState { ... //分发事件 void dispatchEvent(LifecycleOwner owner, Event event) { ... mLifecycleObserver.onStateChanged(owner, event); ... } } } //onStateChanged的实现 class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver { ... @Override public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Event event) { mInfo.invokeCallbacks(source, event, mWrapped); } } static class CallbackInfo { ... //通过反射调用LifecycleObserver被OnLifecycleEvent注解标注的方法 @SuppressWarnings("ConstantConditions") void invokeCallbacks(LifecycleOwner source, Lifecycle.Event event, Object target) { invokeMethodsForEvent(mEventToHandlers.get(event), source, event, target); invokeMethodsForEvent(mEventToHandlers.get(Lifecycle.Event.ON_ANY), source, event, target); } } //事件与状态的对象关系 public abstract class Lifecycle { ... //这个方法结合ReportFragment的dispatch的调用时机，完美的解释了，当回调某个生命周期方法后所对应的状态情况 public State getStateAfter() { switch (this) { case ON_CREATE: // 这个事件时在onActivityPostCreated发送的也就是onCreate执行完后 case ON_STOP: // 这个事件时在onActivityPreStopped时发送的也就是onStop执行之前 return State.CREATED; //综合结论就是onCreate之后和onStop之间进入CREATED状态 case ON_START: case ON_PAUSE: return State.STARTED; case ON_RESUME: return State.RESUMED; case ON_DESTROY: return State.DESTROYED; case ON_ANY: break; } throw new IllegalArgumentException(this + " has no target state"); } ... }
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