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Lifecycle框架用来感知组件执行操作以响应另一个组件(例如Activity和Fragment)的生命周期状态的更改。这些组件可帮助您生成更易于组织且通常更轻量级的代码,这些代码更易于维护。
举个例子: 如果你需要在Activity或者Fragment的声明周期里做一些处理:override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) presenter.onCreate() }override fun onStart(){ super.onStart() presenter.onStart()}...override fun onDestroy(){ super.onDestroy() presenter.onDestroy()}
这种写法需要在Activity或者Fragment的相应生命周期函数里调用presenter的相应方法,这种写法显得比较厚重,不够轻盈也没有解耦presenter和Activity,lifecycle的出现解决了这个问题
class MainActivity : AppCompatActivity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) lifecycle.addObserver(Presenter()) }}class Presenter():LifecycleObserver { @OnLifecycleEvent(Lifecycle.Event.ON_CREATE) fun onCreate(){ println("presenter oncreate") } @OnLifecycleEvent(Lifecycle.Event.ON_START) fun onStart(){ println("presenter onstart") } @OnLifecycleEvent(Lifecycle.Event.ON_RESUME) fun onResumse(){ println("presenter on resume") } @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE) fun onPause(){ println("presenter on pause") } @OnLifecycleEvent(Lifecycle.Event.ON_STOP) fun onStop(){ println("presenter on stop") } @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY) fun onDestroy(){ println("presenter on destroy") }
这样Presenter和Activity的关系就由关联关系转变为依赖关系,降低了耦合度,同时Presenter现在也可以用来感知Activity相应的生命周期,不用再在Activity的声明周期函数里调用Presenter的方法。
通过接口和方法的名字大家就可以发现整个Lifecycle其实就是一个观察者模式,下面就是一个观察者模式的UML类图
Lifecycle是一个生命周期感知的框架,那么它是如何感知Activity的生命周期呢?看SupportActivity的onCretae方法
protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); ReportFragment.injectIfNeededIn(this); }
在这里首先调用了ReportFragment.injectIfNeededIn(this)方法,看这个方法
public static void injectIfNeededIn(Activity activity) { // ProcessLifecycleOwner should always correctly work and some activities may not extend // FragmentActivity from support lib, so we use framework fragments for activities android.app.FragmentManager manager = activity.getFragmentManager(); if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) { manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit(); // Hopefully, we are the first to make a transaction. manager.executePendingTransactions(); } }
很简单,就是在当前的Activity里添加一个ReportFragment。再看ReportFragment的生命周期函数。
@Override public void onActivityCreated(Bundle savedInstanceState) { super.onActivityCreated(savedInstanceState); dispatchCreate(mProcessListener); dispatch(Lifecycle.Event.ON_CREATE); } @Override public void onStart() { super.onStart(); dispatchStart(mProcessListener); dispatch(Lifecycle.Event.ON_START); } @Override public void onResume() { super.onResume(); dispatchResume(mProcessListener); dispatch(Lifecycle.Event.ON_RESUME); } @Override public void onPause() { super.onPause(); dispatch(Lifecycle.Event.ON_PAUSE); } @Override public void onStop() { super.onStop(); dispatch(Lifecycle.Event.ON_STOP); } @Override public void onDestroy() { super.onDestroy(); dispatch(Lifecycle.Event.ON_DESTROY); // just want to be sure that we won't leak reference to an activity mProcessListener = null; } private void dispatch(Lifecycle.Event event) { Activity activity = getActivity(); 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); } } }
你会发现都调用了dispatch方法,而dispatch方法则会判断Activity是否实现了LifecycleOwner接口,如果实现了该接口就调用LifecycleRegister的handleLifecycleEvent,这样生命周期的状态就会借由LifecycleRegistry通知给各个LifecycleObserver从而调用其中对应Lifecycle.Event的方法。这种通过Fragment来感知Activity生命周期的方法其实在Glide的中也是有体现的。这是Activity的声明周期感知,那么Fragment呢?看一下Fragment源码你会发现Fragment也实现了LifecycleOwner接口,也关联了一个LifecycleRegistry对象
void performCreate(Bundle savedInstanceState) { if (this.mChildFragmentManager != null) { this.mChildFragmentManager.noteStateNotSaved(); } this.mState = 1; this.mCalled = false; this.onCreate(savedInstanceState); this.mIsCreated = true; if (!this.mCalled) { throw new SuperNotCalledException("Fragment " + this + " did not call through to super.onCreate()"); } else { this.mLifecycleRegistry.handleLifecycleEvent(Event.ON_CREATE); } }
看它的performCreate里调用了mLifecycleRegistry.handleLifecycleEvent,同理在performStart和performResume方法里也调用了。这里其实有个有趣的现象就是在ReportFragment中dispatch(Lifecycle.Event.ON_CREATE)这个方法是在onActivityCreated里调用的,而fragment的Lifecycle.Event.ON_CREATE的事件的发送是在performCreate里调用的,这就有个问题了,如果你在Activity里的onCreate方法里添加Fragment,同时这个Fragment也添加了LifecycleObserver的监听,那么谁先监听到呢?
举个例子:class MainActivity : AppCompatActivity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) lifecycle.addObserver(Presenter()) supportFragmentManager.beginTransaction().replace(R.id.container,MainFragment()).commit() }}class Presenter:LifecycleObserver { @OnLifecycleEvent(Lifecycle.Event.ON_CREATE) fun onCreate(){ println("presenter oncreate") }class MainFragment:Fragment() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) lifecycle.addObserver(FragmentPresenter()) }}class FragmentPresenter:LifecycleObserver { @OnLifecycleEvent(Lifecycle.Event.ON_CREATE) fun onCreate(){ println("presenter fragment oncreate") }}
运行程序你会发现打印结果是presenter oncreate,然后才是presenter fragment oncreate
这就很疑惑了,我们知道fragment的声明周期onCreate是优先于onActivityCreated的,为什么没有先执行FragmentPresenter的onCreate方法,其实原因就在commit是异步的,提交fragment事物后会在何时的时机调用这个事物,所以此时MainFragment还没有被加入,但是ReportFragment调用了manager.executePendingTransactions()这个方法,这个方法会立即执行现有事物,所以这样ReportFragment就已经添加进FragmentManager了,所以出现了生命周期不对应的情况,如果你在MainActivity里MainFragment commit之后也添加manager.executePendingTransactions(),你就会发现打印结果是倒过来的。LifecycleRegister是重点了先看addObserver方法
@Override public void addObserver(@NonNull LifecycleObserver observer) { State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED; ObserverWithState statefulObserver = new ObserverWithState(observer, initialState); ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver); if (previous != null) { return; } LifecycleOwner lifecycleOwner = mLifecycleOwner.get(); if (lifecycleOwner == null) { // it is null we should be destroyed. Fallback quickly return; } boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent; State targetState = calculateTargetState(observer); mAddingObserverCounter++; while ((statefulObserver.mState.compareTo(targetState) < 0 && mObserverMap.contains(observer))) { pushParentState(statefulObserver.mState); statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState)); popParentState(); // mState / subling may have been changed recalculate targetState = calculateTargetState(observer); } if (!isReentrance) { // we do sync only on the top level. sync(); } mAddingObserverCounter--; }
这里你会发现生成了一个ObserverWithState,然后放入FastSafeIterableMap里,这个类
是一个自定义列表,用于保存观察者并可在遍历期间处理删除/添加。 观察者1和观察者2的任何时刻:如果addition_order(observer1)<addition_order(observer2),那么 state(observer1)> = state(observer2) 接下来看handleLifecycleEvent方法public void handleLifecycleEvent(@NonNull Lifecycle.Event event) { State next = getStateAfter(event); moveToState(next); }
首先会根据传入的Event得到相应的State,具体的对象关系在下图
private void sync() { LifecycleOwner lifecycleOwner = mLifecycleOwner.get(); if (lifecycleOwner == null) { Log.w(LOG_TAG, "LifecycleOwner is garbage collected, you shouldn't try dispatch " + "new events from it."); return; } 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) { backwardPass(lifecycleOwner); } Entrynewest = mObserverMap.newest(); if (!mNewEventOccurred && newest != null && mState.compareTo(newest.getValue().mState) > 0) { forwardPass(lifecycleOwner); } } mNewEventOccurred = false; }
这里可以看到如果ObserverWithState的state小于当前state,那么就调用forwardPass方法,如果大于当前state,那么就调用backwardPass方法,ObserverWithState默认的State是在addObserver方法里确定的
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED; ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
接着看forwardPass方法
private void forwardPass(LifecycleOwner lifecycleOwner) { Iterator> ascendingIterator = mObserverMap.iteratorWithAdditions(); while (ascendingIterator.hasNext() && !mNewEventOccurred) { Entry entry = ascendingIterator.next(); ObserverWithState observer = entry.getValue(); while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred && mObserverMap.contains(entry.getKey()))) { pushParentState(observer.mState); observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState)); popParentState(); } } }
这里调用了ObserverWithState的dispatchEvent方法,看一下这个方法
void dispatchEvent(LifecycleOwner owner, Event event) { State newState = getStateAfter(event); mState = min(mState, newState); mLifecycleObserver.onStateChanged(owner, event); mState = newState; }
真相大白,最终调用了 mLifecycleObserver.onStateChanged(owner, event);同时更新了自己的state,这个mLifecycleObserver是怎么得到的,它又具体做了什么,就不具体说了,有兴趣的可以进去看一看,本片文章中的调用方法会生成一个ReflectiveGenericLifecycleObserver对象,这个对象会持有Presenter中声明了@OnLifecycleEvent注解的方法的Method对象,然后最终调用。
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