1.使用

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。

2.分析

2.1观察者模式

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

那么duiyingLifecycle又是什么样的呢？下面是我自己画的一个Lifecycle的UML类图

我们自定义的Activity都继承自SupportActivity，可以发现SupportActivity实现了LifecycleOwner接口，同时持有实现了Lifecycle的LifecycleRegistry对象，这个对象就可以将其理解为观察者模式中的Subject，LifecycleRegistr聚合多个LifecycleObserver，生命周期改变时通知LifecycleObserver进行相应的方法调用。

2.2生命周期感知

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。

2.3LifecycleRegister分析

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

在moveToState方法里，会调用sync方法

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);
            }
            Entry<LifecycleObserver, ObserverWithState> newest = 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<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);
                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。

android Lifecycle源码分析--源码阅读100天（1）

1.使用

2.分析

2.1观察者模式

2.2生命周期感知

2.3LifecycleRegister分析