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android Lifecycle源码分析--源码阅读100天(1)
阅读量:244 次
发布时间:2019-02-28

本文共 11109 字,大约阅读时间需要 37 分钟。

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

在这里插入图片描述
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
在这里插入图片描述
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

在这里插入图片描述
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
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
> 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。

在这里插入图片描述

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