Android Lifecycle实现原理

2021-06-07 本文已影响0人进击的包籽

1.Lifecycle了解

到官方文档下看 Google Lifecycle，Lifecycle的作用是：生命周期感知型组件可执行操作来响应另一个组件（如 Activity 和 Fragment）的生命周期状态的变化。这些组件有助于您写出更有条理且往往更精简的代码，这样的代码更易于维护。
我们之前开发，因为Activity 或者是 Fragment 的生命周期问题而间接引起的内存问题挺多的，比如每次都要写资源，或者控件工具的回收释放，如果忘记写了，那么可能会引起内存泄漏，而现在搭配 Lifecycle，给我们生命周期的回调，就不必再像以前在某个生命周期加上逻辑代码，而是直接提前写对应的代码，更好解决生命周期问题。

2.生命周期获取对比

2.1 之前的生命周期获取

我们需要Activity重写每一个生命周期的方法，在里面加入逻辑，如果某个回收忘记写了，就可能触发内存泄漏问题。

override fun onPause() {
    super.onPause()
    Log.d(TAG, "onPause")
}

override fun onStop() {
    super.onStop()
    Log.d(TAG, "onStop")
}

override fun onStart() {
    super.onStart()
    Log.d(TAG, "onStart")
}

override fun onResume() {
    super.onResume()
    Log.d(TAG, "onResume")
}

override fun onRestart() {
    super.onRestart()
    Log.d(TAG, "onRestart")
}

override fun onDestroy() {
    super.onDestroy()   
    Log.d(TAG, "onDestroy")
}

之前的生命周期

2.2 Lifecycle回调生命周期

使用Lifecycle，只要拿到Activity的Lifecycle，注册观察，就能回调生命周期了，非常方便，如果写的自定义View或者工具，需要生命周期感知，就可以利用Lifecycle，将逻辑写在内部，代码也更间接，使用者也不要去注意创建回收问题。

lifecycle.addObserver(object : LifecycleEventObserver {
    override fun onStateChanged(source: LifecycleOwner, event: Lifecycle.Event) {
      Log.d(TAG,event.toString())
    }
})

Lifecycle

3.源码分析

3.1 类关系图

Lifecycle

在Activity 获取 Lifecycle，实际上是通过Activity的父类 ComponentActvitiy 获取，父类实现了 LifecycleOwner 接口，就能获取 Lifecycle ,最后注册 LifecycleObserver 就能拿到生命周期回调了。

3.2 ComponentActvitiy.onCreate

在ComponentActvitiy的 onCreate 方法里面可以看到 ReportFragment 的创建。

    /* ComponentActvitiy */
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        ...
        ReportFragment.injectIfNeededIn(this);
        ...
    }

3.3 getLifecycle方法

    /* ComponentActvitiy */
    private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
    
    @NonNull
    @Override
    public Lifecycle getLifecycle() {
        return mLifecycleRegistry;
    }

3.4 Lifecycle.Event

Lifecycle.Event 是个枚举类，这里的生命周期 Event 并不是Fragment的，在后面的生命周期处理时会用上的。

    public enum Event {
        ON_CREATE,
        ON_START,
        ON_RESUME,
        ON_PAUSE,
        ON_STOP,
        ON_DESTROY,
        ON_ANY;
       ...
    }

3.5 ReportFragment的创建

ReportFragment 是一个 没有界面的Fragment，如果有了解过Glide原理的同学，应该也知道这个方法，就是通过看不见的Fragment，来感知生命周期，让使用者无需考虑生命周期的问题。
在SDK29以上的版本使用的是 LifecycleCallbacks.registerIn(activity)。

    /* ReportFragment */
    public static void injectIfNeededIn(Activity activity) {
        if (Build.VERSION.SDK_INT >= 29) {
            // On API 29+, we can register for the correct Lifecycle callbacks directly
            LifecycleCallbacks.registerIn(activity);
        }
        // Prior to API 29 and to maintain compatibility with older versions of
        // ProcessLifecycleOwner (which may not be updated when lifecycle-runtime is updated and
        // need to support activities that don't extend from FragmentActivity from support lib),
        // use a framework fragment to get the correct timing of Lifecycle events
        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();
        }
    }

3.6 LifecycleCallbacks.registerIn(activity)

LifecycleCallbacks 实现了 Application.ActivityLifecycleCallbacks接口，在SDK29以上的生命周期分发是由Application 分发的，activity注册就能回调。
大名鼎鼎的LeakCanary在监听Activity生命周期，也是使用 Application.ActivityLifecycleCallbacks。

    @RequiresApi(29)
    static class LifecycleCallbacks implements Application.ActivityLifecycleCallbacks {
    
        static void registerIn(Activity activity) {
            activity.registerActivityLifecycleCallbacks(new LifecycleCallbacks());
        }
        ...
        @Override
        public void onActivityPostCreated(@NonNull Activity activity,
                @Nullable Bundle savedInstanceState) {
            dispatch(activity, Lifecycle.Event.ON_CREATE);
        }
      ...
    }

3.7 ReportFragment.dispatch 版本兼容

如果SDK版本小于29，ReportFragment的各个生命周期方法里，会调用 dispatch 方法。
比如 onActivityCreated。
反正无论是使用 LifecycleCallbacks.registerIn(activity)，还是 Fragment 的生命周期回调，最后都会dispatch。

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        dispatchCreate(mProcessListener);
        dispatch(Lifecycle.Event.ON_CREATE);
    }

    private void dispatch(@NonNull Lifecycle.Event event) {
        if (Build.VERSION.SDK_INT < 29) {
            // Only dispatch events from ReportFragment on API levels prior
            // to API 29. On API 29+, this is handled by the ActivityLifecycleCallbacks
            // added in ReportFragment.injectIfNeededIn
            dispatch(getActivity(), event);
        }
    }
    
    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);
            }
        }
    }

3.8 Lifecycle.State

这个类跟Lifecycle.Event的关系看图就能理解。
State只有5个但是生命周期可是不止5个，所以Google他们设计时，就创建流程正着走，销毁流程就反正走。

Lifecycle.State

    /* Lifecycle.State */
    public enum State {
        DESTROYED,
      
        INITIALIZED,
      
        CREATED,
     
        STARTED,

        RESUMED;
        
        public boolean isAtLeast(@NonNull State state) {
            return compareTo(state) >= 0;
        }
    }

3.9 handleLifecycleEvent

LifecycleRegistryOwner 也是继承 LifecycleOwner，所以他们最后都会执行 LifecycleRegistry 的 handleLifecycleEvent 方法。
就是把 Lifecycle.Event处理一下，转化成 Lifecycle.State。

    /* Lifecycle.Event */
        @NonNull
        public State getTargetState() {
            switch (this) {
                case ON_CREATE:
                case ON_STOP:
                    return State.CREATED;
                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");
        }

将 Lifecycle.State 继续往下传，先用 mState 保存，再 sync 方法处理。


    /* LifecycleRegistry  */
    public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
        enforceMainThreadIfNeeded("handleLifecycleEvent");
        moveToState(event.getTargetState());
    }

    private void moveToState(State next) {
        if (mState == next) {
            return;
        }
        //保存state状态
        mState = next;
        if (mHandlingEvent || mAddingObserverCounter != 0) {
            mNewEventOccurred = true;
            // we will figure out what to do on upper level.
            return;
        }
        mHandlingEvent = true;
        sync();
        mHandlingEvent = false;
    }

3.10 sync

这里利用上一个方法保存的mState，用于比较，判断是正向执行还是反向执行生命周期。

    /* LifecycleRegistry  */
    private void sync() {
        //这是弱引用包装过的LifecycleOwner 
        LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
        if (lifecycleOwner == null) {
            throw new IllegalStateException("LifecycleOwner of this LifecycleRegistry is already"
                    + "garbage collected. It is too late to change lifecycle state.");
        }
        while (!isSynced()) {
            mNewEventOccurred = false;
            // no need to check eldest for nullability, because isSynced does it for us.
            //上一个方法保存的mState，跟组件之前的的mState对比
            if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
                //返向执行流程
                backwardPass(lifecycleOwner);
            }
           
            Map.Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
            if (!mNewEventOccurred && newest != null
                    && mState.compareTo(newest.getValue().mState) > 0) {
                //正向执行流程
                forwardPass(lifecycleOwner);
            }
        }
        mNewEventOccurred = false;
    }

3.11 forwardPass

反向的逻辑差不多，只是执行 backwardPass ，先转换Stata，最后执行 observer.dispatchEvent。
这里又把 Lifecycle.State 转回 Lifecycle.Event，然后给观察者分发出去。

        /* Lifecycle.Event */
        @Nullable
        public static Event upFrom(@NonNull State state) {
            switch (state) {
                case INITIALIZED:
                    return ON_CREATE;
                case CREATED:
                    return ON_START;
                case STARTED:
                    return ON_RESUME;
                default:
                    return null;
            }
        }

转换 Event.upFrom ，发送 observer.dispatchEvent。

    /* LifecycleRegistry  */
    private void forwardPass(LifecycleOwner lifecycleOwner) {
        Iterator<Map.Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
                mObserverMap.iteratorWithAdditions();
        while (ascendingIterator.hasNext() && !mNewEventOccurred) {
            Map.Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
            ObserverWithState observer = entry.getValue();
            while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
                    && mObserverMap.contains(entry.getKey()))) {
                pushParentState(observer.mState);

                //转化
                final Event event = Event.upFrom(observer.mState);
                if (event == null) {
                    throw new IllegalStateException("no event up from " + observer.mState);
                }
                //发送
                observer.dispatchEvent(lifecycleOwner, event);
                popParentState();
            }
        }
    }

3.12 发送生命周期状态

ObserverWithState 发送出 Lifecycle.Event ，至此就结束了，有注册订阅关系的地方就能收到。

    static class ObserverWithState {
        State mState;
        LifecycleEventObserver mLifecycleObserver;

        ObserverWithState(LifecycleObserver observer, State initialState) {
            mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
            mState = initialState;
        }

        /* 分发生命周期状态 */
        void dispatchEvent(LifecycleOwner owner, Event event) {
            State newState = event.getTargetState();
            mState = min(mState, newState);
            mLifecycleObserver.onStateChanged(owner, event);
            mState = newState;
        }
    }

3.13 简易流程图

简易流程图

Android Lifecycle实现原理

1.Lifecycle了解

2.生命周期获取对比

2.1 之前的生命周期获取

2.2 Lifecycle回调生命周期

3.源码分析

3.1 类关系图

3.2 ComponentActvitiy.onCreate

3.3 getLifecycle方法

3.4 Lifecycle.Event

3.5 ReportFragment的创建

3.6 LifecycleCallbacks.registerIn(activity)

3.7 ReportFragment.dispatch 版本兼容

3.8 Lifecycle.State

3.9 handleLifecycleEvent

3.10 sync

3.11 forwardPass

3.12 发送生命周期状态

3.13 简易流程图

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