Lifecycle的基本使用与原理解析

Lifecycle使用了观察者设计模式，Activity与Fragment这种具有生命周期的页面是被观察者，通过LifecycleObserver来观察生命周期。另外事件与状态的比变化是通过状态机实现的，文末有图总结该方式以及为什么这么设计？

Lifecycle的基本使用

首先需要定义一个LifecycleObserver：

class MyLocationListener : LifecycleObserver {

    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    fun create() = Log.d("zxc", "create 正在启动系统定位服务中...")

    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    fun start() = Log.d("zxc", "start 连接系统定位服务...")

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun resume() = Log.d("zxc", "resume 系统定位的界面展示...")

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun pause() = Log.d("zxc", "pause 系统定位的界面关闭...")

    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    fun stop() = Log.d("zxc", "stop 断开系统定位服务...")

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    fun destroy() = Log.d("zxc", "destroy 正在停止系统定位服务...")
}

然后在Activity中注册他就可以了：

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    // 观察者 == MyLocationListener
    // 1.主线流程，支线流程不管（防止淹死源码）
    // 2.支线流程
    lifecycle.addObserver(MyLocationListener())
}

能这样使用的原因是Activity实现了LifecycleObserver接口。

或者也可以这样使用Lifecycle：

class MyObserver2 : DefaultLifecycleObserver {

    private val TAG = "MyObserver2"

    override fun onCreate(owner: LifecycleOwner) {
        super.onCreate(owner)
        Log.d(TAG,"onCreate run ...")
    }

    override fun onResume(owner: LifecycleOwner) {
        super.onResume(owner)
        Log.d(TAG,"onResume run ...")
    }

    override fun onPause(owner: LifecycleOwner) {
        super.onPause(owner)
        Log.d(TAG,"onPause run ...")
    }
}

DefaultLifecycleObserver 就是对 LifecycleObserver 二次封装 为了用户好用

另外，使用注解的方式也是可以的：

class MyObserver : LifecycleObserver {

    private val TAG = "MyObserver"

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME) // 画面可见 就连接
    fun connectListener() = Log.d(TAG,"connectListener run ...")

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)  // 画面不可见 就断开连接
    fun disconnectListener() = Log.d(TAG,"disconnectListener run ...")

}

但是这种方式有个问题，就是无法拿到当前运行的context，所以Lifecycle的写法有3种。

注册方式都可以通过lifecycle.addObserver(MyObserver())就可以了。

Lifecycle的原理分析：

Lifecycle使用的整体结构：

Activity作为被观察者，实现了LifecycleObserver的类作为观察者，在被观察者中通过添加Observer的方式，完成观察者的注册，然后当Activity生命周期发生变化的时候，这个被观察者就会将自己的状态分发给自己的观察者。

远吗分析，我们需要找一个切入点，对于Lifecycle，最好的切入点就是addObserver

addObserver流程分析：

lifecycle.addObserver(MyLocationListener())

接着进入了

这是一个抽象方法，我们需要去找到他的实现：

public void addObserver(@NonNull LifecycleObserver observer) {
    // 这里源码很多，与我们的主线流程很多关系不大，我们要紧盯着observer不放，看他是怎么进行的。
    Lifecycle.State initialState = this.mState == State.DESTROYED ? State.DESTROYED : State.INITIALIZED;
    // ObserverWithState(observer, initialState)进入看看这里的源码，这段代码会对Observer进行解析
    ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
    ObserverWithState previous = (ObserverWithState)this.mObserverMap.putIfAbsent(observer, statefulObserver);
    if (previous == null) {
        LifecycleOwner lifecycleOwner = (LifecycleOwner)this.mLifecycleOwner.get();
        if (lifecycleOwner != null) {
            boolean isReentrance = this.mAddingObserverCounter != 0 || this.mHandlingEvent;
            Lifecycle.State targetState = this.calculateTargetState(observer);
            ++this.mAddingObserverCounter;

            while(statefulObserver.mState.compareTo(targetState) < 0 && this.mObserverMap.contains(observer)) {
                this.pushParentState(statefulObserver.mState);
                statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState));
                this.popParentState();
                targetState = this.calculateTargetState(observer);
            }

            if (!isReentrance) {
                this.sync();
            }

            --this.mAddingObserverCounter;
        }
    }
}

进入：androidx.lifecycle.LifecycleRegistry.ObserverWithState，ObserverWithState是LifecycleRegistry的一个内部类。

static class ObserverWithState {
    Lifecycle.State mState;
    LifecycleEventObserver mLifecycleObserver;

    ObserverWithState(LifecycleObserver observer, Lifecycle.State initialState) {
        // observer 注册：
        this.mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
        this.mState = initialState;
    }
    // 省略无关代码
}

后边会来到这里：

static LifecycleEventObserver lifecycleEventObserver(Object object) {
    // 省略无关代码
    return new ReflectiveGenericLifecycleObserver(object);
}

在ReflectiveGenericLifecycleObserver里边，

class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {
    private final Object mWrapped;
    private final CallbackInfo mInfo;
    // wrapped就是我们的observer.
    ReflectiveGenericLifecycleObserver(Object wrapped) {
        mWrapped = wrapped;
        // z这里为什么要获取class？因为后边需要使用反射来对observer的每一个方法进行解析。
        mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
    }

    @Override
    public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Event event) {
        mInfo.invokeCallbacks(source, event, mWrapped);
    }
}

这些就是要解析的东西。

getInfo的详细实现在这里：

CallbackInfo getInfo(Class<?> klass) {
    // 为什么使用map，因为反射一般都会消耗性能，这里是出于提高一点点性能的考虑。而且framework中也存在大量的map用来提升性能。
    CallbackInfo existing = mCallbackMap.get(klass);
    if (existing != null) {
        return existing;
    }
    existing = createInfo(klass, null);
    return existing;
}

createInfo的详细实现，核心逻辑用来处理带有OnLifecycleEvent的注解方法，这里边是通过反射来处理的。

private CallbackInfo createInfo(Class<?> klass, @Nullable Method[] declaredMethods) {
    Class<?> superclass = klass.getSuperclass();
    Map<MethodReference, Lifecycle.Event> handlerToEvent = new HashMap<>();
    if (superclass != null) {
        CallbackInfo superInfo = getInfo(superclass);
        if (superInfo != null) {
            handlerToEvent.putAll(superInfo.mHandlerToEvent);
        }
    }

    Class<?>[] interfaces = klass.getInterfaces();
    for (Class<?> intrfc : interfaces) {
        for (Map.Entry<MethodReference, Lifecycle.Event> entry : getInfo(
                intrfc).mHandlerToEvent.entrySet()) {
            verifyAndPutHandler(handlerToEvent, entry.getKey(), entry.getValue(), klass);
        }
    }

    Method[] methods = declaredMethods != null ? declaredMethods : getDeclaredMethods(klass);
    boolean hasLifecycleMethods = false;
    // 遍历所有的方法，解析observer中的方法的注解，只要有@OnLifecycleEvent，都需要处理并保存。
    for (Method method : methods) {
        OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
        if (annotation == null) {
            continue;
        }
        hasLifecycleMethods = true;
        Class<?>[] params = method.getParameterTypes();
        int callType = CALL_TYPE_NO_ARG;
        if (params.length > 0) {
            callType = CALL_TYPE_PROVIDER;
            if (!params[0].isAssignableFrom(LifecycleOwner.class)) {
                throw new IllegalArgumentException(
                        "invalid parameter type. Must be one and instanceof LifecycleOwner");
            }
        }
        Lifecycle.Event event = annotation.value();

        if (params.length > 1) {
            callType = CALL_TYPE_PROVIDER_WITH_EVENT;
            if (!params[1].isAssignableFrom(Lifecycle.Event.class)) {
                throw new IllegalArgumentException(
                        "invalid parameter type. second arg must be an event");
            }
            if (event != Lifecycle.Event.ON_ANY) {
                throw new IllegalArgumentException(
                        "Second arg is supported only for ON_ANY value");
            }
        }
        if (params.length > 2) {
            throw new IllegalArgumentException("cannot have more than 2 params");
        }
        MethodReference methodReference = new MethodReference(callType, method);
        verifyAndPutHandler(handlerToEvent, methodReference, event, klass);
    }
    CallbackInfo info = new CallbackInfo(handlerToEvent);
    // 将所有的方法存储到map中，用来提升一点点性能。
    mCallbackMap.put(klass, info);
    mHasLifecycleMethods.put(klass, hasLifecycleMethods);
    return info;
}

被观察者如何将状态分发给观察者流程分析：

所有的处理最终都会到ComponentActivity，该类实现了LifecycleObserver接口。

先看看他的onCreate方法：

protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    this.mSavedStateRegistryController.performRestore(savedInstanceState);
    ReportFragment.injectIfNeededIn(this);
    if (this.mContentLayoutId != 0) {
        this.setContentView(this.mContentLayoutId);
    }
}

public static void injectIfNeededIn(Activity activity) {
    if (VERSION.SDK_INT >= 29) {
        activity.registerActivityLifecycleCallbacks(new LifecycleCallbacks());
    }
    
    // 生成一个空白的Fragment（ReportFragment）黏贴到用户界面上来。
    FragmentManager manager = activity.getFragmentManager();
    if (manager.findFragmentByTag("androidx.lifecycle.LifecycleDispatcher.report_fragment_tag") == null) {
        manager.beginTransaction().add(new ReportFragment(), "androidx.lifecycle.LifecycleDispatcher.report_fragment_tag").commit();
        manager.executePendingTransactions();
    }
}

其实从这里来看，似乎代码写在Activity中也可以，为什么没有呢？这是以为Google考虑的是，你继承AppCompatActivity 可以，你继承 XXXXActivity 也可以。Fragment是可以放在任何Activity中。就是可以在任何Activity中都可以使用这个Lifecycle。

这里顺便提一下Fragement的生命周期与Activity生命周期的交互：

可以看到Activity先于Fragment创建而创建，后语Fragment的销毁而销毁。

生命周期事件的分发

首先将一些重要的状态列举出来：

事件驱动状态，Lifecycle的设计核心原理之一。

生命周期的留个枚举，也就是6个事件，对应了6个生命周期回调函数：

public enum Event {
    /**
     * Constant for onCreate event of the {@link LifecycleOwner}.
     */
    ON_CREATE,
    /**
     * Constant for onStart event of the {@link LifecycleOwner}.
     */
    ON_START,
    /**
     * Constant for onResume event of the {@link LifecycleOwner}.
     */
    ON_RESUME,
    /**
     * Constant for onPause event of the {@link LifecycleOwner}.
     */
    ON_PAUSE,
    /**
     * Constant for onStop event of the {@link LifecycleOwner}.
     */
    ON_STOP,
    /**
     * Constant for onDestroy event of the {@link LifecycleOwner}.
     */
    ON_DESTROY,
    /**
     * An {@link Event Event} constant that can be used to match all events.
     */
    ON_ANY
}

这里是状态，状态通过上边的事件驱动。

public enum State {
    /**
     * Destroyed state for a LifecycleOwner. After this event, this Lifecycle will not dispatch
     * any more events. For instance, for an {@link android.app.Activity}, this state is reached
     * <b>right before</b> Activity's {@link android.app.Activity#onDestroy() onDestroy} call.
     */
    DESTROYED,

    /**
     * Initialized state for a LifecycleOwner. For an {@link android.app.Activity}, this is
     * the state when it is constructed but has not received
     * {@link android.app.Activity#onCreate(android.os.Bundle) onCreate} yet.
     */
    INITIALIZED,

    /**
     * Created state for a LifecycleOwner. For an {@link android.app.Activity}, this state
     * is reached in two cases:
     * <ul>
     *     <li>after {@link android.app.Activity#onCreate(android.os.Bundle) onCreate} call;
     *     <li><b>right before</b> {@link android.app.Activity#onStop() onStop} call.
     * </ul>
     */
    CREATED,

    /**
     * Started state for a LifecycleOwner. For an {@link android.app.Activity}, this state
     * is reached in two cases:
     * <ul>
     *     <li>after {@link android.app.Activity#onStart() onStart} call;
     *     <li><b>right before</b> {@link android.app.Activity#onPause() onPause} call.
     * </ul>
     */
    STARTED,

    /**
     * Resumed state for a LifecycleOwner. For an {@link android.app.Activity}, this state
     * is reached after {@link android.app.Activity#onResume() onResume} is called.
     */
    RESUMED;

    /**
     * Compares if this State is greater or equal to the given {@code state}.
     *
     * @param state State to compare with
     * @return true if this State is greater or equal to the given {@code state}
     */
    public boolean isAtLeast(@NonNull State state) {
        return compareTo(state) >= 0;
    }
}

在ReportFragment进行事件的分发：

static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) {
    if (activity instanceof LifecycleRegistryOwner) {
        // 处理事件分发，这里就涉及到状态机了，这也是Lifecycle设计的核心原理。
        ((LifecycleRegistryOwner)activity).getLifecycle().handleLifecycleEvent(event);
    } else {
        if (activity instanceof LifecycleOwner) {
            Lifecycle lifecycle = ((LifecycleOwner)activity).getLifecycle();
            if (lifecycle instanceof LifecycleRegistry) {
                ((LifecycleRegistry)lifecycle).handleLifecycleEvent(event);
            }
        }

    }
}
```js
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
    // 通过事件得到状态
    Lifecycle.State next = getStateAfter(event);
    // 状态对齐,就是让被观察者的状态变成与观察者一样的状态。
    this.moveToState(next);
}

事件驱动状态的转化函数：

static Lifecycle.State getStateAfter(Lifecycle.Event event) {
    switch (event) {
        // 前进状态
        case ON_CREATE:
        // 后退状态
        case ON_STOP:
            return State.CREATED;
        // 前进状态
        case ON_START:
        // 后退状态
        case ON_PAUSE:
            return State.STARTED;
        // 只有前进，没有倒退状态。
        case ON_RESUME:
            return State.RESUMED;
       // 只有倒退，没有前进状态。
        case ON_DESTROY:
            return State.DESTROYED;
        case ON_ANY:
        default:
            throw new IllegalArgumentException("Unexpected event value " + event);
    }
}

状态机进行状态转化：

private void moveToState(Lifecycle.State next) {
    if (this.mState != next) {
        this.mState = next;
        if (!this.mHandlingEvent && this.mAddingObserverCounter == 0) {
            this.mHandlingEvent = true;
            this.sync();
            this.mHandlingEvent = false;
        } else {
            this.mNewEventOccurred = true;
        }
    }
}

private void sync() {
    LifecycleOwner lifecycleOwner = (LifecycleOwner)this.mLifecycleOwner.get();
    if (lifecycleOwner == null) {
        throw new IllegalStateException("LifecycleOwner of this LifecycleRegistry is alreadygarbage collected. It is too late to change lifecycle state.");
    } else {
        while(!this.isSynced()) {
            this.mNewEventOccurred = false;
            // 通过枚举的大小来判断当前观察者的状态应该是前进还是后退
            if (this.mState.compareTo(((ObserverWithState)this.mObserverMap.eldest().getValue()).mState) < 0) {
                this.backwardPass(lifecycleOwner);
            }

            Map.Entry<LifecycleObserver, ObserverWithState> newest = this.mObserverMap.newest();
            if (!this.mNewEventOccurred && newest != null && this.mState.compareTo(((ObserverWithState)newest.getValue()).mState) > 0) {
                this.forwardPass(lifecycleOwner);
            }
        }

        this.mNewEventOccurred = false;
    }
}

// 倒退的过程中，我们对齐状态后，需要根据当前的状态获取事件，然后才能回调对应的生命周期函数。同理的前进也是一样的逻辑。
private static Lifecycle.Event downEvent(Lifecycle.State state) {
    switch (state) {
        case INITIALIZED:
            throw new IllegalArgumentException();
        case CREATED:
            return Event.ON_DESTROY;
        case STARTED:
            return Event.ON_STOP;
        case RESUMED:
            return Event.ON_PAUSE;
        case DESTROYED:
            throw new IllegalArgumentException();
        default:
            throw new IllegalArgumentException("Unexpected state value " + state);
    }
}

之后在通过androidx.lifecycle.ReflectiveGenericLifecycleObserver#onStateChanged进行事件的处理

private static void invokeMethodsForEvent(List<MethodReference> handlers,
        LifecycleOwner source, Lifecycle.Event event, Object mWrapped) {
    if (handlers != null) {
        for (int i = handlers.size() - 1; i >= 0; i--) {
            handlers.get(i).invokeCallback(source, event, mWrapped);
        }
    }
}

void invokeCallback(LifecycleOwner source, Lifecycle.Event event, Object target) {
    //noinspection TryWithIdenticalCatches
    try {
        switch (mCallType) {
            case CALL_TYPE_NO_ARG:
                // 通过反射完成生命周期的回调。
                mMethod.invoke(target);
                break;
            case CALL_TYPE_PROVIDER:
                mMethod.invoke(target, source);
                break;
            case CALL_TYPE_PROVIDER_WITH_EVENT:
                mMethod.invoke(target, source, event);
                break;
        }
    } catch (InvocationTargetException e) {
        throw new RuntimeException("Failed to call observer method", e.getCause());
    } catch (IllegalAccessException e) {
        throw new RuntimeException(e);
    }
}

状态变化与事件对应关系图：

mActive只有在STARTED和RESUMED状态下才会是true，其余是false，livedata也是用该变量来刷新数据。

为什么弄这么复杂？因为这个框架需要给LiveData，ViewModel等一起使用，这是一个通用的框架，那么有没有其他方式来实现这么复杂的效果呢？通过平常的if else也可以，但是判断会比较复杂。

首先通过被观察者的生命周期完成状态机的状态变化，然后再通过状态机的状态获取被观察者应该回调的生命周期函数，最后通过反射来进行调用。这个状态机实现了被观察者与观察者的解耦，同时该状态机还可以再livedata， ViewModel等地方使用。

问题

1、如果在onResume中添加observer会怎么样？如果在onStop中注册呢？

onResume中：状态的变化是CREATE->START->RESUME，而不是直接就变成了RESUME；

onStop中：CREATE->DESTROY，整个流程的变化，INTIALIZED 一定会执行，从INTIALIZED->CREATE.因此CREATE状态一定是有的。 addObsever()的时候是初始状态（INTIALIZED），后面只执行了onDestroyed()，所以是onCreate和onDestroyed 详解可以看一下上边的状态机变化图。