Android Architecture Component（Lifecycle）

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简介

Lifecycle，顾名思义，声明周期，简单的说就是将任何java对象都可以感知Activity声明周期。目的是为了减少耦合性。

Lifecycle

LifecycleOwner

先来看 LifecycleOwner 接口，这个接口定义就说明了某样东西是具有生命周期的。getLifecycle() 方法返回生命周期。官方建议除了 Activity 和 Fragment 之外，其他的代码都不应该实现 LifecycleOwner 这个接口。目前 SupportActivity 和 Fragment 都实现了该接口。

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public interface LifecycleOwner {
    /**
     * Returns the Lifecycle of the provider.
     *
     * @return The lifecycle of the provider.
     */
    @NonNull
    Lifecycle getLifecycle();
}

Lifecycle

在上面我们看到 LifecycleOwner 接口的 getLifecycle() 方法返回了 Lifecycle 。Lifecycle 代表着生命周期，那么来看看 Lifecycle 是怎么定义的。

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public abstract class Lifecycle {

    @MainThread
    public abstract void addObserver(@NonNull LifecycleObserver observer);

    @MainThread
    public abstract void removeObserver(@NonNull LifecycleObserver observer);

    @MainThread
    @NonNull
    public abstract State getCurrentState();

    @SuppressWarnings("WeakerAccess")
    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
    }

    @SuppressWarnings("WeakerAccess")
    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;
        }
    }
}

  • addObserver ：增加观察者，观察者可以观察到该生命周期的变化，具体的观察者就是 LifecycleObserver ；
  • removeObserver ：移除观察者 LifecycleObserver ；
  • getCurrentState ：返回当前生命周期的状态；
  • Event ：生命周期事件；
  • State ：生命周期状态；

获取lifecycle使用如下方法：

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@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    getLifecycle().addObserver(new LifecycleTest());
}

父类中的实现方式

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public class ComponentActivity extends Activity implements LifecycleOwner{
 private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
    @CallSuper
    @Override
    protected void onSaveInstanceState(Bundle outState) {
        mLifecycleRegistry.markState(Lifecycle.State.CREATED);
        super.onSaveInstanceState(outState);
    }

    @Override
    public Lifecycle getLifecycle() {
        return mLifecycleRegistry;
    }
}

LifecycleObserver

LifecycleObserver 是个空接口，里面什么都没有。那我们自己定义一个类 LifecycleTest 来实现 LifecycleObserver 接口，以达到观察生命周期的目的。

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@SuppressWarnings("WeakerAccess")
public interface LifecycleObserver {

}
public class LifecycleTest implements LifecycleObserver {

	@OnLifecycleEvent(Lifecycle.Event.ON_ANY)
	void onAny(LifecycleOwner owner, Lifecycle.Event event) {
	    System.out.println("onAny:" + event.name());
	}

	@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
	void onCreate() {
	    System.out.println("onCreate");
	}

	@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
	void onDestroy() {
	    System.out.println("onDestroy");
	}

}

通过之前分析的代码我们可以观察到，getLifecycle() 返回的就是 LifecycleRegistry 对象。所以其实调用的就是 LifecycleRegistry 的 addObserver 方法来添加观察者的。

LifecycleRegistry

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@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--;
}

一开始，针对每个 LifecycleObserver 对象设置了一个初始状态 initialState ，然后结合初始状态 initialState 和 observer ，把它俩包装成一个 ObserverWithState 对象。并保存到 mObserverMap 中。 mObserverMap 缓存了所有的生命周期观察者。
我们来看看 ObserverWithState 里面的操作。

ObserverWithState

ObserverWithState 是 LifecycleRegistry 的静态内部类。

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static class ObserverWithState {
    State mState;
    GenericLifecycleObserver mLifecycleObserver;

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

    void dispatchEvent(LifecycleOwner owner, Event event) {
        State newState = getStateAfter(event);
        mState = min(mState, newState);
        mLifecycleObserver.onStateChanged(owner, event);
        mState = newState;
    }
}

在 ObserverWithState 中，我们有点蹊跷，mLifecycleObserver 的类型是 GenericLifecycleObserver ，但是我们传入的是 LifecycleObserver 类型。所以在 Lifecycling.getCallback(observer) 这句代码中做的事情就是把 LifecycleObserver 转化成 GenericLifecycleObserver ，我们深入了解下。

GenericLifecycleObserver

GenericLifecycleObserver 是一个接口 有一个方法onStateChanged()，有四种实现：

  1. FullLifecycleObserver
  2. SingleGeneratedAdapterObserver
  3. CompositeGeneratedAdaptersObserver
  4. ReflectiveGenericLifecycleObserver

FullLifecycleObserver生成对应的FullLifecycleObserverAdapter类、当使用annotationProcessor “androidx.lifecycle:lifecycle-compiler:2.0.0”时 这是注解处理器的依赖，引入这个之后,会自动生成xxx_LifecycleAdapter的文件。会生成对应的SingleGeneratedAdapterObserver、CompositeGeneratedAdaptersObserver类的实例、ReflectiveGenericLifecycleObserver，通过反射，去调用方法。

Lifecycling

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@NonNull
static GenericLifecycleObserver getCallback(Object object) {
    if (object instanceof FullLifecycleObserver) {
        return new FullLifecycleObserverAdapter((FullLifecycleObserver) object);
    }

    if (object instanceof GenericLifecycleObserver) {
        return (GenericLifecycleObserver) object;
    }

    final Class<?> klass = object.getClass();
    int type = getObserverConstructorType(klass);
    if (type == GENERATED_CALLBACK) {
        List<Constructor<? extends GeneratedAdapter>> constructors =
                sClassToAdapters.get(klass);
        if (constructors.size() == 1) {
            GeneratedAdapter generatedAdapter = createGeneratedAdapter(
                    constructors.get(0), object);
            return new SingleGeneratedAdapterObserver(generatedAdapter);
        }
        GeneratedAdapter[] adapters = new GeneratedAdapter[constructors.size()];
        for (int i = 0; i < constructors.size(); i++) {
            adapters[i] = createGeneratedAdapter(constructors.get(i), object);
        }
        return new CompositeGeneratedAdaptersObserver(adapters);
    }
    return new ReflectiveGenericLifecycleObserver(object);
}

根据代码可以大概知道，在 getCallback 中主要做的事情就是利用适配器 Adapter 把 LifeObserver 转化成 GenericLifecycleObserver 。之前我们定义的 MyLifecycleObserver 是直接实现 LifecycleObserver 接口的，所以它不属于 FullLifecycleObserver 或者 FullLifecycleObserver ，因此它会去执行 getObserverConstructorType(klass) 方法。

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private static int getObserverConstructorType(Class<?> klass) {
    // 如果之前解析过了，直接返回缓存
    if (sCallbackCache.containsKey(klass)) {
        return sCallbackCache.get(klass);
    }
    // 否则调用 resolveObserverCallbackType 进行解析类型
    int type = resolveObserverCallbackType(klass);
    sCallbackCache.put(klass, type);
    return type;
}

在 getObserverConstructorType 中，主要还是要看 resolveObserverCallbackType 方法。

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private static int resolveObserverCallbackType(Class<?> klass) {
    // anonymous class bug:35073837
    if (klass.getCanonicalName() == null) {
        return REFLECTIVE_CALLBACK;
    }

    // 注意这里调用了 generatedConstructor 来生成了 GeneratedAdapter 的构造器
    Constructor<? extends GeneratedAdapter> constructor = generatedConstructor(klass);
    if (constructor != null) {

        // 得到构造器后进行缓存
        sClassToAdapters.put(klass, Collections
                .<Constructor<? extends GeneratedAdapter>>singletonList(constructor));
        return GENERATED_CALLBACK;
    }

    boolean hasLifecycleMethods = ClassesInfoCache.sInstance.hasLifecycleMethods(klass);
    if (hasLifecycleMethods) {
        return REFLECTIVE_CALLBACK;
    }

    Class<?> superclass = klass.getSuperclass();
    List<Constructor<? extends GeneratedAdapter>> adapterConstructors = null;
    if (isLifecycleParent(superclass)) {
        if (getObserverConstructorType(superclass) == REFLECTIVE_CALLBACK) {
            return REFLECTIVE_CALLBACK;
        }
        adapterConstructors = new ArrayList<>(sClassToAdapters.get(superclass));
    }

    for (Class<?> intrface : klass.getInterfaces()) {
        if (!isLifecycleParent(intrface)) {
            continue;
        }
        if (getObserverConstructorType(intrface) == REFLECTIVE_CALLBACK) {
            return REFLECTIVE_CALLBACK;
        }
        if (adapterConstructors == null) {
            adapterConstructors = new ArrayList<>();
        }
        adapterConstructors.addAll(sClassToAdapters.get(intrface));
    }
    if (adapterConstructors != null) {
        sClassToAdapters.put(klass, adapterConstructors);
        return GENERATED_CALLBACK;
    }

    return REFLECTIVE_CALLBACK;
}

resolveObserverCallbackType 方法中调用 generatedConstructor 来生成 MyLifecycleObserver 的 GeneratedAdapter 构造器。看到这里可能很多人会懵逼，什么是 GeneratedAdapter ？

GeneratedAdapter

其实 GeneratedAdapter 可以理解为系统为我们的 MyLifecycleObserver 而设计适配器。
比如，我们在 MyLifecycleObserver 里设计了 onCreate 方法在生命周期的创建状态来回调，但是系统并不知道这个 onCreate 方法。所以需要设计出一套适配器来适配我们的 MyLifecycleObserver 。
那么这个适配器的代码也需要我们来写吗？不需要，在编译期时 apt 自动帮我们生成好了。我们可以在 build/generated/source/apt 目录下找到自动生成的 GeneratedAdapter 。

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public class MyLifecycleObserver_LifecycleAdapter implements GeneratedAdapter {
  final MyLifecycleObserver mReceiver;

  MyLifecycleObserver_LifecycleAdapter(MyLifecycleObserver receiver) {
    this.mReceiver = receiver;
  }

  @Override
  public void callMethods(LifecycleOwner owner, Lifecycle.Event event, boolean onAny,
      MethodCallsLogger logger) {
    boolean hasLogger = logger != null;
    if (onAny) {
      if (!hasLogger || logger.approveCall("onAny", 4)) {
        mReceiver.onAny(owner,event);
      }
      return;
    }
    if (event == Lifecycle.Event.ON_CREATE) {
      if (!hasLogger || logger.approveCall("onCreate", 1)) {
        mReceiver.onCreate();
      }
      return;
    }
    if (event == Lifecycle.Event.ON_DESTROY) {
      if (!hasLogger || logger.approveCall("onDestroy", 1)) {
        mReceiver.onDestroy();
      }
      return;
    }
  }
}

到这里就真相大白了吧，所以在 generatedConstructor 方法中生成的就是 MyLifecycleObserver_LifecycleAdapter 的构造器。

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@Nullable
private static Constructor<? extends GeneratedAdapter> generatedConstructor(Class<?> klass) {
    try {
        Package aPackage = klass.getPackage();
        String name = klass.getCanonicalName();
        final String fullPackage = aPackage != null ? aPackage.getName() : "";
        // 获取apt自动生成的GeneratedAdapter的类名，在这里就是 MyLifecycleObserver_LifecycleAdapter
        final String adapterName = getAdapterName(fullPackage.isEmpty() ? name :
                name.substring(fullPackage.length() + 1));

        @SuppressWarnings("unchecked") final Class<? extends GeneratedAdapter> aClass =
                (Class<? extends GeneratedAdapter>) Class.forName(
                        fullPackage.isEmpty() ? adapterName : fullPackage + "." + adapterName);
        Constructor<? extends GeneratedAdapter> constructor =
                aClass.getDeclaredConstructor(klass);
        if (!constructor.isAccessible()) {
            constructor.setAccessible(true);
        }
        return constructor;
    } catch (ClassNotFoundException e) {
        return null;
    } catch (NoSuchMethodException e) {
        // this should not happen
        throw new RuntimeException(e);
    }
}

我们再回到 resolveObserverCallbackType 方法，获取到 MyLifecycleObserver_LifecycleAdapter 构造器后，直接返回了 GENERATED_CALLBACK 。

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Constructor<? extends GeneratedAdapter> constructor = generatedConstructor(klass);
if (constructor != null) {
    sClassToAdapters.put(klass, Collections
            .<Constructor<? extends GeneratedAdapter>>singletonList(constructor));
    return GENERATED_CALLBACK;
}

然后在 getCallback 方法中会执行：

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if (type == GENERATED_CALLBACK) {
    List<Constructor<? extends GeneratedAdapter>> constructors =
            sClassToAdapters.get(klass);
    // MyLifecycleObserver_LifecycleAdapter 的构造器只有一个，所以适配创建出来的是 SingleGeneratedAdapterObserver
    if (constructors.size() == 1) {
        // 这里的 generatedAdapter 就是 MyLifecycleObserver_LifecycleAdapter
        GeneratedAdapter generatedAdapter = createGeneratedAdapter(
                constructors.get(0), object);

        // 单个构造器
        return new SingleGeneratedAdapterObserver(generatedAdapter);
    }
    // 至于什么时候 MyLifecycleObserver_LifecycleAdapter 会有多个构造器目前我还不清楚，如果有大神知道的话请告知我下
    GeneratedAdapter[] adapters = new GeneratedAdapter[constructors.size()];
    for (int i = 0; i < constructors.size(); i++) {
        adapters[i] = createGeneratedAdapter(constructors.get(i), object);
    }

    // 多个构造器
    return new CompositeGeneratedAdaptersObserver(adapters);
}

因为 MyLifecycleObserver_LifecycleAdapter 的构造器就只有一个，所以 LifecycleObserver 转化成了 SingleGeneratedAdapterObserver 。

SingleGeneratedAdapterObserver

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@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public class SingleGeneratedAdapterObserver implements GenericLifecycleObserver {

    private final GeneratedAdapter mGeneratedAdapter;

    SingleGeneratedAdapterObserver(GeneratedAdapter generatedAdapter) {
        mGeneratedAdapter = generatedAdapter;
    }

    @Override
    public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
        mGeneratedAdapter.callMethods(source, event, false, null);
        mGeneratedAdapter.callMethods(source, event, true, null);
    }
}

SingleGeneratedAdapterObserver 是实现了 GenericLifecycleObserver 这个接口的。经过上面的一系列操作，我们的 MyLifecycleObserver 就被适配成了 SingleGeneratedAdapterObserver 。

ObserverWithState

其实在 ObserverWithState 还有一个方法 ： dispatchEvent 。dispatchEvent 会在生命周期发生改变时，然后通知观察者的时候调用。

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void dispatchEvent(LifecycleOwner owner, Event event) {
    State newState = getStateAfter(event);
    mState = min(mState, newState);
    // mLifecycleObserver 就是上面的 SingleGeneratedAdapterObserver
    mLifecycleObserver.onStateChanged(owner, event);
    mState = newState;
}

所以我们可以理一理调用链：生命周期发生改变 -> ObserverWithState.dispatchEvent -> SingleGeneratedAdapterObserver.onStateChanged -> MyLifecycleObserver_LifecycleAdapter.callMethods -> MyLifecycleObserver.onCreate/onAny/onDestroy

那么什么时候会去调用 ObserverWithState.dispatchEvent 的方法呢？答案就是在 LifecycleRegistry.handleLifecycleEvent 。 handleLifecycleEvent 方法就是被设计为设置生命周期状态并通知观察者的。

LifecycleRegistry的handleLifecycleEvent

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public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
    // 根据 event 来得到下一个生命周期的状态值
    State next = getStateAfter(event);
    // 将当前生命周期状态值改成 next ，并通知观察者
    moveToState(next);
}

在这里正好把 event 和 state 的关系捋一捋，这是官方给出的参考图，简明扼要。

下面就来看看 moveToState 方法。

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

如果当前生命周期的状态已经同步完成了，就直接 return 掉。否则就会同步并调用 sync 方法。

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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;
}

主要做的事情就是比较当前生命周期的状态和我们存放在 mObserverMap 中最早或最新放入的观察者的状态，通过上面的分析，我们知道是 ObserverWithState 里面一开始有我们添加观察者时的初始状态。
假如生命周期当前状态 mState 是 STARTED ,而观察者的状态是 CREATED，那么我们需要通过 forwardPass() 通知所有的观察者当前生命周期的状态改变到了 STARTED ，请同步。

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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();
        }
    }
}

首先循坏遍历存储了所有观察者的 mObserverMap ，第二个 while 是要分发处理各个状态经过的 event 。
比如当前状态 mState 是 RESUMED ，而 ObserverWithState 中的 state 是 INITIALIZED 。那么调用 ObserverWithState 的 dispatchEvent 方法就要分发 ON_CREATE ，ON_START ，ON_RESUME 了。

问题又来了，到底是谁调用了 handleLifecycleEvent 呢？
我们可以在最终 merge 好的 AndroidManifest 中去寻找答案。

我们发现了这货 ：

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<provider
    android:name="android.arch.lifecycle.ProcessLifecycleOwnerInitializer"
    android:authorities="com.test.multiscrolllayout.lifecycle-trojan"
    android:exported="false"
    android:multiprocess="true" />

进 ProcessLifecycleOwnerInitializer 里看看。

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@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public class ProcessLifecycleOwnerInitializer extends ContentProvider {
    @Override
    public boolean onCreate() {
        LifecycleDispatcher.init(getContext());
        ProcessLifecycleOwner.init(getContext());
        return true;
    }

	...

}

里面有个 LifecycleDispatcher ，一听名字上就猜到它做的是生命周期分发的工作。

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class LifecycleDispatcher {

    private static final String REPORT_FRAGMENT_TAG = "android.arch.lifecycle"
            + ".LifecycleDispatcher.report_fragment_tag";

    private static AtomicBoolean sInitialized = new AtomicBoolean(false);

    static void init(Context context) {
        if (sInitialized.getAndSet(true)) {
            return;
        }
        // 注册了ActivityLifecycleCallbacks
        ((Application) context.getApplicationContext())
                .registerActivityLifecycleCallbacks(new DispatcherActivityCallback());
    }

    @SuppressWarnings("WeakerAccess")
    @VisibleForTesting
    static class DispatcherActivityCallback extends EmptyActivityLifecycleCallbacks {
        private final FragmentCallback mFragmentCallback;

        DispatcherActivityCallback() {
            mFragmentCallback = new FragmentCallback();
        }

        @Override
        public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
            // 注册了一个FragmentLifecycleCallbacks，这个是监控fragment的生命周期回调
            if (activity instanceof FragmentActivity) {
                ((FragmentActivity) activity).getSupportFragmentManager()
                        .registerFragmentLifecycleCallbacks(mFragmentCallback, true);
            }
            // 这句代码很关键 
            ReportFragment.injectIfNeededIn(activity);
        }

        @Override
        public void onActivityStopped(Activity activity) {
            if (activity instanceof FragmentActivity) {
                markState((FragmentActivity) activity, CREATED);
            }
        }

        @Override
        public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
            if (activity instanceof FragmentActivity) {
                markState((FragmentActivity) activity, CREATED);
            }
        }
    }

    ...
}

发现有一个 ReportFragment.injectIfNeededIn(activity); 进这里面看看。

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@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public class ReportFragment extends Fragment {
    private static final String REPORT_FRAGMENT_TAG = "android.arch.lifecycle"
            + ".LifecycleDispatcher.report_fragment_tag";

    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();
        }
    }

    static ReportFragment get(Activity activity) {
        return (ReportFragment) activity.getFragmentManager().findFragmentByTag(
                REPORT_FRAGMENT_TAG);
    }

    private ActivityInitializationListener mProcessListener;

    private void dispatchCreate(ActivityInitializationListener listener) {
        if (listener != null) {
            listener.onCreate();
        }
    }

    private void dispatchStart(ActivityInitializationListener listener) {
        if (listener != null) {
            listener.onStart();
        }
    }

    private void dispatchResume(ActivityInitializationListener listener) {
        if (listener != null) {
            listener.onResume();
        }
    }

    @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);
            }
        }
    }

    void setProcessListener(ActivityInitializationListener processListener) {
        mProcessListener = processListener;
    }

    interface ActivityInitializationListener {
        void onCreate();

        void onStart();

        void onResume();
    }
}

把 ReportFragment 加入到 Activity 中,然后在其各个生命周期中都会调用 dispatch() 方法。而 dispatch 方法最后调用了 LifecycleRegistry.RehandleLifecycleEvent 。

至此，Lifecycle 的整个流程都梳理完成了。

您的支持是我原创的动力