Lifecycle 源码

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Lifecycle 类 源码注释

Defines an object that has an Android Lifecycle. Fragment and FragmentActivity classes implement LifecycleOwner interface which has the getLifecycle method to access the Lifecycle. You can also implement LifecycleOwner in your own classes.

Event#ON_CREATE, Event#ON_START, Event#ON_RESUME events in this class are dispatched after the LifecycleOwner‘s related method returns. Event#ON_PAUSE, Event#ON_STOP, Event#ON_DESTROY events in this class are dispatched before the LifecycleOwner‘s related method is called. For instance, Event#ON_START will be dispatched after onStart returns, Event#ON_STOP will be dispatched before onStop is called. This gives you certain guarantees on which state the owner is in.

If you use Java 8 Language, then observe events with DefaultLifecycleObserver. To include it you should add "androidx.lifecycle:lifecycle-common-java8:<version>" to your build.gradle file. 

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class TestObserver implements DefaultLifecycleObserver {
      @Override
      public void onCreate(LifecycleOwner owner) {
          // your code
      }
  }

If you use Java 7 Language, Lifecycle events are observed using annotations. Once Java 8 Language becomes mainstream on Android, annotations will be deprecated, so between DefaultLifecycleObserver and annotations, you must always prefer DefaultLifecycleObserver. 

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class TestObserver implements LifecycleObserver {
    @OnLifecycleEvent(ON_STOP)
    void onStopped() {}
  }

Observer methods can receive zero or one argument. If used, the first argument must be of type LifecycleOwner. Methods annotated with Event#ON_ANY can receive the second argument, which must be of type Event. 

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class TestObserver implements LifecycleObserver {
    @OnLifecycleEvent(ON_CREATE)
    void onCreated(LifecycleOwner source) {}
    @OnLifecycleEvent(ON_ANY)
    void onAny(LifecycleOwner source, Event event) {}
  }

These additional parameters are provided to allow you to conveniently observe multiple providers and events without tracking them manually.

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Lifecycle 使用 Event 和 State 来管理生命周期状态的变化。

Lifecycle

LifecycleOnwer

接口。表示一个有 Android lifecycle 的类。

可以通过 getLifecycle() 获得 Lifecycle。

androidx 中的 ComponentActivity 和 Fragment 都实现了 LifecycleOwner 接口。

ComponentActivity 中的实现（androidx.activity:activity:1.1.0）：

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@NonNull
@Override
public Lifecycle getLifecycle() {
    return mLifecycleRegistry;
}
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private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
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@CallSuper
@Override
protected void onSaveInstanceState(@NonNull Bundle outState) {
    Lifecycle lifecycle = getLifecycle();
    if (lifecycle instanceof LifecycleRegistry) {
        ((LifecycleRegistry) lifecycle).setCurrentState(Lifecycle.State.CREATED);
    }
    super.onSaveInstanceState(outState);
    mSavedStateRegistryController.performSave(outState);
}

LifecycleRegistry 是抽象类 Lifecycle 的实现类，可以处理多个 observer。

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

ReportFragment#injectIfNeededIn

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public static void injectIfNeededIn(Activity activity) {
    if (Build.VERSION.SDK_INT >= 29) {
        // On API 29+, we can register for the correct Lifecycle callbacks directly
        activity.registerActivityLifecycleCallbacks(
                new LifecycleCallbacks());
    }
    // 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();
    }
}

这里创建了一个不可见的 ReportFragment 来监听 Activity 的生命周期。Fragment 是一个调度初始化事件的内部类。

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@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;
}

在 ReportFragment 中调用这些生命周期方法来 dispatch 生命周期事件。

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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);
    }
}
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@SuppressWarnings("deprecation")
static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) {
    // LifecycleRegistryOwner 已经 deprecated
    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);
        }
    }
}

LifecycleRegistry#handleLifecycleEvent

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public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
    State next = getStateAfter(event);
    moveToState(next);
}

getStateAfter 获得 event 后的 state：

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static State getStateAfter(Event event) {
    switch (event) {
        case ON_CREATE:
        case ON_STOP:
            return CREATED;
        case ON_START:
        case ON_PAUSE:
            return STARTED;
        case ON_RESUME:
            return RESUMED;
        case ON_DESTROY:
            return DESTROYED;
        case ON_ANY:
            break;
    }
    throw new IllegalArgumentException("Unexpected event value " + event);
}

LifecycleRegistry#moveToState

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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;
}
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// happens only on the top of stack (never in reentrance),
// so it doesn't have to take in account parents
private void sync() {
    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.
        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;
}
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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();
        }
    }
}
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private void backwardPass(LifecycleOwner lifecycleOwner) {
    Iterator<Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
            mObserverMap.descendingIterator();
    while (descendingIterator.hasNext() && !mNewEventOccurred) {
        Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
        ObserverWithState observer = entry.getValue();
        while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
                && mObserverMap.contains(entry.getKey()))) {
            Event event = downEvent(observer.mState);
            pushParentState(getStateAfter(event));
            observer.dispatchEvent(lifecycleOwner, event);
            popParentState();
        }
    }
}