2.1.1.Architecture components_Lifecycles

参考

https://developer.android.com/topic/libraries/architecture/lifecycle

https://developer.android.com/jetpack/androidx/releases/lifecycle

配置

需要在本目录的build.gradle里添加google maven 仓库，

allprojects {
    repositories {
        google()
        // If you're using a version of Gradle lower than 4.1, you must instead use:
        // maven {
        //     url 'https://maven.google.com'
        // }
        // An alternative URL is 'https://dl.google.com/dl/android/maven2/'
    }
}

lifecycle相关的库如下：

dependencies {
    def lifecycle_version = "2.1.0"

    // ViewModel and LiveData
    implementation "androidx.lifecycle:lifecycle-extensions:$lifecycle_version"
    // alternatively - just ViewModel
    implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version" // For Kotlin use lifecycle-viewmodel-ktx
    // alternatively - just LiveData
    implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
    // alternatively - Lifecycles only (no ViewModel or LiveData). Some UI AndroidX libraries use this lightweight import for Lifecycle
    implementation "androidx.lifecycle:lifecycle-runtime:$lifecycle_version"

    annotationProcessor "androidx.lifecycle:lifecycle-compiler:$lifecycle_version" // For Kotlin use kapt instead of annotationProcessor
    // alternately - if using Java8, use the following instead of lifecycle-compiler
使用Lifecycle-common-Java8库提供的Lifecycle Java8 API，而不是使用Lifecycle annotations，以获得更快的增量构建
    implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"

    // optional - ReactiveStreams support for LiveData
    implementation "androidx.lifecycle:lifecycle-reactivestreams:$lifecycle_version" // For Kotlin use lifecycle-reactivestreams-ktx

    // optional - Test helpers for LiveData
    testImplementation "androidx.arch.core:core-testing:$lifecycle_version"
}

Lifecycle的Event和state

Lifecycle是用两个枚举来管理生命周期组件的状态，一个是Event，一个是State，如下图：

所以操作ui需要是isAtLeast(STARTED)

几个相关类

Lifecycle

Lifecycle类

Lifecycle是一个抽象类，他的实现类为LifecycleRegistry。

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

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

LifecycleRegistry.addObserver

@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;
    }
...
}

可以看出是可以用同一个LifecycleObserver 重复调用的，mObserverMap.putIfAbsent表示如果没有添加过则添加，否则返回之前添加的value（并不会更新），里边会进行检查如果之前已经添加过了就直接return。

Lifecycle.State

public enum State {
    /**
LifecycleOwner的Destroyed 状态。 此事件之后，此生命周期将不再分派任何事件。 
例如，对于android.app.Activity，此状态在Activity的onDestroy调用之前就已经达到。
     */
    DESTROYED,

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

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

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

    /**
     * Compares if this State is greater or equal to the given {@code state}.
     * 这个方法表示State要大于等于当前的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;
    }
}

Lifecycle.Event

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
}

其中ON_ANY事件是不管什么事件发生都会被调用。

LifecycleRegistry

他是Lifecycle的实现类，除了lifecycle接口定义的方法外还有几个public方法：

l setCurrentState

/**
 * Moves the Lifecycle to the given state and dispatches necessary events to the observers.
 *
 * @param state new state
 */
@MainThread
public void setCurrentState(@NonNull State state) {
    moveToState(state);
}

l handleLifecycleEvent

/**
 * Sets the current state and notifies the observers.
 * <p>
 * Note that if the {@code currentState} is the same state as the last call to this method, calling this method has no effect.
 *
 * @param event The event that was received
 */
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
    State next = getStateAfter(event);
    moveToState(next);
}

l moveToState

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;
}

l getStateAfter

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);
}

LifecycleObserver

public interface LifecycleObserver {
}

是一个空接口

public class MyObserver implements LifecycleObserver {
    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    public void connectListener() {
        ...
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    public void disconnectListener() {
        ...
    }
}

myLifecycleOwner.getLifecycle().addObserver(new MyObserver());

• 为了实现监听生命周期事件，那么可以implement lifecycleObserver，
• 同时在相关方法上，要加上@OnLifecycleEvent(Lifecycle.Event.xxx)，来表示什么时机要执行的方法，
• 被@OnLifecycleEvent修饰的方法 不能有参数，不能是private的

OnLifecycleEvent注解

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface OnLifecycleEvent {
    Lifecycle.Event value();
}

继承LifecycleObserver 的接口

LifecycleObserver 单独使用的话，就必须要和OnLifecycleEvent注解一起用，如果不想用注解，那么可以用它的几个Lifecycle库支持的继承接口。

LifecycleEventObserver

public interface LifecycleEventObserver extends LifecycleObserver {
    /**
     * Called when a state transition event happens.
     *
     * @param source The source of the event
     * @param event The event
     */
    void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event);
}

• 该类可以接收任何生命周期更改并将其分发给接收者。
• 如果一个类同时实现了这个接口和DefaultLifecycleObserver，那么首先调用DefaultLifecycleObserver的方法，然后再调用LifecycleEventObserver.onStateChanged(LifecycleOwner, Lifecycle.Event)。
• 如果一个类实现了这个接口，同时使用了OnLifecycleEvent注解，那么注解将被忽略。

FullLifecycleObserver

interface FullLifecycleObserver extends LifecycleObserver {

    void onCreate(LifecycleOwner owner);

    void onStart(LifecycleOwner owner);

    void onResume(LifecycleOwner owner);

    void onPause(LifecycleOwner owner);

    void onStop(LifecycleOwner owner);

    void onDestroy(LifecycleOwner owner);
}

DefaultLifecycleObserver

需要导入

implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"

public interface DefaultLifecycleObserver extends FullLifecycleObserver {

    @Override
    default void onCreate(@NonNull LifecycleOwner owner) {
    }

    @Override
    default void onStart(@NonNull LifecycleOwner owner) {
    }

    @Override
    default void onResume(@NonNull LifecycleOwner owner) {
    }

    @Override
    default void onPause(@NonNull LifecycleOwner owner) {
    }

    @Override
    default void onStop(@NonNull LifecycleOwner owner) {
    }

    @Override
    default void onDestroy(@NonNull LifecycleOwner owner) {
    }
}

用于监听LifecycleOwner状态更改的回调接口。

• l 如果您使用Java 8语言，请始终优先选择它而不是annotations。
• l 如果一个类同时实现了这个接口和LifecycleEventObserver，那么首先调用DefaultLifecycleObserver的方法，然后再调用LifecycleEventObserver.onstatechange (LifecycleOwner Lifecycle.Event)。
• l 如果一个类实现了这个接口，同时使用了OnLifecycleEvent注解，那么注解将被忽略。

LifecycleOwner

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

实现这个的类，生命周期的所有者，需要创建一个Lifecycle实例，通过这个来通知观察者状态改变。

• l 如果要尝试管理整个应用程序流程的生命周期，请参阅ProcessLifecycleOwner。
• l 在support库26.1.0及之后的Fragment和AppCompatActivity都已经实现了LifecycleOwner接口了，所以就可以直接在里边获取Lifecycle添加观察者。

实现自定义的LifecycleOwner

如果您有想要创建LifecycleOwner的自定义类，则可以使用LifecycleRegistry类，但是您需要将事件转发到该类中，如以下代码示例所示：

public class MyActivity extends Activity implements LifecycleOwner {
    private LifecycleRegistry lifecycleRegistry;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        lifecycleRegistry = new LifecycleRegistry(this);
        lifecycleRegistry.setCurrentState(Lifecycle.State.CREATED);
    }

    @Override
    public void onStart() {
        super.onStart();
        lifecycleRegistry.setCurrentState(Lifecycle.State.STARTED);
    }

    @NonNull
    @Override
    public Lifecycle getLifecycle() {
        return lifecycleRegistry;
    }
}

处理onStop事件

当Lifecycle 属于AppCompatActivity或Fragment时，在调用AppCompatActivity或Fragment的onSaveInstanceState（）时，Lifecycle's 的状态更改为CREATED，并且调度ON_STOP事件。

AppCompatActivity继承自FragmentActivity，FragmentActivity继承自ComponentActivity，

在ComponentActivity.onSaveInstanceState中

@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);
}

对于fragment，在FragmentActivity

@Override
protected void onSaveInstanceState(@NonNull Bundle outState) {
    super.onSaveInstanceState(outState);
// 会把此activity的所有fragment都标记为Lifecycle.State.CREATED
    markFragmentsCreated();
    mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_STOP);

// 会调用fragment的onSaveInstanceState
    Parcelable p = mFragments.saveAllState();
    ...
}

private void markFragmentsCreated() {
    boolean reiterate;
    do {
        reiterate = markState(getSupportFragmentManager(), Lifecycle.State.CREATED);
    } while (reiterate);
}

private static boolean markState(FragmentManager manager, Lifecycle.State state) {
    boolean hadNotMarked = false;
    Collection<Fragment> fragments = manager.getFragments();
    for (Fragment fragment : fragments) {
        if (fragment == null) {
            continue;
        }
        if (fragment.getLifecycle().getCurrentState().isAtLeast(Lifecycle.State.STARTED)) {
            fragment.mLifecycleRegistry.setCurrentState(state);
            hadNotMarked = true;
        }

        if (fragment.getHost() != null) {
            FragmentManager childFragmentManager = fragment.getChildFragmentManager();
            hadNotMarked |= markState(childFragmentManager, state);
        }
    }
    return hadNotMarked;
}

activity的lifecycle

activity的lifecycle的event是由lifecycle库中的ReportFragment来dispatch的，

ReportFragment是在ComponentActivity的onCreate中加入的：

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

在ReportFragment的各生命周期方法中会dispatch lifecycle event：

@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);
        }
    }
}

fragment的lifecycle

fragment是有两套lifecycle。

fragment的lifecycle

是用来管理从onCreate-》onDestory的。

可以通过fragment.getLifecycle()获得。

它的Lifecycle的生命周期event调用顺序：

• l onCreate 在fragment.onCreate 之后
• l onStart 在fragment.onStart之后，先于view的lifecycle
• l onResume 在fragment.onResume 之后，先于view的lifecycle
• l onPause 在fragment.onPause 之前，后于view的lifecycle
• l onStop 在fragment.onStop 之前，后于view的lifecycle
• l onDestroy 在fragment.onDestroy 之前

所以想要观察此lifecycle范围的 生命周期的对象（比如LiveData）时，可以在fragment.onCreate 中创建观察。

view的lifecycle

是用来管理从onCreateView-》onDestoryView的。

fragment.getViewLifecycleOwner()获得LifecycleOwner，然后通过LifecycleOwner.getLifecycle()获取Lifecycle。

它的Lifecycle的生命周期event调用顺序：

• l onCreate 在fragment.onViewStateRestored之后，而fragment.onViewStateRestored是在fragment.onActivityCreated之后。
• l onStart 在fragment.onStart之后，后于fragment的lifecycle
• l onResume 在fragment.onResume 之后，后于fragment的lifecycle
• l onPause 在fragment.onPause 之前，先于fragment的lifecycle
• l onStop 在fragment.onStop 之前，先于fragment的lifecycle
• l onDestroy 在fragment.onDestroyView之前

所以想要观察此lifecycle范围的 生命周期的对象（比如LiveData）时，可以在fragment.onCreateView、onViewCreated 、onActivityCreated中创建观察。

问题

在lifecycle.addObserver(this)后是否会立即回调observer

会的，

LifecycleRegistry中

@Override
public void addObserver(@NonNull LifecycleObserver observer) {
// 设置初始状态
    State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;

    ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
    ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);

    // 如果之前加入过就直接return
    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--;
}

在其中的wihle循环逻辑，表示一直正向dispatch到当前state状态为止，比如当前state时Resumed，那么就会回调observer的onCreate、onStart、onResume，

所以当我们调用addObserver后会立马接收到回调。

ObserverWithState.dispatchEvent

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

LifecycleRegistry中有两个转换state为event的方法，需要结合状态图更好理解

downEvent表示反过程，即销毁过程

private static Event downEvent(State state) {
    switch (state) {
        case INITIALIZED:
            throw new IllegalArgumentException();
        case CREATED:
            return ON_DESTROY;
        case STARTED:
            return ON_STOP;
        case RESUMED:
            return ON_PAUSE;
        case DESTROYED:
            throw new IllegalArgumentException();
    }
    throw new IllegalArgumentException("Unexpected state value " + state);
}

upEvent表示正过程，即创建过程

private static Event upEvent(State state) {
    switch (state) {
        case INITIALIZED:
        case DESTROYED:
            return ON_CREATE;
        case CREATED:
            return ON_START;
        case STARTED:
            return ON_RESUME;
        case RESUMED:
            throw new IllegalArgumentException();
    }
    throw new IllegalArgumentException("Unexpected state value " + state);
}