Jetpack Lifecycle

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引言

  • Lifecycle 是一个类,用于存储有关组件(如 Activity 或 Fragment)的生命周期状态的信息,并允许其他对象观察此状态。
  • Lifecycle 使用两种主要枚举跟踪其关联组件的生命周期状态。
  • Lifecycle.Event 从框架和 Lifecycle 类分派的生命周期事件。这些事件映射到 Activity 和 Fragment 中的回调事件。
  • Lifecycle.State 由 Lifecycle 对象跟踪的组件的当前状态。

预览图

Jetpack Lifecycle 概览图

1. 诞生

 在应用开发中,处理生命周期相关业务,一种常见的模式是在 Activity 和 Fragment 的生命周期方法中实现依赖组件的操作。我们就拿大家习惯的 MVP 举例:

class LifecycleActivity : AppCompatActivity() {

    private lateinit var commonPresenter: CommonPresenter

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_lifecycle)
        commonPresenter = CommonPresenter()
    }

    override fun onResume() {
        super.onResume()
        commonPresenter.onResume()
    }

    override fun onStop() {
        super.onStop()
        commonPresenter.onStop()
    }
}


private const val TAG = "CommonPresenter"

class CommonPresenter {
    fun onResume() {
        Log.i(TAG, "onResume: ")
        checkConfig(object : CheckResultCallback {
            override fun success() {
                Log.i(TAG, "onResume: success to work")
                work()
            }
        })
    }

    fun onStop() {
        Log.i(TAG, "onStop: ")
    }

    private fun checkConfig(callback: CheckResultCallback) {
        Log.i(TAG, "checkConfig: ")
        // TODO: check username and password, need 500ms
        callback.success()
    }

    private fun work() {
        Log.i(TAG, "work: ")
    }
}

 从上面的代码中,会有诸多问题:

  • 会在 Activity 的生命周期中(如 onResumeonStop )中放置大量的代码,难以维护。
  • 在我们需要执行长时间运行的操作(如 onResume() 中的 checkConfig() ), onStop() 方法会在 onResume() 之前结束,这使得组件留存的时间比所需的时间要长。

诞生日记 :于是它来了,androidx.lifecycle 软件包提供的类和接口可帮助以弹性和隔离的方式解决这些问题。

2. 使用

2.1 依赖添加

 官方指导文档

    dependencies {
        val lifecycle_version = "2.4.0-alpha02"
        val arch_version = "2.1.0"

        // ViewModel
        implementation("androidx.lifecycle:lifecycle-viewmodel-ktx:$lifecycle_version")
        // LiveData
        implementation("androidx.lifecycle:lifecycle-livedata-ktx:$lifecycle_version")
        // Lifecycles only (without ViewModel or LiveData)
        implementation("androidx.lifecycle:lifecycle-runtime-ktx:$lifecycle_version")

        // Saved state module for ViewModel
        implementation("androidx.lifecycle:lifecycle-viewmodel-savedstate:$lifecycle_version")

        // Annotation processor
        kapt("androidx.lifecycle:lifecycle-compiler:$lifecycle_version")
        // alternately - if using Java8, use the following instead of lifecycle-compiler
        implementation("androidx.lifecycle:lifecycle-common-java8:$lifecycle_version")

        // optional - helpers for implementing LifecycleOwner in a Service
        implementation("androidx.lifecycle:lifecycle-service:$lifecycle_version")

        // optional - ProcessLifecycleOwner provides a lifecycle for the whole application process
        implementation("androidx.lifecycle:lifecycle-process:$lifecycle_version")
        // optional - ReactiveStreams support for LiveData
        implementation("androidx.lifecycle:lifecycle-reactivestreams-ktx:$lifecycle_version")

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

2.2 使用方法

  • 添加观察者:getLifecycle() 获取 Lifecycle 实例,然后调用 addObserve() 添加观察者。
  • 观察者注解:观察者实现 LifecycleObserver,使用 OnLifecycleEvent 注解添加对应生命周期的方法。

 我们依然优化上面的 诞生 例子。

class LifecycleActivity : AppCompatActivity() {

    private lateinit var lifePresenter: LifePresenter

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_lifecycle)
        lifePresenter = LifePresenter()
        lifecycle.addObserver(lifePresenter)
    }
}


private const val TAG = "LifePresenter"

class LifePresenter : LifecycleObserver {

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun onResume(lifecycleOwner: LifecycleOwner) {
        Log.i(TAG, "onResume() called with: lifecycleOwner = $lifecycleOwner")
        checkConfig(object : CheckResultCallback {
            override fun success() {
                if (lifecycleOwner.lifecycle.currentState.isAtLeast(Lifecycle.State.STARTED)) {
                    Log.i(TAG, "onResume: success to work")
                    work()
                }
            }
        })
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    fun onStop() {
        Log.i(TAG, "onStop: ")
    }

    private fun checkConfig(callback: CheckResultCallback) {
        Log.i(TAG, "checkConfig: ")
        // TODO: check username and password, need 500ms
        callback.success()
    }

    private fun work() {
        Log.i(TAG, "work: ")
    }
}

 LifePresenter 实现了接口 LifecycleObserver,LifecycleObserver 用于标记一个类是生命周期的观察者。然后在具体的方法上面,添加上 @OnLifecycleEvent注解,便可以 follow 生命周期拥有者(Activity)的生命周期回调。ps: 方法的参数可以选配一个 LifecycleOwner 类型的参数; 如果 Event 是 ON_ANY, 还可以接受一个 Event 类型的参数

打开应用并退出,观察 log 输出:

LifePresenter: onResume()
LifePresenter: checkConfig: 
LifePresenter: onResume: success to work
LifePresenter: work:
LifePresenter: onStop: 

2.3 自定义 LifecycleOwner

  • 支持库 26.1.0 及更高版本中的 Fragment 和 Activity 已实现 LifecycleOwner 接口。
  • 如果有一个自定义类并希望使其成为 LifecycleOwner,可以使用 LifecycleRegistry 类,但需要将事件转发到该类,如以下代码示例中所示:
class UserDefineActivity : Activity(), LifecycleOwner {
    private lateinit var lifecycleRegistry: LifecycleRegistry
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_user_define)
        lifecycleRegistry = LifecycleRegistry(this)
        lifecycleRegistry.currentState = Lifecycle.State.CREATED
    }

    override fun onStart() {
        super.onStart()
        lifecycleRegistry.currentState = Lifecycle.State.STARTED
    }

    override fun getLifecycle(): Lifecycle {
        return lifecycleRegistry
    }
}

 需要调用 setCurrentState() 来设置生命周期状态。

2.4 Application 生命周期

  • Lifecycle 也为 Application 提供了生命周期的拥有者:ProcessLifecycleOwner。
  • 需要引入独立依赖:lifecycle-process
  • ProcessLifecycleOwner 依然是通过计数器来维护 Application 的生命周期,获取前后台切换状态。
  • 使用方式如下:
private const val TAG = "MyApplication"

class MyApplication : Application() {
    override fun onCreate() {
        super.onCreate()
        ProcessLifecycleOwner.get().lifecycle.addObserver(ApplicationLifecycleObserver)
    }

    object ApplicationLifecycleObserver : LifecycleObserver {
        @OnLifecycleEvent(Lifecycle.Event.ON_START)
        private fun onFront() {
            Log.i(TAG, "onFront: ")
        }

        @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
        private fun onBack() {
            Log.i(TAG, "onBack: ")
        }
    }
}

3. 关系

3.1 类关系

Jetpck Lifecycle 类关系图

3.2 State & Event 对应关系

官网对照图:状态,事件,流向

Jetpack Lifecycle State & Event 官网 对应关系图

私房图:状态,事件,流向,升级,降级(利于辅助阅读源码)

Jetpck Lifecycle State & Event 对应关系

3.3 调用时序关系

Jetpck Lifecycle 调用时序关系

4. 原理

4.1 角色扮演

  • 观察者LifecycleObserver 接口。通过 Lifecycle 的 addObserver() 注册监听。实现该接口的类使用注解方式,会被反射生成对应类,在生命周期变化时,调用对应类的方法,从而使得实现了该接口的类可以收到 生命周期事件的回调。
  • 持有者LifecycleOwner 接口。关联了 Lifecycle(核心实现在 LifecycleRegistry 中),LifecycleOwner 可以添加观察者(LifecycleObserver),在生命周期变更时,通过到观察者。
  • 分发者ReportFragment 。接受 AMS 侧的生命周期事件,并分发到 LifecycleRegistry。
  • 定义者Lifecycle 。生命状态和生命周期事件集合定义。
  • 同步者LifecycleRegistry 。接受新的生命周期事件, 与之前的留存状态对比,进行同步,然后分发到 观察者(LifecycleObserver)。

4.2 持有者 LifecycleOwner

 AppCompatActivity 的基类 ComponentActivity 实现了 LifecycleOwner,关联了 LifecycleRegistry。

public class ComponentActivity extends androidx.core.app.ComponentActivity implements
        LifecycleOwner,  //持有者
        ViewModelStoreOwner,
        SavedStateRegistryOwner,
        OnBackPressedDispatcherOwner {
        
    //同步者
    private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);

    @NonNull
    @Override
    public Lifecycle getLifecycle() {
        return mLifecycleRegistry;
    }
    
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mSavedStateRegistryController.performRestore(savedInstanceState);
        //分发者
        ReportFragment.injectIfNeededIn(this);
        if (mContentLayoutId != 0) {
            setContentView(mContentLayoutId);
        }
    }
}

4.3 分发者 ReportFragment

  ReportFragment 统筹 AMS 侧的生命周期分发,最终会调用到 dispatch() ,调用到同步者(LifecycleRegistry)。

public class ReportFragment extends android.app.Fragment {
    public static void injectIfNeededIn(Activity activity) {
        //版本 不低于 29, 使用 ActivityLifecycleCallbacks 是进行分发
        if (Build.VERSION.SDK_INT >= 29) {
            // On API 29+, we can register for the correct Lifecycle callbacks directly
            LifecycleCallbacks.registerIn(activity);
        }
        // 添加空的 Fragment(ReportFragment)
        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();
        }
    }

    //分发到 同步者(LifecycleRegistry)
    @SuppressWarnings("deprecation")
    static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) {
        //向前兼容,LifecycleRegistryOwner已不在使用
        if (activity instanceof LifecycleRegistryOwner) {
            ((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
            return;
        }

        //进行分发
        if (activity instanceof LifecycleOwner) {
            Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
            if (lifecycle instanceof LifecycleRegistry) {
                ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
            }
        }
    }
    
    //Fragment 生命周期事件分发
    @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);
    }

    //省略其他生命周期分发代码

    //Fragment 生命周期分发,版本低于 29 才会执行
    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);
        }
    }

    @RequiresApi(29)
    static class LifecycleCallbacks implements Application.ActivityLifecycleCallbacks {

        //LifecycleCallbacks 注册监听
        static void registerIn(Activity activity) {
            activity.registerActivityLifecycleCallbacks(new LifecycleCallbacks());
        }

        //LifecycleCallbacks 生命周期事件分发
        @Override
        public void onActivityPostCreated(@NonNull Activity activity,
                @Nullable Bundle savedInstanceState) {
            dispatch(activity, Lifecycle.Event.ON_CREATE);
        }

        @Override
        public void onActivityPostStarted(@NonNull Activity activity) {
            dispatch(activity, Lifecycle.Event.ON_START);
        }

        //省略其他生命周期分发代码
    }

}

4.4 定义者 Lifecycle

 Lifecycle 包含生命周期事件和状态。

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();

    //生命周期事件
    public enum Event {
        ON_CREATE,
        ON_START,
        ON_RESUME,
        ON_PAUSE,
        ON_STOP,
        ON_DESTROY,
        ON_ANY;

        //降级使用
        @Nullable
        public static Event downFrom(@NonNull State state) {
            switch (state) {
                case CREATED:
                    return ON_DESTROY;
                case STARTED:
                    return ON_STOP;
                case RESUMED:
                    return ON_PAUSE;
                default:
                    return null;
            }
        }

        //可以忽略
        @Nullable
        public static Event downTo(@NonNull State state) {
            switch (state) {
                case DESTROYED:
                    return ON_DESTROY;
                case CREATED:
                    return ON_STOP;
                case STARTED:
                    return ON_PAUSE;
                default:
                    return null;
            }
        }

        //升级使用
        @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;
            }
        }

        //可以忽略
        @Nullable
        public static Event upTo(@NonNull State state) {
            switch (state) {
                case CREATED:
                    return ON_CREATE;
                case STARTED:
                    return ON_START;
                case RESUMED:
                    return ON_RESUME;
                default:
                    return null;
            }
        }

        //获取事件对应状态
        @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");
        }
    }

    //状态(状态越来越大)
    @SuppressWarnings("WeakerAccess")
    public enum State {
        DESTROYED,
        INITIALIZED,
        CREATED,
        STARTED,
        RESUMED;

        //状态不低于指定状态
        public boolean isAtLeast(@NonNull State state) {
            return compareTo(state) >= 0;
        }
    }

}

 还是针对 Event & State 的关系图来说明。

Jetpck Lifecycle State & EVENT 对应关系

事件:这个大家很好理解,就是我们常见的生命周期。

状态:聚合了生命周期的事件,给出的优先级状态。比如我们只会在 RESUMED 状态处理一些显示,在 CREATE 状态就要关闭一些后台处理。

状态升级:对应方法 upFrom(),计算从当前状态向上升一级,对应什么事件,然后进行分发。比如:当前状态是 STARTED,状态升级,调用 upFrom() 计算出来的事件是 ON_RESUME。

状态降级:对应方法 downFrom(),计算从当前状态向下降一级,对应什么事件,然后进行分发。比如:当前状态是 STARTED,状态降级,调用 downFrom() 计算出来的事件是 ON_STOP。

前提: 事件,状态,升级,降级。理解了这些,才能真正理解下面的状态同步处理。

4.5 同步者 LifecycleRegistry

 继承自 Lifecycle。接收了分发者(ReportFragment)的事件分发。进行状态的同步和对应的生命周期事件分发给观察者(继承了 LifecycleObserver 的类)。

public class LifecycleRegistry extends Lifecycle {

    //接受了分发过来的生命周期事件
    public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
        enforceMainThreadIfNeeded("handleLifecycleEvent");
        moveToState(event.getTargetState());
    }

    //状态跳转判断,以便安全同步
    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;
    }

    //是否已经同步完成
    private boolean isSynced() {
        if (mObserverMap.size() == 0) {
            return true;
        }
        State eldestObserverState = mObserverMap.eldest().getValue().mState;
        State newestObserverState = mObserverMap.newest().getValue().mState;
        return eldestObserverState == newestObserverState && mState == newestObserverState;
    }

    //同步处理
    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.
            //如果目标状态比历史状态底,那么就要状态降级,调用 backwardPass
            if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
                backwardPass(lifecycleOwner);
            }
            Map.Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
            //如果目标状态比历史状态高,那么就要状态升级,调用 forwardPass
            if (!mNewEventOccurred && newest != null
                    && mState.compareTo(newest.getValue().mState) > 0) {
                forwardPass(lifecycleOwner);
            }
        }
        mNewEventOccurred = false;
    }

    //状态升级
    private void forwardPass(LifecycleOwner lifecycleOwner) {
        Iterator<Map.Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
                mObserverMap.iteratorWithAdditions();
        //遍历所有所有观察者,进行状态升级
        while (ascendingIterator.hasNext() && !mNewEventOccurred) {
            Map.Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
            ObserverWithState observer = entry.getValue();
            //如果状态升级需要跨级,那么就需要一层一层去回调
            while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
                    && mObserverMap.contains(entry.getKey()))) {
                pushParentState(observer.mState);
                //状态升级计算
                final Event event = Event.upFrom(observer.mState);
                if (event == null) {
                    throw new IllegalStateException("no event up from " + observer.mState);
                }
                //每调整一层状态,就需要对观察者分发一次
                observer.dispatchEvent(lifecycleOwner, event);
                popParentState();
            }
        }
    }

    //状态降级
    private void backwardPass(LifecycleOwner lifecycleOwner) {
        Iterator<Map.Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
                mObserverMap.descendingIterator();
        //遍历所有所有观察者,进行状态降级
        while (descendingIterator.hasNext() && !mNewEventOccurred) {
            Map.Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
            ObserverWithState observer = entry.getValue();
            //如果状态降级需要跨级,那么就需要一层一层去回调
            while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
                    && mObserverMap.contains(entry.getKey()))) {
                //状态降级计算
                Event event = Event.downFrom(observer.mState);
                if (event == null) {
                    throw new IllegalStateException("no event down from " + observer.mState);
                }
                pushParentState(event.getTargetState());
                //每调整一层状态,就需要对观察者分发一次
                observer.dispatchEvent(lifecycleOwner, event);
                popParentState();
            }
        }
    }

    //这个类的处理,会在后面单独说明
    static class ObserverWithState {
        State mState;
        LifecycleEventObserver mLifecycleObserver;

        ObserverWithState(LifecycleObserver observer, State initialState) {
            //反射获取 LifecycleEventObserver 的实例
            mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
            mState = initialState;
        }

        void dispatchEvent(LifecycleOwner owner, Event event) {
            State newState = event.getTargetState();
            mState = min(mState, newState);
            //调用观察者回调方法
            mLifecycleObserver.onStateChanged(owner, event);
            mState = newState;
        }
    }

}

4.6 观察者 LifecycleObserver

 继承了 LifecycleObserver 接口的实现类(添加了 @OnLifecycleEvent 的注解)。通过 Lifecycle.addObserver() 的添加,会调用到 LifecycleRegistry.addObserver() 方法,会创建 ObserverWithState 类,也会进行状态校准,进行同步分发。当然,我们本次主要来分析 ObserverWithState。

4.6.1 ObserverWithState 生成

 LifecycleRegistry.java

static class ObserverWithState {
    State mState;
     LifecycleEventObserver mLifecycleObserver;

    //反射获取 LifecycleEventObserver 的实例
    ObserverWithState(LifecycleObserver observer, State initialState) {
        mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
        mState = initialState;
    }

    void dispatchEvent(LifecycleOwner owner, Event event) {
        State newState = event.getTargetState();
        mState = min(mState, newState);
        //调用观察者回调方法
        mLifecycleObserver.onStateChanged(owner, event);
        mState = newState;
    }
}

4.6.2 LifecycleEventObserver 生成

 Lifecycling.java

@NonNull
static LifecycleEventObserver lifecycleEventObserver(Object object) {
    boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;
    boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver;
    if (isLifecycleEventObserver && isFullLifecycleObserver) {
        return new FullLifecycleObserverAdapter((FullLifecycleObserver) object,
                (LifecycleEventObserver) object);
    }
    if (isFullLifecycleObserver) {
        return new FullLifecycleObserverAdapter((FullLifecycleObserver) object, null);
    }

    if (isLifecycleEventObserver) {
        return (LifecycleEventObserver) 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);
    }
    //上面处理流程忽略,直接到要生成的具体类
    return new ReflectiveGenericLifecycleObserver(object);
}

4.6.3 ReflectiveGenericLifecycleObserver 生成

 ReflectiveGenericLifecycleObserver.java

class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {
    private final Object mWrapped;
    private final CallbackInfo mInfo;

    ReflectiveGenericLifecycleObserver(Object wrapped) {
        mWrapped = wrapped;
        // CallbackInfo 生成
        mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
    }

    //事件分发调用
    @Override
    public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Event event) {
        //回调方法调用
        mInfo.invokeCallbacks(source, event, mWrapped);
    }
}

4.6.4 CallbackInfo 生成 & 事件分发调用

 ClassesInfoCache.java

CallbackInfo getInfo(Class<?> klass) {
    CallbackInfo existing = mCallbackMap.get(klass);
    if (existing != null) {
        return existing;
    }
    //创建 CallbackInfo
    existing = createInfo(klass, null);
    return existing;
}

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);
        }
    }

    //注解方法
    Method[] methods = declaredMethods != null ? declaredMethods : getDeclaredMethods(klass);
    boolean hasLifecycleMethods = false;
    for (Method method : methods) {
        OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
        if (annotation == null) {
            continue;
        }
        hasLifecycleMethods = true;
        //注解方法参数
        Class<?>[] params = method.getParameterTypes();
        int callType = CALL_TYPE_NO_ARG;
        //如果有参数,第一个参数的类型不是 LifecycleOwner,那么报非法异常
        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();

        //如果存在第二个参数的话
        if (params.length > 1) {
            callType = CALL_TYPE_PROVIDER_WITH_EVENT;
            //第二个参数,如果不是 Lifecycle.Event 类型,那么报异常
            if (!params[1].isAssignableFrom(Lifecycle.Event.class)) {
                throw new IllegalArgumentException(
                        "invalid parameter type. second arg must be an event");
            }
            //第二个参数,如果是 Lifecycle.Event 类型,当方法注解不是 ON_ANY 时,报异常
            if (event != Lifecycle.Event.ON_ANY) {
                throw new IllegalArgumentException(
                        "Second arg is supported only for ON_ANY value");
            }
        }
        //多余两个参数,直接报异常
        if (params.length > 2) {
            throw new IllegalArgumentException("cannot have more than 2 params");
        }
        //创建类型和方法
        MethodReference methodReference = new MethodReference(callType, method);
        //缓存校验与设置
        verifyAndPutHandler(handlerToEvent, methodReference, event, klass);
    }
    //创建 CallbackInfo
    CallbackInfo info = new CallbackInfo(handlerToEvent);
    mCallbackMap.put(klass, info);
    mHasLifecycleMethods.put(klass, hasLifecycleMethods);
    return info;
}

@SuppressWarnings("WeakerAccess")
static class CallbackInfo {
    final Map<Lifecycle.Event, List<MethodReference>> mEventToHandlers;
    final Map<MethodReference, Lifecycle.Event> mHandlerToEvent;

    CallbackInfo(Map<MethodReference, Lifecycle.Event> handlerToEvent) {
        mHandlerToEvent = handlerToEvent;
        mEventToHandlers = new HashMap<>();
        for (Map.Entry<MethodReference, Lifecycle.Event> entry : handlerToEvent.entrySet()) {
            Lifecycle.Event event = entry.getValue();
            List<MethodReference> methodReferences = mEventToHandlers.get(event);
            if (methodReferences == null) {
                methodReferences = new ArrayList<>();
                mEventToHandlers.put(event, methodReferences);
            }
            methodReferences.add(entry.getKey());
        }
    }

    //调用事件
    @SuppressWarnings("ConstantConditions")
    void invokeCallbacks(LifecycleOwner source, Lifecycle.Event event, Object target) {
        invokeMethodsForEvent(mEventToHandlers.get(event), source, event, target);
        invokeMethodsForEvent(mEventToHandlers.get(Lifecycle.Event.ON_ANY), source, event,
                target);
    }

    //调用事件
    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--) {
                //方法调用
                handlers.get(i).invokeCallback(source, event, mWrapped);
            }
        }
    }
}

@SuppressWarnings("WeakerAccess")
static final class MethodReference {
    final int mCallType;
    final Method mMethod;

    MethodReference(int callType, Method method) {
        mCallType = callType;
        mMethod = method;
        mMethod.setAccessible(true);
    }

    //方法调用
    void invokeCallback(LifecycleOwner source, Lifecycle.Event event, Object target) {
        //noinspection TryWithIdenticalCatches
        try {
            //方法的参数调用类型,总共三种
            switch (mCallType) {
                case CALL_TYPE_NO_ARG:
                    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);
        }
    }
}

5.小结

  • 使界面控制器(Activity 和 Fragment)尽可能保持精简。它们不应试图获取自己的数据,而应使用 ViewModel 执行此操作,并观察 LiveData 对象以将更改体现到视图中。
  • 设法编写数据驱动型界面,对于此类界面,界面控制器的责任是随着数据更改而更新视图,或者将用户操作通知给 ViewModel
  • 将数据逻辑放在 ViewModel 类中。ViewModel 应充当界面控制器与应用其余部分之间的连接器。不过要注意,ViewModel 不负责获取数据(例如,从网络获取)。但是,ViewModel 应调用相应的组件来获取数据,然后将结果提供给界面控制器。
  • 使用DataBinding在视图与界面控制器之间维持干净的接口。这样一来,可以使视图更具声明性,并尽量减少需要在 Activity 和 Fragment 中编写的更新代码。
  • 如果界面很复杂,不妨考虑创建 presenter 类来处理界面的修改。这可能是一项艰巨的任务,但这样做可使界面组件更易于测试。
  • 避免在 ViewModel 中引用 View 或 Activity 上下文。如果 ViewModel 存在的时间比 Activity 更长(在配置更改的情况下),Activity 将泄漏并且不会获得垃圾回收器的妥善处置。

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