Android Jetpack组件之Lifecycles库详解

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Android Jetpack 组件是库的集合，这些库是为了协同工作而构建的，不过也可以单独采用，接下来会一一详细地学习这些库, 下面源码版本是com.android.support:appcompat-v7:28.0.0, 以及库android.arch.lifecycle:extensions:1.1.1

Lifecycles库是拿来干什么的

这个库从系统框架层去管理具有生命周期的组件，例如activity, fragment。让开发更方便地去管理自己应用里需要和activity或者fragment绑定的组件，让代码更容易维护。

也许有点抽象，举个例子说明一下，比如有个需求，需要在一个界面比较频繁更新地理位置信息。当Activity走了onstop之后，你应该也要暂停更新地理位置，或者当Activity走destroy后，你要释放一些资源。下面用一些代码实例解析一下，你的代码也许是这样的:

class MyLocationListener {
    public MyLocationListener(Context context, Callback callback) {
        // ...
    }

    void start() {
        // 开始连接位置服务
    }

    void stop() {
        // 停止连接位置服务
    }
    
    void destroy(){
        //释放资源
    }
}

class MyActivity extends AppCompatActivity {
    private MyLocationListener myLocationListener;

    @Override
    public void onCreate(...) {
        myLocationListener = new MyLocationListener(this, new Callback(){
            //回调更新UI
        });
    }

    @Override
    public void onStart() {
        super.onStart();
        myLocationListener.start();
        //绑定actiivty的onStart周期函数
    }

    @Override
    public void onStop() {
        super.onStop();
        myLocationListener.stop();
        //绑定actiivty的onStop周期函数
    }
    
    @Override
    public void onDestroy() {
        super.onDestroy();
        myLocationListener.destroy();
        //绑定actiivty的onDestroy周期函数
    }
}

上面的代码在简单app看起来也许还好，但是当你activity业务逻辑比较多，可能包含很多和生命周期绑定的自定义组件,代码长期积累就很难维护啦。

下面在看看使用Lifecycles库的代码做对比:

class MyLocationListener implements LifecycleObserver{
    public MyLocationListener(Context context, Callback callback) {
        // ...
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    void start() {
        // 开始连接位置服务
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    void stop() {
        // 停止连接位置服务
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    void destroy(){
        //释放资源
    }
}

class MyActivity extends AppCompatActivity {
    private MyLocationListener myLocationListener;

    @Override
    public void onCreate(...) {
        myLocationListener = new MyLocationListener(this, new Callback(){
            //回调更新UI
        });
        getLifecycle().addObserver(myLocationListener);
    }
}

MyLocationListener实现LifecycleObserver, 在相应的方法添加OnLifecycleEvent注解就可以收到相应的回调，在Activity的onCreate方法里调用 getLifecycle().addObserver(myLocationListener)即可。下面结合源码分析Lifecycles库, 去更好地学习这个库。

Lifecycles库核心类与结构

Support Library 26.1.0 版本以及之后的版本，AppCompatActivity和Fragment实现了LifecycleOwner。类图如下所示:

jetpack_lifecycle_01

Lifecycles库核心就是订阅者模式。

LifecycleOberver类：只是个空接口, 安卓生命周期观察者，

Lifecycle类: 是个抽象类，定义了安卓生命周期对象，有3个方法，添加观察者，移除观察者，获取当前状态。

LifecycleOwner类: 是个接口, 安卓生命周期的拥有者

LifecycleRegistry: Lifecycle的实现类，实现了添加、移除观察者，分派观察者状态等

自定义类实现LifecycleOberver接口，在方法中添加OnLifecycleEvent注解就可以收到相应生命周期的状态

public @interface OnLifecycleEvent {
    Lifecycle.Event value();
}

public enum Event { 
    ON_CREATE,
    ON_START,
    ON_RESUME,
    ON_PAUSE,
    ON_STOP,
    ON_DESTROY,
    ON_ANY
}
//Event是Lifecycle内部类一个枚举类, 分别定义了onCreate, onStart, onResume, onPause,onStop,onDestroy, onAny这几个Event

public enum State {
    DESTROYED,
    INITIALIZED,
    CREATED,
    STARTED,
    RESUMED;

    public boolean isAtLeast(@NonNull State state) {
    	return compareTo(state) >= 0;
    }
}//State也是Lifecycle内部类一个枚举类, 定义了INITIALIZED, CREATED, STARTED, RESUMED, DESTROYED几种状态

Lifecycle的各个状态以及事件分发过程如下图所示: (该图来自google官网文档)

lifecycle_states
矩形代表状态，一共有5个状态，记录在枚举State中， 依次是DESTROYED, INITIALIZED, CREATED, STARTED, RESUMED;

箭头上面代表分发的Event:

  • 当分发ON_CREATE事件时，State由INITIALIZED -> CREATED;
  • 当分发ON_START事件时, State由CREATED -> STARTED
  • 当分发ON_RESUME事件时, State由STARTED -> RESUMED
  • 当分发ON_PAUSE事件时, State由RESUMED -> STARTED
  • 当分发ON_STOP事件时, State由STARTED -> CREATED
  • 当分发ON_DESTROY事件时, State由CREATED -> DESTROYED

你会发现State没STOPED和PAUSED的状态, 当State=CREATED时, Activity大概是在onCreate调用后或者onStop调用后;当State=STARTED时, Activity大概是在onStart调用后或者onPause调用后

ComponentActivity分发Event的过程

下面截取部分ComponentActivity的关键代码

public class ComponentActivity extends Activity implements LifecycleOwner{
    private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
    
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ReportFragment.injectIfNeededIn(this); //利用Fragment来分发
    }
    
        @CallSuper
    @Override
    protected void onSaveInstanceState(Bundle outState) {
        mLifecycleRegistry.markState(Lifecycle.State.CREATED); //onSaveInstanceState是用来恢复Activity状态的, 这里记录的状态是CREATED
        super.onSaveInstanceState(outState);
    }
    
        @Override
    public Lifecycle getLifecycle() {
        return mLifecycleRegistry; //返回LifecycleRegistry
    }
}

下面再看ReportFragment类关键代码：

    public static void injectIfNeededIn(Activity activity) { 
        // 为当前activity add 一个ReportFragment,用于分发event
        android.app.FragmentManager manager = activity.getFragmentManager();
        if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
            manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
            manager.executePendingTransactions();
        }
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        dispatch(Lifecycle.Event.ON_CREATE); //分发ON_CREATE Event
    }

    @Override
    public void onStart() {
        super.onStart();
        dispatch(Lifecycle.Event.ON_START);//分发ON_START Event
    }

    @Override
    public void onResume() {
        super.onResume();
        dispatch(Lifecycle.Event.ON_RESUME);//分发ON_RESUME Event
    }

    @Override
    public void onPause() {
        super.onPause();
        dispatch(Lifecycle.Event.ON_PAUSE);//分发ON_PAUSE Event
    }

    @Override
    public void onStop() {
        super.onStop();
        dispatch(Lifecycle.Event.ON_STOP);//分发ON_STOP Event
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        dispatch(Lifecycle.Event.ON_DESTROY);//分发ON_DESTROY Event
    }

    private void dispatch(Lifecycle.Event event) {
        Activity activity = getActivity();
        // Activity是实现LifecycleOwner接口，这里可以跳过
        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);
                //最终调用LifecycleRegistry.handleLifecycleEvent(event)处理event分发
            }
        }
    }

ComponentActivity的是Event分发是通过添加一个ReportFragment, 通过重写ReportFragment的onActivityCreated, onStart, onResume, onStop, onPause, onDestroy方法，最终交给LifecycleRegistry.handleLifecycleEvent(event)处理。

Fragment分发Event的过程

下面也是截去相关v4里Fragment的相关源码

    LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);

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

   void performCreate(Bundle savedInstanceState) {
        ...
        onCreate(savedInstanceState);
        ...
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
    }

    void performStart() {
        ...
        onStart();
 		...
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START);
    }

    void performResume() {
        ...
        onResume();
        ...
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
    }

    void performPause() {
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE);
        ...
        onPause();
        ...
    }

    void performStop() {
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_STOP);
        ...
        onStop();
        ...
    }

    void performDestroy() {
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY);
        ...
        onDestroy();
        ...
    }

LifecycleRegistry.handleLifecycleEvent(event)处理。 至于LifecycleRegistry这个类更多的细节就不展开啦

自定义LifecycleOwner

前面提到Support Library 26.1.0 版本以及之后的版本，AppCompatActivity和Fragment实现了LifecycleOwner, 如果你还用旧的版本或者继承Activity, 你可以通过自定义Activity或者Fragment实现。自定义实现代码如下：

//可以单独引入androidx.lifecycle:lifecycle-runtime:$lifecycle_version库
public class MyActivity extends Activity implements LifecycleOwner {
    private LifecycleRegistry mLifecycleRegistry;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mLifecycleRegistry = new LifecycleRegistry(this);
        mLifecycleRegistry.markState(Lifecycle.State.CREATED);
    }

    @Override
    public void onStart() {
        super.onStart();
        mLifecycleRegistry.markState(Lifecycle.State.STARTED);
    }
    
    ....

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

使用ProcessLifecycleOwner监听整个App进程的前后台

要注意ON_CREATE的Event之后分发一次，ON_DESTROY不会分发

public class App extends Application {
    @Override
    public void onCreate() {
        super.onCreate();
        ProcessLifecycleOwner.get().getLifecycle().addObserver(new LifecycleObserver() {

            private static final String TAG = "ProcessLifecycleOwner";

            @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
            public void onCreate(){
                Log.d(TAG, "onCreate: "); //应用启动只被调用一次
            }
            
            @OnLifecycleEvent(Lifecycle.Event.ON_START)
            public void onStart(){
                Log.d(TAG, "onStart: "); //应用启动会调用一次, 从后台回来也会调用
            }

            @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
            public void onResume(){
                Log.d(TAG, "onResume: "); //应用启动会调用一次, 从后台回来也会调用
            }

            @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
            public void onPause(){
                Log.d(TAG, "onPause: "); //按home键或者切换应用会调用
            }

            @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
            public void onStop(){
                Log.d(TAG, "onStop: "); //按home键或者切换应用会调用
            }

        });
}

要注意ON_PAUSE, ON_STOP的回调会有700毫秒的延迟, 官方的解析是保证不要由于配置更改而销毁和重新Activity时不会分发任何事件。还有一点，如果你的app是多进程应用，ProcessLifecycleOwner只能用来监听主进程。

更多细节参考：developer.android.google.cn/reference/a…

下面简单说一下ProcessLifecycleOwner的工作原理:

// ProcessLifecycleOwner 关键代码
public class ProcessLifecycleOwner implements LifecycleOwner{
    
    private int mStartedCounter = 0; // 计数器
    private int mResumedCounter = 0; // 计数器
    
    void activityResumed() {
        mResumedCounter++;
        if (mResumedCounter == 1) {
            if (mPauseSent) {
                mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
                mPauseSent = false;
            } else {
                mHandler.removeCallbacks(mDelayedPauseRunnable);
            }
        }
    }

    void activityPaused() {
        mResumedCounter--;
        if (mResumedCounter == 0) {
            mHandler.postDelayed(mDelayedPauseRunnable, TIMEOUT_MS);
        }
    }
    
    void attach(Context context) {
        mHandler = new Handler();
        //mRegistry是LifecycleRegistry对象，依靠LifecycleRegistry分发Event，不多说
        mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
        Application app = (Application) context.getApplicationContext();
        //利用registerActivityLifecycleCallbacks注册callback监听activity生命周期
        //细看activityResumed和activityPaused方法，通过Activity计数法来实现应用前后台的监听
        app.registerActivityLifecycleCallbacks(new EmptyActivityLifecycleCallbacks() {
            @Override
            public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
                ReportFragment.get(activity).setProcessListener(mInitializationListener);
            }

            @Override
            public void onActivityPaused(Activity activity) {
                activityPaused();
            }

            @Override
            public void onActivityStopped(Activity activity) {
                activityStopped();
            }
        });
    }
    
    static void init(Context context) { // 初始化
        sInstance.attach(context);
    }
}

public class ProcessLifecycleOwnerInitializer extends ContentProvider {
    @Override
    public boolean onCreate() {
        LifecycleDispatcher.init(getContext());
        //这是个ContentProvider，在onCreate方法初始化ProcessLifecycleOwner
        //主进程的第一个ContentProvider.onCreate是比Application.onCreate先调用的
        //这个ContentProvider会注册在Androidmenifest中，从而不用再Application中进行ProcessLifecycleOwner初始化
        ProcessLifecycleOwner.init(getContext()); 
        return true;
    }
}

LifecycleService的使用

LifecycleService 继承Service, 并实现LifecycleOwner, 可以自定义一个服务继承LifecycleService来使用,下面是代码实例：

public class MyService extends LifecycleService {

    public MyService() {
    }

    @Override
    public void onCreate() {
        super.onCreate();
        getLifecycle().addObserver(new LifecycleObserver() {

            @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
            public void onCreate(){
                Log.d(TAG, "onCreate: ");
            }

            @OnLifecycleEvent(Lifecycle.Event.ON_START)
            public void onStart(){
                Log.d(TAG, "onStart: ");
            }

            @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
            public void onStop(){
                Log.d(TAG, "onStop: ");
            }
            
            @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
            public void onDestroy(){
                Log.d(TAG, "onDestroy: ");
            }

        });
    }
}

总结以及其他的Tips

Lifecycles库为Jetpack其他组件打下了基础，通过LifecycleObserver观察者减少对Activity, Fragment, 和Service这些具有生命周期类的依赖。

  • 是使用LifecycleService和ProcessLifecycleOwner需要引入android.arch.lifecycle:extensions:1.1.1库，它并没有包含在com.android.support:appcompat-v7:version中
  • 在使用到Lifecycles库时最好在gradle引入apt编译器库annotationProcessor "android.arch.lifecycle:compiler:1.1.1", 没引入这库,对应注解@OnLifecycleEvent的方法就是使用反射来实现的，当引入这库后，会在编译时期自动生成YourObserverName_LifecycleAdapter类实现0反射提高性能。
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