Android Jetpack組件之Lifecycles庫詳解

時間  2019-12-05
標籤 android jetpack 組件 lifecycles 詳解 欄目 Android 简体版
原文   原文鏈接

Android Jetpack 組件是庫的集合，這些庫是爲了協同工做而構建的，不過也能夠單獨採用，接下來會一一詳細地學習這些庫, 下面源碼版本是com.android.support:appcompat-v7:28.0.0, 以及庫android.arch.lifecycle:extensions:1.1.1java

Lifecycles庫是拿來幹什麼的

這個庫從系統框架層去管理具備生命週期的組件，例如activity, fragment。讓開發更方便地去管理本身應用裏須要和activity或者fragment綁定的組件，讓代碼更容易維護。android

也許有點抽象，舉個例子說明一下，好比有個需求，須要在一個界面比較頻繁更新地理位置信息。當Activity走了onstop以後，你應該也要暫停更新地理位置，或者當Activity走destroy後，你要釋放一些資源。下面用一些代碼實例解析一下，你的代碼也許是這樣的:bash

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

    void start() {
        // 開始鏈接位置服務
    }

    void stop() {
        // 中止鏈接位置服務
    }
    
    void destroy(){
        //釋放資源
    }
}

class MyActivity extends AppCompatActivity {
    private MyLocationListener myLocationListener;

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

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

    @Override
    public void onStop() {
        super.onStop();
        myLocationListener.stop();
        //綁定actiivty的onStop周期函數
    }
    
    @Override
    public void onDestroy() {
        super.onDestroy();
        myLocationListener.destroy();
        //綁定actiivty的onDestroy周期函數
    }
}
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上面的代碼在簡單app看起來也許還好，可是當你activity業務邏輯比較多，可能包含不少和生命週期綁定的自定義組件,代碼長期積累就很難維護啦。app

下面在看看使用Lifecycles庫的代碼作對比:框架

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

    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    void start() {
        // 開始鏈接位置服務
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    void stop() {
        // 中止鏈接位置服務
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    void destroy(){
        //釋放資源
    }
}

class MyActivity extends AppCompatActivity {
    private MyLocationListener myLocationListener;

    @Override
    public void onCreate(...) {
        myLocationListener = new MyLocationListener(this, new Callback(){
            //回調更新UI
        });
        getLifecycle().addObserver(myLocationListener);
    }
}    
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MyLocationListener實現LifecycleObserver, 在相應的方法添加OnLifecycleEvent註解就能夠收到相應的回調，在Activity的onCreate方法裏調用 getLifecycle().addObserver(myLocationListener)便可。下面結合源碼分析Lifecycles庫, 去更好地學習這個庫。ide

Lifecycles庫核心類與結構

Support Library 26.1.0 版本以及以後的版本，AppCompatActivity和Fragment實現了LifecycleOwner。類圖以下所示:函數

jetpack_lifecycle_01

Lifecycles庫核心就是訂閱者模式。源碼分析

LifecycleOberver類：只是個空接口, 安卓生命週期觀察者，post

Lifecycle類: 是個抽象類，定義了安卓生命週期對象，有3個方法，添加觀察者，移除觀察者，獲取當前狀態。性能

LifecycleOwner類: 是個接口, 安卓生命週期的擁有者

LifecycleRegistry: Lifecycle的實現類，實現了添加、移除觀察者，分派觀察者狀態等

自定義類實現LifecycleOberver接口，在方法中添加OnLifecycleEvent註解就能夠收到相應生命週期的狀態

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內部類一個枚舉類, 分別定義了onCreate, onStart, onResume, onPause,onStop,onDestroy, onAny這幾個Event

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

    public boolean isAtLeast(@NonNull State state) {
    	return compareTo(state) >= 0;
    }
}//State也是Lifecycle內部類一個枚舉類, 定義了INITIALIZED, CREATED, STARTED, RESUMED, DESTROYED幾種狀態
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Lifecycle的各個狀態以及事件分發過程以下圖所示: (該圖來自google官網文檔)

lifecycle_states
矩形表明狀態，一共有5個狀態，記錄在枚舉State中， 依次是DESTROYED, INITIALIZED, CREATED, STARTED, RESUMED;

箭頭上面表明分發的Event:

  • 當分發ON_CREATE事件時，State由INITIALIZED -> CREATED;
  • 當分發ON_START事件時, State由CREATED -> STARTED
  • 當分發ON_RESUME事件時, State由STARTED -> RESUMED
  • 當分發ON_PAUSE事件時, State由RESUMED -> STARTED
  • 當分發ON_STOP事件時, State由STARTED -> CREATED
  • 當分發ON_DESTROY事件時, State由CREATED -> DESTROYED

你會發現State沒STOPED和PAUSED的狀態, 當State=CREATED時, Activity大概是在onCreate調用後或者onStop調用後;當State=STARTED時, Activity大概是在onStart調用後或者onPause調用後

ComponentActivity分發Event的過程

下面截取部分ComponentActivity的關鍵代碼

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來分發
    }
    
        @CallSuper
    @Override
    protected void onSaveInstanceState(Bundle outState) {
        mLifecycleRegistry.markState(Lifecycle.State.CREATED); //onSaveInstanceState是用來恢復Activity狀態的, 這裏記錄的狀態是CREATED
        super.onSaveInstanceState(outState);
    }
    
        @Override
    public Lifecycle getLifecycle() {
        return mLifecycleRegistry; //返回LifecycleRegistry
    }
}
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下面再看ReportFragment類關鍵代碼：

public static void injectIfNeededIn(Activity activity) { 
        // 爲當前activity add 一個ReportFragment,用於分發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); //分發ON_CREATE Event
    }

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

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

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

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

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

    private void dispatch(Lifecycle.Event event) {
        Activity activity = getActivity();
        // Activity是實現LifecycleOwner接口，這裏能夠跳過
        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);
                //最終調用LifecycleRegistry.handleLifecycleEvent(event)處理event分發
            }
        }
    }
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ComponentActivity的是Event分發是經過添加一個ReportFragment, 經過重寫ReportFragment的onActivityCreated, onStart, onResume, onStop, onPause, onDestroy方法，最終交給LifecycleRegistry.handleLifecycleEvent(event)處理。

Fragment分發Event的過程

下面也是截去相關v4裏Fragment的相關源碼

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();
        ...
    }
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LifecycleRegistry.handleLifecycleEvent(event)處理。 至於LifecycleRegistry這個類更多的細節就不展開啦

自定義LifecycleOwner

前面提到Support Library 26.1.0 版本以及以後的版本，AppCompatActivity和Fragment實現了LifecycleOwner, 若是你還用舊的版本或者繼承Activity, 你能夠經過自定義Activity或者Fragment實現。自定義實現代碼以下：

//能夠單獨引入androidx.lifecycle:lifecycle-runtime:$lifecycle_version庫
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;
    }
}
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使用ProcessLifecycleOwner監聽整個App進程的先後臺

要注意ON_CREATE的Event以後分發一次，ON_DESTROY不會分發

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: "); //應用啓動只被調用一次
            }
            
            @OnLifecycleEvent(Lifecycle.Event.ON_START)
            public void onStart(){
                Log.d(TAG, "onStart: "); //應用啓動會調用一次, 從後臺回來也會調用
            }

            @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
            public void onResume(){
                Log.d(TAG, "onResume: "); //應用啓動會調用一次, 從後臺回來也會調用
            }

            @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
            public void onPause(){
                Log.d(TAG, "onPause: "); //按home鍵或者切換應用會調用
            }

            @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
            public void onStop(){
                Log.d(TAG, "onStop: "); //按home鍵或者切換應用會調用
            }

        });
}    
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要注意ON_PAUSE, ON_STOP的回調會有700毫秒的延遲, 官方的解析是保證不要因爲配置更改而銷燬和從新Activity時不會分發任何事件。還有一點，若是你的app是多進程應用，ProcessLifecycleOwner只能用來監聽主進程。

更多細節參考：developer.android.google.cn/reference/a…

下面簡單說一下ProcessLifecycleOwner的工做原理:

// ProcessLifecycleOwner 關鍵代碼
public class ProcessLifecycleOwner implements LifecycleOwner{
    
    private int mStartedCounter = 0; // 計數器
    private int mResumedCounter = 0; // 計數器
    
    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對象，依靠LifecycleRegistry分發Event，很少說
        mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
        Application app = (Application) context.getApplicationContext();
        //利用registerActivityLifecycleCallbacks註冊callback監聽activity生命週期
        //細看activityResumed和activityPaused方法，經過Activity計數法來實現應用先後臺的監聽
        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());
        //這是個ContentProvider，在onCreate方法初始化ProcessLifecycleOwner
        //主進程的第一個ContentProvider.onCreate是比Application.onCreate先調用的
        //這個ContentProvider會註冊在Androidmenifest中，從而不用再Application中進行ProcessLifecycleOwner初始化
        ProcessLifecycleOwner.init(getContext()); 
        return true;
    }
}    
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LifecycleService的使用

LifecycleService 繼承Service, 並實現LifecycleOwner, 能夠自定義一個服務繼承LifecycleService來使用,下面是代碼實例：

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: ");
            }

        });
    }
}
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總結以及其餘的Tips

Lifecycles庫爲Jetpack其餘組件打下了基礎，經過LifecycleObserver觀察者減小對Activity, Fragment, 和Service這些具備生命週期類的依賴。

  • 是使用LifecycleService和ProcessLifecycleOwner須要引入android.arch.lifecycle:extensions:1.1.1庫，它並無包含在com.android.support:appcompat-v7:version中
  • 在使用到Lifecycles庫時最好在gradle引入apt編譯器庫annotationProcessor "android.arch.lifecycle:compiler:1.1.1", 沒引入這庫,對應註解@OnLifecycleEvent的方法就是使用反射來實現的，當引入這庫後，會在編譯時期自動生成YourObserverName_LifecycleAdapter類實現0反射提升性能。
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