Android Jetpack -- Lifecycles篇

• Android Jetpack架构组件
  • Android Jetpack – ViewModel篇
  • Android Jetpack – LiveDate篇
  • Android Jetpack – DataBinding篇
  • Android Jetpack – Navigation篇
  • Android Jetpack – Lifecycles篇

基本概念

Lifecycles其实从名字看肯定是与生命周期相关，那它与生命周期又有什么联系？
先参考一下官方文档：
Lifecycles是一个生命周期感知组件，当Activity或者Fragment的生命周期发生改变的时会，Lifecycles也会做出相应的生命周期状态的改变，它保存关于组件生命周期状态的信息(比如活动或片段)，并允许其他对象观察这种状态。
可以看出Lifecycles是一个组件，具有感知生命周期的功能，既然是个组件，那就说明可以嵌入到其他地方，比如VewModel便是其中之一。
我们知道，当我们需要处理与生命周期相关的组件的时候，在没有Lifecycles提供的时候，需要设置各种回调，如下所示：

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

    void start() {
     
        // connect to system location service
    }

    void stop() {
     
        // disconnect from system location service
    }
}

class MyActivity extends AppCompatActivity {
     
    private MyLocationListener myLocationListener;

    @Override
    public void onCreate(...) {
     
        myLocationListener = new MyLocationListener(this, (location) -> {
     
            // update UI
        });
    }

    @Override
    public void onStart() {
     
        super.onStart();
        myLocationListener.start();
        // manage other components that need to respond
        // to the activity lifecycle
    }

    @Override
    public void onStop() {
     
        super.onStop();
        myLocationListener.stop();
        // manage other components that need to respond
        // to the activity lifecycle
    }
}

当组件数目过多的时候，便会需要在生命周期回调中管理多个组件从而徒增代码量，使得项目难以维护，而且在生命周期中执行过多的耗时操作极易引起内存泄漏，而这些都可以通过Lifecycles来解决。
看下Lifecycles的源码吧，反正也不长

public abstract class Lifecycle {
     

    /**
     * Lifecycle coroutines extensions stashes the CoroutineScope into this field.
     *
     * @hide used by lifecycle-common-ktx
     */
    @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
    @NonNull
    AtomicReference<Object> mInternalScopeRef = new AtomicReference<>();

    //添加生命周期状态的观察者
    @MainThread
    public abstract void addObserver(@NonNull LifecycleObserver observer);

    //这个也就是移除观察者
    @MainThread
    public abstract void removeObserver(@NonNull LifecycleObserver observer);

    //获得当前生命周期状态
    @MainThread
    @NonNull
    public abstract State getCurrentState();

	//再熟悉不过的生命周期回调
	//是从框架和生命周期类分派的生命周期事件，这些事件映射到活动和片段中的回调事件。
    @SuppressWarnings("WeakerAccess")
    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
    }

    //这个也就此当前所处于的状态了
    @SuppressWarnings("WeakerAccess")
    public enum State {
     
        /**
         * Destroyed state for a LifecycleOwner. After this event, this Lifecycle will not dispatch
         * any more events. For instance, for an {@link android.app.Activity}, this state is reached
         * right before Activity's {@link android.app.Activity#onDestroy() onDestroy} call.
         */
        DESTROYED,

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

        /**
         * Created state for a LifecycleOwner. For an {@link android.app.Activity}, this state
         * is reached in two cases:
         * 

         *     
after {@link android.app.Activity#onCreate(android.os.Bundle) onCreate} call;
         *     
right before {@link android.app.Activity#onStop() onStop} call.
         * 
         */
        CREATED,

        /**
         * Started state for a LifecycleOwner. For an {@link android.app.Activity}, this state
         * is reached in two cases:
         * 

         *     
after {@link android.app.Activity#onStart() onStart} call;
         *     
right before {@link android.app.Activity#onPause() onPause} call.
         * 
         */
        STARTED,

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

        /**
         * Compares if this State is greater or equal to the given {@code 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;
        }
    }
}

使用示例

以Chronometer为例子来示范生命周期监听

public class MyChronometer extends Chronometer implements LifecycleObserver {
     

    private long time;


    public MyChronometer(Context context) {
     
        super(context);
    }

    public MyChronometer(Context context, AttributeSet attrs) {
     
        super(context, attrs);
    }

    public MyChronometer(Context context, AttributeSet attrs, int defStyleAttr) {
     
        super(context, attrs, defStyleAttr);
    }

	//以注解的形式完成监听
    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    private void stopMeter(){
     
        time = SystemClock.elapsedRealtime() - getBase();
        stop();
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    private void resumeMeter(){
     
        setBase(SystemClock.elapsedRealtime() - time);
        start();
    }
}

然后Activity如下：只需要添加观察者即可

public class LifecyclesActivity extends AppCompatActivity {
     

    MyChronometer chronometer;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
     
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_lifecycles);

        chronometer = findViewById(R.id.meter);
        getLifecycle().addObserver(chronometer);
    }

}

是不是感觉Activity与Chronometer之间解耦了，Chrononmeter自我实现了良好的封装，也方便了移植。
这也就是使用Lifecycles带来的好处。