Android app生命周期全局监听(判断app是否处于后台)

### 一、在Application中注册ActivityLifecycleCallbacks：

##### registerActivityLifecycleCallbacks(myActivityLifecycle);

```
public class MyApplication extends Application {

    private static MyActivityLifecycle myActivityLifecycle;

    @Override
    public void onCreate() {
        super.onCreate();
        myActivityLifecycle = new MyActivityLifecycle();
        registerActivityLifecycleCallbacks(myActivityLifecycle);
    }

    /**
     *
     * @return  true  处于后台   false  前台
     */
    public static boolean isAppBackground() {
        if (myActivityLifecycle.getStartCount() == 0) {
            return true;
        }
        return false;
    }
}
```

### 二、MyActivityLifecycle实现Application.ActivityLifecycleCallbacks

```
public class MyActivityLifecycle implements Application.ActivityLifecycleCallbacks {
    private int startCount;

    @Override
    public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
        Log.e("==============","======>onActivityCreated");
    }

    @Override
    public void onActivityStarted(Activity activity) {
        Log.e("==============","======>onActivityStarted");
        startCount++;
    }

    @Override
    public void onActivityResumed(Activity activity) {
        Log.e("==============","======>onActivityResumed");
    }

    @Override
    public void onActivityPaused(Activity activity) {
        Log.e("==============","======>onActivityPaused");
    }

    @Override
    public void onActivityStopped(Activity activity) {
        Log.e("==============","======>onActivityStopped");
        startCount--;
    }

    @Override
    public void onActivityDestroyed(Activity activity) {
        Log.e("==============","======>onActivityDestroyed");
    }

    @Override
    public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
        Log.e("==============","======>onActivitySaveInstanceState");
    }

    public int getStartCount(){
        return startCount;
    }
}
```

===============================分割线======================================

上面是以前的老方法（也可以说是核心原理），现在在AndroidX中有新的（google帮我们写好了的）API

AppBackgroundObserver 实现了 LifecycleObserver 接口，通过注解监听生命周期。
```
class AppBackgroundObserver : LifecycleObserver {

    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    fun onAppBackground() {
        Log.e("AppBackgroundObserver", "onAppBackground")
        AppBackgroundUtil.isBackground = true
        AppBackgroundUtil.dispatchBackgroundStatus(true)
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    fun onAppForeground() {
        Log.e("AppBackgroundObserver", "onAppForeground")
        AppBackgroundUtil.isBackground = false
        AppBackgroundUtil.dispatchBackgroundStatus(false)
    }

}
```

AppBackgroundUtil 工具类
```
object AppBackgroundUtil {

    internal var isBackground: Boolean = true

    private val observerList = mutableListOf<Observer>()

    fun isAppBackground(): Boolean {
        return isBackground
    }

    fun observe(observer: Observer) {
        observerList.add(observer)
    }

    fun stopObserve(observer: Observer) {
        observerList.remove(observer)
    }

    fun clearObserver(observer: Observer) {
        observerList.clear()
    }

    internal fun dispatchBackgroundStatus(isAppBackground: Boolean) {
        observerList.forEach {
            it.onChange(isAppBackground)
        }
    }

    interface Observer {
        fun onChange(isAppBackground: Boolean)
    }

}
```

在Application的onCreate()方法中注册：

```
ProcessLifecycleOwner.get().getLifecycle().addObserver(new AppBackgroundObserver());
```

上面是通过变量静态判断，我们还可以通过注册监听来动态观测。

```
AppBackgroundUtil.observe(object : AppBackgroundUtil.Observer {
            override fun onChange(isAppBackground: Boolean) {
                if (isAppBackground) {
                    // 后台
                } else {
                    // 前台
                }
            }
        })
```

ps：最后有兴趣的可以看看实现原理，其实核心还是最开始说的方式。