Android监听应用切换到后台和前台

一个是注册监听：

@HiltAndroidApp
class MyApplication :Application{
    constructor() : super()
    override fun onCreate() {
        super.onCreate()
         Log.e("MyApplication","=====MyApplication")
         registerActivityLifecycleCallbacks(activityLifecycleCallbacks)
    }


    val activityLifecycleCallbacks:ActivityLifecycleCallbacks=   object:ActivityLifecycleCallbacks{

        private var activityStartCount = 0
        override fun onActivityCreated(activity: Activity, savedInstanceState: Bundle?) {
        }

        override fun onActivityStarted(activity: Activity) {
            activityStartCount++
            if(activityStartCount==1){
                //从后台切换到前台
            }

        }

        override fun onActivityResumed(activity: Activity) {
        }

        override fun onActivityPaused(activity: Activity) {
        }

        override fun onActivityStopped(activity: Activity) {
            activityStartCount--
            if (activityStartCount == 0){
                //从前台切换到后台
            }
        }

        override fun onActivitySaveInstanceState(activity: Activity, outState: Bundle) {
        }

        override fun onActivityDestroyed(activity: Activity) {
        }

    }
}

registerActivityLifecycleCallbacks(activityLifecycleCallbacks)这个是监听所有的activity的生命周期的，为什么能做到呢？接下来我们慢慢分析。

ActivityLifecycleCallbacks是Application一个内部的接口。

Application提供了

registerActivityLifecycleCallbacks(ActivityLifecycleCallbacks callback)

unregisterActivityLifecycleCallbacks(ActivityLifecycleCallbacks callback)

两个方法。

我们分析其中registerActivityLifecycleCallbacks（）方法

   public void registerActivityLifecycleCallbacks(ActivityLifecycleCallbacks callback) {
        synchronized (mActivityLifecycleCallbacks) {
            mActivityLifecycleCallbacks.add(callback);
        }
    }

发现是将接口的实现添加到Application的一个集合中

private ArrayList mActivityLifecycleCallbacks =
        new ArrayList();

这又是一个典型的观察者模式。

那这个集合肯定是在哪里调用的呢？全局搜索发现在这个方法中转成了数组

    @UnsupportedAppUsage
    private Object[] collectActivityLifecycleCallbacks() {
        Object[] callbacks = null;
        synchronized (mActivityLifecycleCallbacks) {
            if (mActivityLifecycleCallbacks.size() > 0) {
                callbacks = mActivityLifecycleCallbacks.toArray();
            }
        }
        return callbacks;
    }

这个数组又在application中的

dispatchActivityCreated

dispatchActivityStarted

dispatchActivityResumed

等方法中被调用，如下所示：

  /* package */ void dispatchActivityDestroyed(@NonNull Activity activity) {
        Object[] callbacks = collectActivityLifecycleCallbacks();
        if (callbacks != null) {
            for (int i=0; i

上面的几个方法明显对应的是activity的生命周期，那么很显然这些方法肯定是得在activity的生命周期中调用，不然是无法监听activity的生命周期的。我们到activity的最parent的Activity中去寻找。

我们分析其中

Resume的一个生命周期（其他的大同小异），如下所示：

   protected void onResume() {
        if (DEBUG_LIFECYCLE) Slog.v(TAG, "onResume " + this);
        dispatchActivityResumed();  //调用这方法
        mActivityTransitionState.onResume(this);
        enableAutofillCompatibilityIfNeeded();
        if (mAutoFillResetNeeded) {
            if (!mAutoFillIgnoreFirstResumePause) {
                View focus = getCurrentFocus();
                if (focus != null && focus.canNotifyAutofillEnterExitEvent()) {
                    // TODO(b/148815880): Bring up keyboard if resumed from inline authentication.
                    // TODO: in Activity killed/recreated case, i.e. SessionLifecycleTest#
                    // testDatasetVisibleWhileAutofilledAppIsLifecycled: the View's initial
                    // window visibility after recreation is INVISIBLE in onResume() and next frame
                    // ViewRootImpl.performTraversals() changes window visibility to VISIBLE.
                    // So we cannot call View.notifyEnterOrExited() which will do nothing
                    // when View.isVisibleToUser() is false.
                    getAutofillManager().notifyViewEntered(focus);
                }
            }
        }

        notifyContentCaptureManagerIfNeeded(CONTENT_CAPTURE_RESUME);

        mCalled = true;
    }

 果然是调用了Application中对象的方法

  
// getApplication().dispatchActivityResumed(this); 调用了Application中的分发方法

  private void dispatchActivityResumed() {
        getApplication().dispatchActivityResumed(this);
        Object[] callbacks = collectActivityLifecycleCallbacks();
        if (callbacks != null) {
            for (int i = 0; i < callbacks.length; i++) {
                ((Application.ActivityLifecycleCallbacks) callbacks[i]).onActivityResumed(this);
            }
        }
    }

2.利用Lifecycle的扩展方法

添加依赖：
 

    //只能适用于androidX项目
    implementation("androidx.lifecycle:lifecycle-process:2.2.0")

在mainActivity或者application中添加如下代码： 

   ProcessLifecycleOwner.get().lifecycle.addObserver(object:DefaultLifecycleObserver{
            override fun onStop(owner: LifecycleOwner) {
                super.onStop(owner)
                //后台
            }


            override fun onStart(owner: LifecycleOwner) {
                super.onStart(owner)
                //前台
            }

        })