我在谷歌Analytics EasyTracker中使用了这个，它起作用了。可以将其扩展为使用简单整数来完成您想要的任务。

public class MainApplication extends Application {

    int isAppBackgrounded = 0;

    @Override
    public void onCreate() {
        super.onCreate();
        appBackgroundedDetector();
    }

    private void appBackgroundedDetector() {
        registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() {
            @Override
            public void onActivityCreated(Activity activity, Bundle bundle) {

            }

            @Override
            public void onActivityStarted(Activity activity) {
                EasyTracker.getInstance(MainApplication.this).activityStart(activity);
            }

            @Override
            public void onActivityResumed(Activity activity) {
                isAppBackgrounded++;
                if (isAppBackgrounded > 0) {
                    // Do something here
                }
            }

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

            @Override
            public void onActivityStopped(Activity activity) {
                EasyTracker.getInstance(MainApplication.this).activityStop(activity);
            }

            @Override
            public void onActivitySaveInstanceState(Activity activity, Bundle bundle) {

            }

            @Override
            public void onActivityDestroyed(Activity activity) {

            }
        });
    }
}

我们可以使用LiveData扩展这个解决方案:

class AppForegroundStateLiveData : LiveData<AppForegroundStateLiveData.State>() {

    private var lifecycleListener: LifecycleObserver? = null

    override fun onActive() {
        super.onActive()
        lifecycleListener = AppLifecycleListener().also {
            ProcessLifecycleOwner.get().lifecycle.addObserver(it)
        }
    }

    override fun onInactive() {
        super.onInactive()
        lifecycleListener?.let {
            this.lifecycleListener = null
            ProcessLifecycleOwner.get().lifecycle.removeObserver(it)
        }
    }

    internal inner class AppLifecycleListener : LifecycleObserver {

        @OnLifecycleEvent(Lifecycle.Event.ON_START)
        fun onMoveToForeground() {
            value = State.FOREGROUND
        }

        @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
        fun onMoveToBackground() {
            value = State.BACKGROUND
        }
    }

    enum class State {
        FOREGROUND, BACKGROUND
    }
}

现在我们可以订阅这个LiveData并捕获所需的事件。例如:

appForegroundStateLiveData.observeForever { state ->
    when(state) {
        AppForegroundStateLiveData.State.FOREGROUND -> { /* app move to foreground */ }
        AppForegroundStateLiveData.State.BACKGROUND -> { /* app move to background */ }
    }
}

我知道有点晚了，但我认为所有这些答案都有一些问题，而我这样做了，这是完美的。

创建一个活动生命周期回调，如下所示:

 class ActivityLifeCycle implements ActivityLifecycleCallbacks{

    @Override
    public void onActivityCreated(Activity activity, Bundle savedInstanceState) {

    }

    @Override
    public void onActivityStarted(Activity activity) {

    }

    Activity lastActivity;
    @Override
    public void onActivityResumed(Activity activity) {
        //if (null == lastActivity || (activity != null && activity == lastActivity)) //use this condition instead if you want to be informed also when  app has been killed or started for the first time
        if (activity != null && activity == lastActivity) 
        {
            Toast.makeText(MyApp.this, "NOW!", Toast.LENGTH_LONG).show();
        }

        lastActivity = activity;
    }

    @Override
    public void onActivityPaused(Activity activity) {

    }

    @Override
    public void onActivityStopped(Activity activity) {

    }

    @Override
    public void onActivitySaveInstanceState(Activity activity, Bundle outState) {

    }

    @Override
    public void onActivityDestroyed(Activity activity) {

    }
}

然后像下面这样在你的应用类上注册它:

public class MyApp extends Application {

@Override
public void onCreate() {
    super.onCreate();
    registerActivityLifecycleCallbacks(new ActivityLifeCycle());
}

编辑:新的体系结构组件带来了一些有希望的东西:ProcessLifecycleOwner，参见@vokilam的回答

实际解决方案根据谷歌I/O谈话:

class YourApplication : Application() {

  override fun onCreate() {
    super.onCreate()
    registerActivityLifecycleCallbacks(AppLifecycleTracker())
  }

}


class AppLifecycleTracker : Application.ActivityLifecycleCallbacks  {

  private var numStarted = 0

  override fun onActivityStarted(activity: Activity?) {
    if (numStarted == 0) {
      // app went to foreground
    }
    numStarted++
  }

  override fun onActivityStopped(activity: Activity?) {
    numStarted--
    if (numStarted == 0) {
      // app went to background
    }
  }

}

是的。我知道很难相信这个简单的解决方案有效，因为我们有这么多奇怪的解决方案。

但还是有希望的。

我设法监控应用导航到后台和回前台通过实现一个BaseActivity，利用onResume, onPause和onStop活动回调的使用。这是我的实现。

override fun onResume() {
    super.onResume()
    if (AppActivityState.state == AppState.ON_LAUNCHED) {
        // We are in the first launch.
        onLaunched()
    } else {
        if (AppActivityState.state == AppState.ON_BACKGROUND) {
            // We came from background to foreground.
            AppActivityState.state = AppState.ON_FOREGROUND
            onForeground()
        } else {
            // We are just navigating through pages.
            AppActivityState.state = AppState.RESUMED
        }
    }
}

override fun onPause() {
    super.onPause()
    // If state is followed by onStop then it means we will going to background.
    AppActivityState.state = AppState.PAUSED
}

override fun onStop() {
    super.onStop()

    // App will go to background base on the 'pause' cue.
    if (AppActivityState.state == AppState.PAUSED) {
        AppActivityState.state = AppState.ON_BACKGROUND
        onBackground()
    }
}

在创建BaseActivity之后，你只需要将这个activity扩展到应用程序中的任何activity。

在这些类型的实现中，您可以准确地检测到以下内容: onBackground >应用程序将进入后台 onForeground >应用将返回前台 onLaunch >应用程序刚刚打开

我希望这对你有帮助:)

您可以在应用程序类中简单地调用此方法

ProcessLifecycleOwner.get().getLifecycle().addObserver(new LifecycleEventObserver() {
            @Override
            public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event) {
                Log.e(TAG, "onStateChanged: " + event.toString());
            }
        });

生命周期。事件将返回应用程序的状态

ON_CREATE
ON_START
ON_RESUME
ON_PAUSE
ON_STOP
ON_DESTROY
ON_ANY

当应用程序进入后台时，它将返回ON_PAUSE & ON_STOP 当应用程序进入前台时，将返回ON_START & ON_RESUME