使用ProcessLifecycleOwner在Activity(或任何类)中从后台到前台检测app的示例。 当应用程序启动时，我缓存启动时间，然后在每个活动中，我将检查应用程序时间，以知道活动是否在第一时间启动或从后台启动

class MyApplication : Application(), LifecycleObserver {

    var appStartBeginTime: Long? = null

    override fun onCreate() {
        super.onCreate()
        ProcessLifecycleOwner.get().lifecycle.addObserver(this);
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    fun onMoveToForeground() {
        Log.i("TAG", "onMoveToForeground")
        appStartBeginTime = System.currentTimeMillis()
    }
}

LoginActivity

class LoginActivity : AppCompatActivity() {
    var localAppStartBeginTime: Long? = null

    ...
    
    // Detect in onResume() instead of onStart because 
    // onMoveToForeground() in MyApplication will fired before onStart 
    override fun onResume() {
        super.onResume()
        if (isOpenedFirstTimeOrFromBackground()) {
            Log.i("TAG", "open first time or from background")

            // do something: eg, call API
        } else {
            Log.i("TAG", "on in another time")
        }
    }

    private fun isOpenedFirstTimeOrFromBackground(): Boolean {
        val globalStartBeginTime = (application as MyApplication).appStartBeginTime
        if (localAppStartBeginTime != globalStartBeginTime) {
            localAppStartBeginTime = globalStartBeginTime
            return true
        }
        return false
    }
}

安卓清单

<manifest ...>

    <application
        android:name=".MyApplication"
        ...>
            
    </application>

</manifest>

演示 https://github.com/PhanVanLinh/AndroidDetectAppFromBackgroundToForeground

这个解决方案呢

public class BaseActivity extends Activity
{

    static String currentAct = "";

    @Override
    protected void onStart()
    {
        super.onStart();

        if (currentAct.equals(""))
            Toast.makeText(this, "Start", Toast.LENGTH_LONG).show();

        currentAct = getLocalClassName();
    }

    @Override
    protected void onStop()
    {
        super.onStop();

        if (currentAct.equals(getLocalClassName()))
        {
            currentAct = "";
            Toast.makeText(this, "Stop", Toast.LENGTH_LONG).show();
        }
    }
}

所有的Activity都需要扩展BaseActivity。

当一个活动调用另一个(A->B)时，currentAct不等于getLocalClassName()，因为第二个活动(B)的onStart()在第一个(A) (https://developer.android.com/guide/components/activities.html#CoordinatingActivities)的onStop()之前被调用。

当用户按下home键或应用程序之间的变化将只调用onStop()，然后currentAct等于getLocalClassName()。

这是我的解决方案https://github.com/doridori/AndroidUtils/blob/master/App/src/main/java/com/doridori/lib/app/ActivityCounter.java

基本上涉及到用计时器计算所有Activity的生命周期方法，以捕捉当前前台没有活动但应用程序(即在旋转)的情况

考虑使用onUserLeaveHint。它只会在应用进入后台时被调用。onPause会有一些极端情况需要处理，因为它可以被用于其他原因;例如，如果用户在你的应用程序中打开另一个活动，比如你的设置页面，你的主活动的onPause方法将被调用，即使它们仍然在你的应用程序中;当你可以简单地使用onUserLeaveHint回调函数来做你所要求的事情时，跟踪正在进行的事情将导致错误。

当调用on UserLeaveHint时，你可以设置一个boolean inBackground标志为true。当onResume被调用时，如果inBackground标志被设置，只假设你回到前台。这是因为onResume也将被调用在你的主要活动，如果用户只是在你的设置菜单，从来没有离开应用程序。

Remember that if the user hits the home button while in your settings screen, onUserLeaveHint will be called in your settings activity, and when they return onResume will be called in your settings activity. If you only have this detection code in your main activity you will miss this use case. To have this code in all your activities without duplicating code, have an abstract activity class which extends Activity, and put your common code in it. Then each activity you have can extend this abstract activity.

例如:

public abstract AbstractActivity extends Activity {
    private static boolean inBackground = false;

    @Override
    public void onResume() {
        if (inBackground) {
            // You just came from the background
            inBackground = false;
        }
        else {
            // You just returned from another activity within your own app
        }
    }

    @Override
    public void onUserLeaveHint() {
        inBackground = true;
    }
}

public abstract MainActivity extends AbstractActivity {
    ...
}

public abstract SettingsActivity extends AbstractActivity {
    ...
}

onPause()和onResume()方法在应用程序被带到后台并再次进入前台时被调用。但是，在应用程序第一次启动时和关闭之前也会调用它们。你可以在活动中阅读更多。

没有任何直接的方法来获得应用程序的状态，而在后台或前台，但即使我已经面临这个问题，并找到解决方案与onWindowFocusChanged和onStop。

更多细节请查看这里Android:解决方案检测当一个Android应用程序去后台，回到前台没有getRunningTasks或getRunningAppProcesses。

在你的应用程序中添加回调并检查根活动，如下所示:

@Override
public void onCreate() {
    super.onCreate();
    registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() {
        @Override
        public void onActivityStopped(Activity activity) {
        }

        @Override
        public void onActivityStarted(Activity activity) {
        }

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

        @Override
        public void onActivityResumed(Activity activity) {
        }

        @Override
        public void onActivityPaused(Activity activity) {
        }

        @Override
        public void onActivityDestroyed(Activity activity) {
        }

        @Override
        public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
            if (activity.isTaskRoot() && !(activity instanceof YourSplashScreenActivity)) {
                Log.e(YourApp.TAG, "Reload defaults on restoring from background.");
                loadDefaults();
            }
        }
    });
}