查明活动是否正在从服务运行

创建自己的类Application，在其中实现接口Application.ActivityLifecycleCallbacks并注册为侦听器。在其中，您可以跟踪打开或关闭了哪些活动：

public class MyApplication extends Application 
        implements Application.ActivityLifecycleCallbacks {

    private int activityCount = 0;

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

    public boolean isAppForeground() {
        return activityCount > 0;
    }

    @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) {
        activityCount++;
    }

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

    @Override
    public void onActivityDestroyed(Activity activity) {

    }

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

    }
}

并且在必要的时候，获取服务中的应用，检查应用是否打开：

((MyApplication) getApplicationContext()).isAppForeground();

不要忘记在清单中声明您的应用程序类：

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

如果您只需要跟踪特定活动，请检查相应方法中的类类型if (activity instanceof YourTargetActivity)：

public boolean isTargetActivityStarted = false;

@Override
public void onActivityResumed(Activity activity) {
    if (activity instanceof YourTargetActivity) {
        isTargetActivityStarted = true;
    }
}

@Override
public void onActivityPaused(Activity activity) {
    if (activity instanceof YourTargetActivity) {
        isTargetActivityStarted = false;
    }
}

在服务中检查变量：

if (((MyApplication) getApplicationContext()).isTargetActivityStarted) {
    // Активити запущена и находится на переднем плане
} else {
    // Активити не запущена или свёрнута
}

有点特殊情况，但作为一种选择，您可以：

派送服务Broadcast

public class MessagingService extends FirebaseMessagingService {
    public static final String ACTION_FILTER = "com.exampe.EVENT";
    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        Intent intent = new Intent(ACTION_FILTER);
        intent.putExtra(...);
        sendOrderedBroadcast(intent, null);
    }
}

receiver低AndroidManifest.xml优先级注册

<receiver
    android:name=".services.NotificationsReceiver"
    android:priority="0">
    <intent-filter>
        <action android:name="com.exampe.EVENT" />
    </intent-filter>
</receiver>

它发送一个通知

在Activity以高优先级注册，OnResume()注册。receiverOnPause()

IntentFilter intentFilter = new IntentFilter(MessagingService.ACTION_FILTER);
intentFilter.setPriority(IntentFilter.SYSTEM_HIGH_PRIORITY);
registerReceiver(broadcastReceiver, intentFilter);

在这个receiver'e中做你在Activity跑步时需要做的事

private BroadcastReceiver broadcastReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        //do something
        abortBroadcast(); //отменяет дальнейшую обработку бродкаста
    }
};

服务可以通过 Notification 发送通知

示例代码 - 来自我的项目：

public void sendMessage(int countNews){
    Resources resources = getResources();
    Intent i = NewsListActivity.newIntent(this);
    PendingIntent pi = PendingIntent.getActivity(this, 0, i, 0);

    Resources res = getApplicationContext().getResources();

    Notification notification = new NotificationCompat.Builder(this)
            .setTicker(resources.getString(R.string.get_new_news))
            .setSmallIcon(R.drawable.ic_notif_logo)
            .setLargeIcon(BitmapFactory.decodeResource(res, R.mipmap.ic_launcher))
            .setNumber(countNews)
            .setContentTitle(resources.getString(R.string.get_new_news))
            .setContentText(resources.getQuantityString(R.plurals.news_plurals, countNews, countNews))
            .setContentIntent(pi)
            .setAutoCancel(true)
            .build();

    NotificationManagerCompat notificationManager = NotificationManagerCompat.from(this);
    notificationManager.notify(0, notification);
}

意图包含单击通知时将启动的活动。

不难判断一个activity是否在屏幕上运行

@Override
public void onPause() {
    super.onPause();
    _show_form = false;
}

@Override
public void onDestroy() {
    super.onDestroy();
    _show_form = false;
}

@Override
protected void onStart() {
    super.onStart();
    _show_form = true;
}

@Override
protected void onResume () {
    super.onResume();
    _show_form = true;
}