我建议你仔细阅读这个页面:http://developer.android.com/reference/android/app/Activity.html

简而言之，你的活动在onStop()被调用后不再可见。

由于没有提到它，我将建议读者探索通过Android架构组件提供的ProcessLifecycleOwner

您应该使用共享首选项来存储属性，并使用来自活动的服务绑定对其进行操作。如果你只使用绑定(也就是从不使用startService)，那么你的服务将只在你绑定它时运行，(绑定onResume和取消绑定onPause)，这将使它只在前台运行，如果你确实想在后台工作，你可以使用常规的启动停止服务。

我认为这个问题应该更清楚。什么时候?在哪里?你想知道应用是否处于后台的具体情况是什么?

我只是用我的方法介绍我的解决方案。 我通过在我的应用程序中的每个活动的onStop方法中使用RunningAppProcessInfo类的字段“重要性”来完成这一点，这可以简单地通过为其他活动提供一个BaseActivity来扩展，实现onStop方法来检查“重要性”的值。代码如下:

public static boolean isAppRunning(Context context) {
    ActivityManager activityManager = (ActivityManager) context
        .getSystemService(Context.ACTIVITY_SERVICE);
    List<RunningAppProcessInfo> appProcesses = activityManager
        .getRunningAppProcesses();
    for (RunningAppProcessInfo appProcess : appProcesses) {
        if (appProcess.processName.equals(context.getPackageName())) {
            if (appProcess.importance != RunningAppProcessInfo.IMPORTANCE_PERCEPTIBLE) {
                return true;
            } 
        }
    }
    return false;
}

这篇旧文章的另一个解决方案(对那些可能有帮助的人来说):

<application android:name=".BaseApplication" ... >

public class BaseApplication extends Application {

    private class Status {
        public boolean isVisible = true;
        public boolean isFocused = true;
    }

    private Map<Activity, Status> activities;

    @Override
    public void onCreate() {
        activities = new HashMap<Activity, Status>();
        super.onCreate();
    }

    private boolean hasVisibleActivity() {
        for (Status status : activities.values())
            if (status.isVisible)
                return true;
        return false;
    }

    private boolean hasFocusedActivity() {
        for (Status status : activities.values())
            if (status.isFocused)
                return true;
        return false;
    }

    public void onActivityCreate(Activity activity, boolean isStarting) {
        if (isStarting && activities.isEmpty())
            onApplicationStart();
        activities.put(activity, new Status());
    }

    public void onActivityStart(Activity activity) {
        if (!hasVisibleActivity() && !hasFocusedActivity())
            onApplicationForeground();
        activities.get(activity).isVisible = true;
    }

    public void onActivityWindowFocusChanged(Activity activity, boolean hasFocus) {
        activities.get(activity).isFocused = hasFocus;
    }

    public void onActivityStop(Activity activity, boolean isFinishing) {
        activities.get(activity).isVisible = false;
        if (!isFinishing && !hasVisibleActivity() && !hasFocusedActivity())
            onApplicationBackground();
    }

    public void onActivityDestroy(Activity activity, boolean isFinishing) {
        activities.remove(activity);
        if(isFinishing && activities.isEmpty())
            onApplicationStop();
    }

    private void onApplicationStart() {Log.i(null, "Start");}
    private void onApplicationBackground() {Log.i(null, "Background");}
    private void onApplicationForeground() {Log.i(null, "Foreground");}
    private void onApplicationStop() {Log.i(null, "Stop");}

}

public class MyActivity extends BaseActivity {...}

public class BaseActivity extends Activity {

    private BaseApplication application;

    @Override
    protected void onCreate(Bundle state) {
        application = (BaseApplication) getApplication();
        application.onActivityCreate(this, state == null);
        super.onCreate(state);
    }

    @Override
    protected void onStart() {
        application.onActivityStart(this);
        super.onStart();
    }

    @Override
    public void onWindowFocusChanged(boolean hasFocus) {
        application.onActivityWindowFocusChanged(this, hasFocus);
        super.onWindowFocusChanged(hasFocus);
    }

    @Override
    protected void onStop() {
        application.onActivityStop(this, isFinishing());
        super.onStop();
    }

    @Override
    protected void onDestroy() {
        application.onActivityDestroy(this, isFinishing());
        super.onDestroy();
    }

}

我建议你仔细阅读这个页面:http://developer.android.com/reference/android/app/Activity.html

简而言之，你的活动在onStop()被调用后不再可见。