这样怎么样:

boolean isBackgrounded() {
    try {
        context.startService(new Intent(action));
        return false;
    }
    catch (IllegalStateException exc) {
        // "Not allowed to start service Intent: app is in background"
        return true;
    }
}

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();
    }

}

根据CommonsWare和Key所说的，你可以扩展Application类，并让你所有的活动在onPause/onResume方法上调用它。这将允许您知道哪些活动是可见的，但这可能会得到更好的处理。

你能详细说明一下你的想法吗?当你说在后台运行，你的意思是简单地让你的应用程序仍然在内存中，即使它目前不在屏幕上?你有没有考虑过使用服务作为一种更持久的方式来管理你的应用程序，当它不在焦点?

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;
}

这样怎么样:

boolean isBackgrounded() {
    try {
        context.startService(new Intent(action));
        return false;
    }
    catch (IllegalStateException exc) {
        // "Not allowed to start service Intent: app is in background"
        return true;
    }
}

当一个对话框出现在活动上方时，活动将被暂停，因此所有推荐的解决方案都是半解决方案。您还需要为对话框创建钩子。