如何获得当前界面Activity对象？

我有一个需求：需要知道android手机上当前显示的界面是哪个Activity（该Activity不一定是自己的App，可能是其他第三方的App，也可能是系统的App，比如桌面，记事本等等）
我现在通过ActivityManager得到了当前界面的ComponentName对象。
_ActivityManager am = (ActivityManager)getSystemService(Context.ACTIVITY_SERVICE);
ComponentName cn = am.getRunningTasks(1).get(0).topActivity;
但是那也仅仅是ComponentName对象，或者通过cn.getClassName()获得类的名字String。如何得到它的Activity实例对象呢？

老外的这个方法只能获得自己App的Activity对象，不是我想要的。

 public static Activity getActivity() throws ClassNotFoundException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchMethodException, NoSuchFieldException {
        Class activityThreadClass = Class.forName("android.app.ActivityThread");
        Object activityThread = activityThreadClass.getMethod("currentActivityThread").invoke(null);
        Field activitiesField = activityThreadClass.getDeclaredField("mActivities");
        activitiesField.setAccessible(true);
        HashMap activities = (HashMap) activitiesField.get(activityThread);
        for (Object activityRecord : activities.values()) {
            Class activityRecordClass = activityRecord.getClass();
            Field pausedField = activityRecordClass.getDeclaredField("paused");
            pausedField.setAccessible(true);
            if (!pausedField.getBoolean(activityRecord)) {
                Field activityField = activityRecordClass.getDeclaredField("activity");
                activityField.setAccessible(true);
                Activity activity = (Activity) activityField.get(activityRecord);
                return activity;
            }
        }
        return null;
    }