android 如何判断应用是否在前台运行

private boolean isTopActivity(){ 
        List<RunningTaskInfo>  tasksInfo = activityManager.getRunningTasks(1); 
        if(tasksInfo.size() > 0){ 
            //应用程序位于堆栈的顶层 
            if(packageName.equals(tasksInfo.get(0).topActivity.getPackageName())){ 
                return true; 
            } 
        } 
        return false; 
    }