Android如何得到当前正在屏幕上运行的程序包名？

ActivityManager mActivityManager =(ActivityManager)this.getSystemService(Context.ACTIVITY_SERVICE);
if(Build.VERSION.SDK_INT > 20){
    String mPackageName = mActivityManager.getRunningAppProcesses().get(0).processName;
}else{ 
  String mpackageName =  mActivityManager.getRunningTasks(1).get(0).topActivity.getPackageName();
}




ActivityManager activityManager = (ActivityManager) mContext.getSystemService(Context.ACTIVITY_SERVICE);
RunningAppProcessInfo  cn = activityManager.getRunningAppProcesses().get(0)
mForegroundUidPids.get(uid, new SparseBooleanArray()).put(pid, true);