怎么在Android应用中利用TextSwitcher实现一个上下滚动功能

textSwitcher = (TextSwitcher)findViewById(R.id.text_switcher);     textSwitcher.setFactory(new ViewFactory() {              @Override       public View makeView() {         TextView tv = new  TextView(MainActivity.this);         tv.setTextSize(TypedValue.COMPLEX_UNIT_DIP, 16.0F);         tv.setTextColor(Color.RED);         return tv;       }     });          textSwitcher.setInAnimation(AnimationUtils.loadAnimation(this, R.anim.anim_in));     textSwitcher.setOutAnimation(AnimationUtils.loadAnimation(this, R.anim.anim_out));