Android侧边栏滑动切换的view效果

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.drawable.BitmapDrawable;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
public class SideBar extends View {
  private static final int MARGIN_BOTTOM = 2;
  private int mItemHeight = 18;
  private OnLetterTouchListener mOnLetterTouchListener;
  private Paint mPaint;
  private float mWidthCenter;
  private int backgroundColor = 0x00F0F0F0;
  private char[] chars = AppCommon.INDEXER_CHARS;
  public interface OnLetterTouchListener {
    public abstract void onPressedDown(int index, char ch);
    public abstract void onPressedUp(int index, char ch);
  }
  public SideBar(Context context) {
    this(context, null);
  }
  public SideBar(Context context, AttributeSet attrs) {
    this(context, attrs, -1);
  }
  public SideBar(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
    init();
  }
  private int startY = 0;//其实位置的y轴坐标
  public SideBar setChars(char[] chars) {
    this.chars = chars;
//   for(int i = 0 ; i <(26 - chars.length) / 2 ; i++){
//     startY += i * mItemHeight;
//   }
    init();
    postInvalidate();
    return this;
  }
  private void init() {
    mPaint = new Paint();
    mPaint.setColor(0xFFCFCFD0);
    mPaint.setAntiAlias(true);
    mPaint.setFakeBoldText(true);
    mPaint.setTextSize(32);
    mPaint.setColor(getResources().getColor(R.color.blue));
    mPaint.setSubpixelText(true);
    mPaint.setTextAlign(Paint.Align.CENTER);
    setBackgroundColor(backgroundColor);
  }
  public boolean onTouchEvent(MotionEvent event) {
    super.onTouchEvent(event);
    int idx = (int) event.getY() / mItemHeight;
    if (idx >= chars.length) {
      idx = chars.length - 1;
    } else if (idx <0) {
      idx = 0;
    }
    if (event.getAction() == MotionEvent.ACTION_DOWN || event.getAction() == MotionEvent.ACTION_MOVE) {
      setBackgroundResource(R.color.sidebar_select_background);
      if (mOnLetterTouchListener != null) {
        mOnLetterTouchListener.onPressedDown(idx, chars[idx]);
      }
    } else if (event.getAction() == MotionEvent.ACTION_UP) {
      setBackgroundDrawable(new BitmapDrawable());
      setBackgroundColor(backgroundColor);
      if (mOnLetterTouchListener != null) {
        mOnLetterTouchListener.onPressedUp(idx, chars[idx]);
      }
    }
    return true;
  }
  @Override
  protected void onSizeChanged(int w, int h, int oldw, int oldh) {
    mItemHeight = (h - MARGIN_BOTTOM) / 26;
    mWidthCenter = getMeasuredWidth() / 2;
    super.onSizeChanged(w, h, oldw, oldh);
  }
  protected void onDraw(Canvas canvas) {
    for (int i = 0; i