Android自定义ViewPager实例

package com.rong.activity;
import android.content.Context;
import android.graphics.Color;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.view.View;
import android.view.ViewGroup;
import android.widget.LinearLayout;
import android.widget.Scroller;
public class CustomViewPager extends ViewGroup {
  private Context mContext;
  private int screenWidth;
  private int screenHight;
  private int lastMoveX = 0;
  private VelocityTracker velocityTracker;
  private int MAX_VELOCITY=600;
  private int curScreen=0;
  private Scroller scroller;
  public CustomViewPager(Context context) {
    super(context);
    mCOntext= context;
    initView();
  }
  public CustomViewPager(Context context, AttributeSet attrs) {
    super(context, attrs);
    mCOntext= context;
    initView();
  }
  public CustomViewPager(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
    mCOntext= context;
    initView();
  }
  @Override
  public boolean onTouchEvent(MotionEvent event) {
    if (velocityTracker == null) {
      velocityTracker = VelocityTracker.obtain();
    }
    velocityTracker.addMovement(event);
    int x = (int) event.getX();
    switch (event.getAction()) {
    case MotionEvent.ACTION_DOWN:
      lastMoveX = x;
      break;
    case MotionEvent.ACTION_MOVE:
      int dis = lastMoveX - x;
      scrollBy(dis, 0);
      lastMoveX = x;
      break;
    case MotionEvent.ACTION_UP:
      velocityTracker.computeCurrentVelocity(1000);
      int velocityX=(int)velocityTracker.getXVelocity();
      if(velocityX>MAX_VELOCITY&&curScreen>0){
        jump2Screen(curScreen-1);
      }else if(velocityX<-MAX_VELOCITY&&curScreengetChildCount()-1){
      curScreen=getChildCount()-1;
    }
    int dis=curScreen*screenWidth-getScrollX();
    scroller.startScroll(getScrollX(), 0, dis, 0);
    invalidate();
  }
  @Override
  public void computeScroll() {
    if(scroller.computeScrollOffset()){
      scrollTo(scroller.getCurrX(), 0);
      postInvalidate();
    }
  }
  public void initView() {
    scroller=new Scroller(mContext);
    LinearLayout layout1 = new LinearLayout(getContext());
    layout1.setBackgroundColor(Color.RED);
    addView(layout1);
    LinearLayout layout2 = new LinearLayout(getContext());
    layout2.setBackgroundColor(Color.GREEN);
    addView(layout2);
    LinearLayout layout3 = new LinearLayout(getContext());
    layout3.setBackgroundColor(Color.BLUE);
    addView(layout3);
  }
  @Override
  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    screenWidth = MeasureSpec.getSize(widthMeasureSpec);
    screenHight = MeasureSpec.getSize(heightMeasureSpec);
    setMeasuredDimension(screenWidth, screenHight);
    for (int i = 0; i