Android编程ViewPager回弹效果实例分析

package com.freesonfish.viewpager_2; 
import android.content.Context; 
import android.graphics.Rect; 
import android.support.v4.view.ViewPager; 
import android.util.AttributeSet; 
import android.view.KeyEvent; 
import android.view.MotionEvent; 
import android.view.animation.TranslateAnimation; 
public class MyViewPager extends ViewPager { 
  private Rect mRect = new Rect();//用来记录初始位置 
  private int pagerCount = 3; 
  private int currentItem = 0; 
  private boolean handleDefault = true; 
  private float preX = 0f; 
  private static final float RATIO = 0.5f;//摩擦系数 
  private static final float SCROLL_WIDTH = 30f; 
  public MyViewPager(Context context) { 
    super(context); 
  } 
  public MyViewPager(Context context, AttributeSet attrs) { 
    super(context, attrs); 
  } 
  //设置总共有多少页,请记得调用它 
  public void setpagerCount(int pagerCount) { 
    this.pagerCount = pagerCount; 
  } 
  //这是当前是第几页，请在onPageSelect方法中调用它。 
  public void setCurrentIndex(int currentItem) { 
    this.currentItem = currentItem; 
  } 
  @Override 
  public boolean dispatchKeyEvent(KeyEvent event) { 
    return super.dispatchKeyEvent(event); 
  } 
  @Override 
  public boolean onInterceptTouchEvent(MotionEvent arg0) { 
    if (arg0.getAction() == MotionEvent.ACTION_DOWN) { 
      preX = arg0.getX();//记录起点 
    } 
    return super.onInterceptTouchEvent(arg0); 
  } 
  @Override 
  public boolean onTouchEvent(MotionEvent arg0) { 
    switch (arg0.getAction()) { 
    case MotionEvent.ACTION_UP: 
      onTouchActionUp(); 
      break; 
    case MotionEvent.ACTION_MOVE: 
      //当时滑到第一项或者是最后一项的时候。 
      if ((currentItem == 0 || currentItem == pagerCount - 1)) { 
        float nowX = arg0.getX(); 
        float offset = nowX - preX; 
        preX = nowX; 
        if (currentItem == 0) { 
          if (offset > SCROLL_WIDTH) {//手指滑动的距离大于设定值 
            whetherConditionIsRight(offset); 
          } else if (!handleDefault) {//这种情况是已经出现缓冲区域了，手指慢慢恢复的情况 
            if (getLeft() + (int) (offset * RATIO) >= mRect.left) {
              layout(getLeft() + (int) (offset * RATIO), getTop(), getRight() + (int) (offset * RATIO), getBottom()); 
            } 
          } 
        } else { 
          if (offset < -SCROLL_WIDTH) { 
            whetherConditionIsRight(offset); 
          } else if (!handleDefault) { 
            if (getRight() + (int) (offset * RATIO) <= mRect.right) { 
              layout(getLeft() + (int) (offset * RATIO), getTop(), getRight() + (int) (offset * RATIO), getBottom()); 
            } 
          } 
        } 
      } else { 
        handleDefault = true; 
      } 
      if (!handleDefault) { 
        return true; 
      } 
      break; 
    default: 
      break; 
    } 
    return super.onTouchEvent(arg0); 
  } 
  private void whetherConditionIsRight(float offset) { 
    if (mRect.isEmpty()) { 
      mRect.set(getLeft(), getTop(), getRight(), getBottom()); 
    } 
    handleDefault = false; 
    layout(getLeft() + (int) (offset * RATIO), getTop(), getRight() + (int) (offset * RATIO), getBottom()); 
  } 
  private void onTouchActionUp() { 
    if (!mRect.isEmpty()) { 
      recoveryPosition(); 
    } 
  } 
  private void recoveryPosition() { 
    TranslateAnimation ta = null; 
    ta = new TranslateAnimation(getLeft(), mRect.left, 0, 0); 
    ta.setDuration(300); 
    startAnimation(ta); 
    layout(mRect.left, mRect.top, mRect.right, mRect.bottom); 
    mRect.setEmpty(); 
    handleDefault = true; 
  } 
}