android自定义ViewPager水平滑动弹性效果

android ViewPager是一个经常要用到的组件，但android系统本身为我们提供的ViewPager是没有任何效果的，只能是一页一页的滑动，这样会让人感觉很死板，在看一些知名大公司的App时，看到了他们的ViewPager在滑动到最开始或者最后的时候是有一个弹性效果的，使用起来感觉非常的好，于是乎就是百度搜了一下，在StackOverflow中看到一篇文章就是讲如何实现这个效果的。

先看下效果图：滑动到最后一页时仍然可以拉动……

代码如下：

package com.example.myviewpager; 

import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Camera;
import android.support.v4.view.MotionEventCompat;
import android.support.v4.view.ViewConfigurationCompat;
import android.support.v4.view.ViewPager;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewConfiguration;
import android.view.animation.DecelerateInterpolator;
import android.view.animation.Transformation; 

import com.nineoldandroids.animation.Animator;
import com.nineoldandroids.animation.Animator.AnimatorListener;
import com.nineoldandroids.animation.ObjectAnimator; 

public class BounceBackViewPager extends ViewPager
{ 

 /**
  * maximum z distance to translate child view
  */
 final static int DEFAULT_OVERSCROLL_TRANSLATION = 500; 

 /**
  * duration of overscroll animation in ms
  */
 final private static int DEFAULT_OVERSCROLL_ANIMATION_DURATION = 400; 

 @SuppressWarnings("unused")
 private final static String DEBUG_TAG = ViewPager.class.getSimpleName();
 private final static int INVALID_POINTER_ID = -1; 

 /**
  *
  * @author renard, extended by Piotr Zawadzki
  *
  */
 private class OverscrollEffect
 {
  private float mOverscroll;
  private Animator mAnimator; 

  /**
   * @param deltaDistance [0..1] 0->no overscroll, 1>full overscroll
   */
  public void setPull(final float deltaDistance)
  {
   mOverscroll = deltaDistance;
   invalidateVisibleChilds(mLastPosition);
  } 

  /**
   * called when finger is released. starts to animate back to default position
   */
  private void onRelease()
  {
   if (mAnimator != null && mAnimator.isRunning())
   {
    mAnimator.addListener(new AnimatorListener()
    { 

     @Override
     public void onAnimationStart(Animator animation)
     {
     } 

     @Override
     public void onAnimationRepeat(Animator animation)
     {
     } 

     @Override
     public void onAnimationEnd(Animator animation)
     {
      startAnimation(0);
     } 

     @Override
     public void onAnimationCancel(Animator animation)
     {
     }
    });
    mAnimator.cancel();
   }
   else
   {
    startAnimation(0);
   }
  } 

  private void startAnimation(final float target)
  {
   mAnimator = ObjectAnimator.ofFloat(this, "pull", mOverscroll, target);
   mAnimator.setInterpolator(new DecelerateInterpolator());
   final float scale = Math.abs(target - mOverscroll);
   mAnimator.setDuration((long) (mOverscrollAnimationDuration * scale));
   mAnimator.start();
  } 

  private boolean isOverscrolling()
  {
   if (mScrollPosition == 0 && mOverscroll < 0)
   {
    return true;
   }
   final boolean isLast = (getAdapter().getCount() - 1) == mScrollPosition;
   if (isLast && mOverscroll > 0)
   {
    return true;
   }
   return false;
  } 

 } 

 final private OverscrollEffect mOverscrollEffect = new OverscrollEffect();
 final private Camera mCamera = new Camera(); 

 private OnPageChangeListener mScrollListener;
 private float mLastMotionX;
 private int mActivePointerId;
 private int mScrollPosition;
 private float mScrollPositionOffset;
 final private int mTouchSlop; 

 private float mOverscrollTranslation;
 private int mOverscrollAnimationDuration; 

 public BounceBackViewPager(Context context, AttributeSet attrs)
 {
  super(context, attrs);
  setStaticTransformationsEnabled(true);
  final ViewConfiguration configuration = ViewConfiguration.get(context);
  mTouchSlop = ViewConfigurationCompat.getScaledPagingTouchSlop(configuration);
  super.setOnPageChangeListener(new MyOnPageChangeListener());
  init(attrs);
 } 

 private void init(AttributeSet attrs)
 {
  TypedArray a = getContext().obtainStyledAttributes(attrs, R.styleable.BounceBackViewPager);
  mOverscrollTranslation = a.getDimension(R.styleable.BounceBackViewPager_overscroll_translation_bounce,
    DEFAULT_OVERSCROLL_TRANSLATION);
  mOverscrollAnimationDuration = a.getInt(R.styleable.BounceBackViewPager_overscroll_animation_duration_bounce,
    DEFAULT_OVERSCROLL_ANIMATION_DURATION);
  a.recycle();
 } 

 public int getOverscrollAnimationDuration()
 {
  return mOverscrollAnimationDuration;
 } 

 public void setOverscrollAnimationDuration(int mOverscrollAnimationDuration)
 {
  this.mOverscrollAnimationDuration = mOverscrollAnimationDuration;
 } 

 public float getOverscrollTranslation()
 {
  return mOverscrollTranslation;
 } 

 public void setOverscrollTranslation(int mOverscrollTranslation)
 {
  this.mOverscrollTranslation = mOverscrollTranslation;
 } 

 @Override
 public void setOnPageChangeListener(OnPageChangeListener listener)
 {
  mScrollListener = listener;
 }; 

 private void invalidateVisibleChilds(final int position)
 {
  for (int i = 0; i < getChildCount(); i++)
  {
   getChildAt(i).invalidate(); 

  }
  // this.invalidate();
  // final View child = getChildAt(position);
  // final View previous = getChildAt(position - 1);
  // final View next = getChildAt(position + 1);
  // if (child != null) {
  // child.invalidate();
  // }
  // if (previous != null) {
  // previous.invalidate();
  // }
  // if (next != null) {
  // next.invalidate();
  // }
 } 

 private int mLastPosition = 0; 

 private class MyOnPageChangeListener implements OnPageChangeListener
 { 

  @Override
  public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels)
  {
   if (mScrollListener != null)
   {
    mScrollListener.onPageScrolled(position, positionOffset, positionOffsetPixels);
   }
   mScrollPosition = position;
   mScrollPositionOffset = positionOffset;
   mLastPosition = position;
   invalidateVisibleChilds(position);
  } 

  @Override
  public void onPageSelected(int position)
  { 

   if (mScrollListener != null)
   {
    mScrollListener.onPageSelected(position);
   }
  } 

  @Override
  public void onPageScrollStateChanged(final int state)
  { 

   if (mScrollListener != null)
   {
    mScrollListener.onPageScrollStateChanged(state);
   }
   if (state == SCROLL_STATE_IDLE)
   {
    mScrollPositionOffset = 0;
   }
  }
 } 

 @Override
 public boolean onInterceptTouchEvent(MotionEvent ev)
 {
  try
  {
   final int action = ev.getAction() & MotionEventCompat.ACTION_MASK;
   switch (action)
   {
   case MotionEvent.ACTION_DOWN:
   {
    mLastMotionX = ev.getX();
    mActivePointerId = MotionEventCompat.getPointerId(ev, 0);
    break;
   }
   case MotionEventCompat.ACTION_POINTER_DOWN:
   {
    final int index = MotionEventCompat.getActionIndex(ev);
    final float x = MotionEventCompat.getX(ev, index);
    mLastMotionX = x;
    mActivePointerId = MotionEventCompat.getPointerId(ev, index);
    break;
   }
   }
   return super.onInterceptTouchEvent(ev);
  }
  catch (IllegalArgumentException e)
  {
   e.printStackTrace();
   return false;
  }
  catch (ArrayIndexOutOfBoundsException e)
  {
   e.printStackTrace();
   return false;
  }
 } 

 @Override
 public boolean dispatchTouchEvent(MotionEvent ev)
 {
  try
  {
   return super.dispatchTouchEvent(ev);
  }
  catch (IllegalArgumentException e)
  {
   e.printStackTrace();
   return false;
  }
  catch (ArrayIndexOutOfBoundsException e)
  {
   e.printStackTrace();
   return false;
  }
 } 

 @Override
 public boolean onTouchEvent(MotionEvent ev)
 {
  boolean callSuper = false; 

  final int action = ev.getAction();
  switch (action)
  {
  case MotionEvent.ACTION_DOWN:
  {
   callSuper = true;
   mLastMotionX = ev.getX();
   mActivePointerId = MotionEventCompat.getPointerId(ev, 0);
   break;
  }
  case MotionEventCompat.ACTION_POINTER_DOWN:
  {
   callSuper = true;
   final int index = MotionEventCompat.getActionIndex(ev);
   final float x = MotionEventCompat.getX(ev, index);
   mLastMotionX = x;
   mActivePointerId = MotionEventCompat.getPointerId(ev, index);
   break;
  }
  case MotionEvent.ACTION_MOVE:
  {
   if (mActivePointerId != INVALID_POINTER_ID)
   {
    // Scroll to follow the motion event
    final int activePointerIndex = MotionEventCompat.findPointerIndex(ev, mActivePointerId);
    final float x = MotionEventCompat.getX(ev, activePointerIndex);
    final float deltaX = mLastMotionX - x;
    final float oldScrollX = getScrollX();
    final int width = getWidth();
    final int widthWithMargin = width + getPageMargin();
    final int lastItemIndex = getAdapter().getCount() - 1;
    final int currentItemIndex = getCurrentItem();
    final float leftBound = Math.max(0, (currentItemIndex - 1) * widthWithMargin);
    final float rightBound = Math.min(currentItemIndex + 1, lastItemIndex) * widthWithMargin;
    final float scrollX = oldScrollX + deltaX;
    if (mScrollPositionOffset == 0)
    {
     if (scrollX < leftBound)
     {
      if (leftBound == 0)
      {
       final float over = deltaX + mTouchSlop;
       mOverscrollEffect.setPull(over / width);
      }
     }
     else if (scrollX > rightBound)
     {
      if (rightBound == lastItemIndex * widthWithMargin)
      {
       final float over = scrollX - rightBound - mTouchSlop;
       mOverscrollEffect.setPull(over / width);
      }
     }
    }
    else
    {
     mLastMotionX = x;
    }
   }
   else
   {
    mOverscrollEffect.onRelease();
   }
   break;
  }
  case MotionEvent.ACTION_UP:
  case MotionEvent.ACTION_CANCEL:
  {
   callSuper = true;
   mActivePointerId = INVALID_POINTER_ID;
   mOverscrollEffect.onRelease();
   break;
  }
  case MotionEvent.ACTION_POINTER_UP:
  {
   final int pointerIndex = (ev.getAction() & MotionEvent.ACTION_POINTER_INDEX_MASK) >> MotionEvent.ACTION_POINTER_INDEX_SHIFT;
   final int pointerId = MotionEventCompat.getPointerId(ev, pointerIndex);
   if (pointerId == mActivePointerId)
   {
    // This was our active pointer going up. Choose a new
    // active pointer and adjust accordingly.
    final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
    mLastMotionX = ev.getX(newPointerIndex);
    mActivePointerId = MotionEventCompat.getPointerId(ev, newPointerIndex);
    callSuper = true;
   }
   break;
  }
  } 

  if (mOverscrollEffect.isOverscrolling() && !callSuper)
  {
   return true;
  }
  else
  {
   return super.onTouchEvent(ev);
  }
 } 

 @Override
 protected boolean getChildStaticTransformation(View child, Transformation t)
 {
  if (child.getWidth() == 0)
  {
   return false;
  }
  final int position = child.getLeft() / child.getWidth();
  final boolean isFirstOrLast = position == 0 || (position == getAdapter().getCount() - 1);
  if (mOverscrollEffect.isOverscrolling() && isFirstOrLast)
  {
   final float dx = getWidth() / 2;
   final int dy = getHeight() / 2;
   t.getMatrix().reset();
   final float translateX = (float) mOverscrollTranslation
     * (mOverscrollEffect.mOverscroll > 0 ? Math.min(mOverscrollEffect.mOverscroll, 1) : Math.max(
       mOverscrollEffect.mOverscroll, -1));
   mCamera.save();
   mCamera.translate(-translateX, 0, 0);
   mCamera.getMatrix(t.getMatrix());
   mCamera.restore();
   t.getMatrix().preTranslate(-dx, -dy);
   t.getMatrix().postTranslate(dx, dy); 

   if (getChildCount() == 1)
   {
    this.invalidate();
   }
   else
   {
    child.invalidate();
   }
   return true;
  }
  return false;
 }
}

自定义属性如下：

<declare-styleable name="BounceBackViewPager">
  <attr name="overscroll_translation_bounce" format="dimension" /> 

  <!-- Duration of animation when user releases the over scroll. Default is 400 ms. -->
  <attr name="overscroll_animation_duration_bounce" format="integer" />
 </declare-styleable>

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