android自定义ViewPager之——水平滑动弹性效果

　　android ViewPager是一個经常要用到的组件，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>