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(){@Overridepublic void onAnimationStart(Animator animation){}@Overridepublic void onAnimationRepeat(Animator animation){}@Overridepublic void onAnimationEnd(Animator animation){startAnimation(0);}@Overridepublic 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;}@Overridepublic 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{@Overridepublic void onPageScrolled(int position, float positionOffset, int positionOffsetPixels){if (mScrollListener != null){mScrollListener.onPageScrolled(position, positionOffset, positionOffsetPixels);}mScrollPosition = position;mScrollPositionOffset = positionOffset;mLastPosition = position;invalidateVisibleChilds(position);}@Overridepublic void onPageSelected(int position){if (mScrollListener != null){mScrollListener.onPageSelected(position);}}@Overridepublic void onPageScrollStateChanged(final int state){if (mScrollListener != null){mScrollListener.onPageScrollStateChanged(state);}if (state == SCROLL_STATE_IDLE){mScrollPositionOffset = 0;}}}@Overridepublic 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;}}@Overridepublic boolean dispatchTouchEvent(MotionEvent ev){try{return super.dispatchTouchEvent(ev);}catch (IllegalArgumentException e){e.printStackTrace();return false;}catch (ArrayIndexOutOfBoundsException e){e.printStackTrace();return false;}}@Overridepublic 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 eventfinal 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);}}@Overrideprotected 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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android自定义ViewPager之——水平滑动弹性效果

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