带你彻彻底底弄懂Scroller

Scroller的使用

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这是一个滑动帮助类。并不能够使View真正的滑动，而是依据时间的流逝。获取插值器中的数据。传递给我们。让我们去配合scrollTo/scrollBy去让view产生缓慢滑动，产生动画的效果。事实上是和属性动画同一个原理。以下是官方文档对于这个类所给的解释：

This class encapsulates scrolling. You can use scrollers (Scroller or OverScroller) to collect the data you need to produce a scrolling animation—for example, in response to a fling gesture. Scrollers track scroll offsets for you over time, but they don’t automatically apply those positions to your view. It’s your responsibility to get and apply new coordinates at a rate that will make the scrolling animation look smooth.

首先。我们要先获得这个对象，我们一起看看它的构造方法：

public Scroller (Context context)

public Scroller (Context context, Interpolator interpolator)

public Scroller (Context context, Interpolator interpolator, boolean flywheel)

一共同拥有三个构造方法。我们通经常使用第二个比較多，给定Scroller一个插值器，让其从这样的插值器中取值。

那么，怎样使用Scroller呢，仅仅需调用以下的代码就可以：

Scroller.(int startX, int startY, int dx, int dy, int duration)。
invalidate();

就能够为Scroller指定从起始位置和结束位置以及滑动的时间。Scroller就能够帮助我们获取某一时刻，我们的view所在的位置了。

接着我们须要在view的computeScroll()的方法中推断scroller是否结束，假设没有结束就用scrollTo方法使view处于正确的位置就可以。

    @Override
    public void computeScroll() {
        //推断是否还在滚动，还在滚动为true
        if (mScroller.computeScrollOffset()) {
            scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
            //更新界面
            postInvalidate();
            isMove = true;
        }
        super.computeScroll();
    }

怎么样，用起来是不是非常easy粗暴。

Scroller的源代码分析

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仅仅是会使用可不行。我们不仅要知其然还要知其所以然。接下来，我们从源代码的角度来分析下Scroller的工作原理。从哪里分析呢。事实上也不用从Scroller类的第一行代码開始看。捡重要的看就可以了。

首先看看startScroll()这种方法吧：

    public void startScroll(int startX, int startY, int dx, int dy, int duration) {
        mMode = SCROLL_MODE;
        mFinished = false;
        mDuration = duration;
        mStartTime = AnimationUtils.currentAnimationTimeMillis();
        mStartX = startX;
        mStartY = startY;
        mFinalX = startX + dx;
        mFinalY = startY + dy;
        mDeltaX = dx;
        mDeltaY = dy;
        mDurationReciprocal = 1.0f / (float) mDuration;
    }

我们能够从中看到，Scroller仅仅是给他的成员变量一一赋值而已，比方模式啊，位置信息，时间等。并没有做关于view滑动的不论什么工作。我们接下来调用了View的invalidate()方法，让View树又一次绘制，让它绘制什么呢？什么东西都没有，这不是坑我们么？事实上。View的draw方法中都会调用我们上面说的computeScroll() 方法，可是这种方法在View中却是一个空方法。接下来。我们就继续分析Scroller的computeScrollOffset()方法：

    public boolean computeScrollOffset() {
        if (mFinished) {
            return false;
        }

        int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);

        if (timePassed < mDuration) {
            switch (mMode) {
            case SCROLL_MODE:
                float x = timePassed * mDurationReciprocal;

                if (mInterpolator == null)
                    x = viscousFluid(x); 
                else
                    x = mInterpolator.getInterpolation(x);

                mCurrX = mStartX + Math.round(x * mDeltaX);
                mCurrY = mStartY + Math.round(x * mDeltaY);
                break;
            case FLING_MODE:
                final float t = (float) timePassed / mDuration;
                final int index = (int) (NB_SAMPLES * t);
                float distanceCoef = 1.f;
                float velocityCoef = 0.f;
                if (index < NB_SAMPLES) {
                    final float t_inf = (float) index / NB_SAMPLES;
                    final float t_sup = (float) (index + 1) / NB_SAMPLES;
                    final float d_inf = SPLINE_POSITION[index];
                    final float d_sup = SPLINE_POSITION[index + 1];
                    velocityCoef = (d_sup - d_inf) / (t_sup - t_inf);
                    distanceCoef = d_inf + (t - t_inf) * velocityCoef;
                }

                mCurrVelocity = velocityCoef * mDistance / mDuration * 1000.0f;

                mCurrX = mStartX + Math.round(distanceCoef * (mFinalX - mStartX));
                // Pin to mMinX <= mCurrX <= mMaxX
                mCurrX = Math.min(mCurrX, mMaxX);
                mCurrX = Math.max(mCurrX, mMinX);

                mCurrY = mStartY + Math.round(distanceCoef * (mFinalY - mStartY));
                // Pin to mMinY <= mCurrY <= mMaxY
                mCurrY = Math.min(mCurrY, mMaxY);
                mCurrY = Math.max(mCurrY, mMinY);

                if (mCurrX == mFinalX && mCurrY == mFinalY) {
                    mFinished = true;
                }

                break;
            }
        }
        else {
            mCurrX = mFinalX;
            mCurrY = mFinalY;
            mFinished = true;
        }
        return true;
    }

从这种方法中，我们能够看出。Scroller先推断滑动有没有结束，假设没有结束就去获取View此时应该所处的位置信息。这种方法中重要的是它的返回值。假设完毕了返回false,没有完毕才返回true。继续接着Scroller的使用段落往下看。假设mScroller.computeScrollOffset()的返回值是true的话，也就是Scroller还没结束，我们就让我们的View滑动到这里，并刷新。值得注意的是，View中的computeScroll()方法并非执行在主线程中的，所以我们要使postInvalidate()方法来调用重绘。

接着重绘又会调用View的draw方法，draw方法又会调用computeScroll()方法，直至Scroller结束。到这里我们依据平时使用的代码的走向，了解了Scroller的大致工作流程。

怎么样。是不是对Scroller的理解更深刻一些了呢？

Scroller的实例

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仅仅是理解还不够啊，我们得从实际开发中使用Scroller。才干真正的会用他。接下来，我会用两个实例，来升华你对Scroller的理解。

实例一：仿微信朋友圈刷新

废话不多说，直接上代码吧！

/**
 * 仿微信刷新
 * @author Nipuream
 */
public class WXLayout extends LinearLayout{

    private static final String TAG = "WXLayout";
    private int mTouchSlop;
    private boolean mIsBeingDragged = false;
    private float mLastMotionY;
    private float  mInitialMotionY;
    private float resistance = 0.6f;
    private Scroller mScroller;
    private ListView mListView;
    private boolean isMove = false;
    private int duration = 300;
    private ScrollRershListener l;
    private boolean isRersh = false;

    public WXLayout(Context context, AttributeSet attrs) {
        super(context, attrs);
        // TODO Auto-generated constructor stub
        init(context);
    }

    private void init(final Context context){
        ViewConfiguration config = ViewConfiguration.get(context);
        mTouchSlop = config.getScaledTouchSlop();
        DecelerateInterpolator interpolator = new DecelerateInterpolator();
        mScroller = new Scroller(context,interpolator);
        post(new Runnable() {
            @Override
            public void run() {
                // TODO Auto-generated method stub
                mListView = (ListView) WXLayout.this.getChildAt(0);
            }
        });
    }


    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        // TODO Auto-generated method stub
        final  int action = ev.getAction();

        if(action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP){
            mIsBeingDragged = false;
            return false;
        }

        if (action != MotionEvent.ACTION_DOWN && mIsBeingDragged) {
            return true;
        }

        switch(action){
        case MotionEvent.ACTION_DOWN:{
            mLastMotionY = mInitialMotionY = ev.getY();
            mIsBeingDragged = false;
            break;
        }
        case MotionEvent.ACTION_MOVE:{
            final float y = ev.getY(), x = ev.getX();
            final float diff, absDiff;
            diff = y - mLastMotionY;
            absDiff = Math.abs(diff);
            if(absDiff > mTouchSlop){
                if(diff > 1){
                    if(mListView.getFirstVisiblePosition()==0){
                        View view = mListView.getChildAt(0);
                        Rect rect = new Rect ();
                        view.getLocalVisibleRect(rect);
                        if(rect.top == 0){
                            mLastMotionY = y;
                            mIsBeingDragged = true;
                        }
                    }
                }
            }
            break;
        }
        }
        return mIsBeingDragged;
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        // TODO Auto-generated method stub

        //假设碰触到控件的边缘。就不接受这一系列的action了
        if (event.getAction() == MotionEvent.ACTION_DOWN && event.getEdgeFlags() != 0) {
            return false;
        }

        //假设Scroller正在滑动。就不接受这次事件了
        if(isMove){
            return false;
        }

        switch(event.getAction()){
        case MotionEvent.ACTION_DOWN:{
            mLastMotionY = mInitialMotionY = event.getY();
            return true;
        }
        case MotionEvent.ACTION_MOVE:{
            if (mIsBeingDragged) {
                if(l!=null && !isRersh){
                    l.startRersh();
                    isRersh = true;
                }
                mLastMotionY = event.getY();
                float moveY = mLastMotionY - mInitialMotionY;
                pullEvent(moveY);
                return true;
            }
            break;
        }
        case MotionEvent.ACTION_CANCEL:
        case MotionEvent.ACTION_UP:{
            if(mIsBeingDragged){
                mIsBeingDragged = false;
                startMoveAnim(getScrollY(), Math.abs(getScrollY()), duration);
                if(l!= null && isRersh && (event.getY() - mInitialMotionY) > 0){
                    l.endRersh(event.getY() - mInitialMotionY);
                    isRersh = false;
                }
                return true;
            }
            break;
        }
        }
        return super.onTouchEvent(event);
    }

    private void pullEvent(float moveY){
        if(l != null){
            l.Rersh(moveY);
        }
        if(moveY > 0){
            int value = (int) Math.abs(moveY);
            scrollTo(0, - (int)(value*resistance));
        }
    }

    public void startMoveAnim(int startY, int dy, int duration) {
        isMove = true;
        mScroller.startScroll(0, startY, 0, dy, duration);
        invalidate();//通知UI线程的更新
    }

    @Override
    public void computeScroll() {
        //推断是否还在滚动，还在滚动为true
        if (mScroller.computeScrollOffset()) {
            scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
            //更新界面
            postInvalidate();
            isMove = true;
        } else {
            isMove = false;
        }
        super.computeScroll();
    }

    public interface ScrollRershListener{
        void Rersh(float value);
        void startRersh();
        void endRersh(float value);
    }

    public static int dip2px(Context context, float dpValue) {
        final float scale = context.getResources().getDisplayMetrics().density;
        return (int) (dpValue * scale + 0.5f);
    }

    public void setOnScrollRershListener(ScrollRershListener l){
        this.l = l;
    }

}

实例二：仿QQ側滑删除

得多谢夏安明大神的博客，让我有了清晰的思路，对他的代码加以改造。从而完毕了这个实例。

/**
 * 仿QQ側滑删除
 * @author Nipuream
 *
 */
public class SlideListView extends ListView implements OnTouchListener,OnClickListener{

    private static final String TAG = "SlideListView";
    private Context mContext;
    private Scroller mScroller; 
    /**
     * 初始值
     */
    private float initalXvalue,mLastXvalue;
    private float initalYvalue,mLastYvalue;
    /**
     * 确认滑动的最小速度
     */
    private int MIN_VELOCITY = 800;
    /**
     * 速度跟踪器
     */
    private VelocityTracker velocityTracker;
    /**
     * 正在被拖动的view
     */
    private View dragView;
    /**
     * 正在被拖动的position
     */
    private int touchPos;
    /**
     * 默认的最小滑动距离
     */
    private int mTouchSlop;
    /**
     * 滑动时间
     */
    private int DURATION_TIME = 300;
    /**
     * 出现删除button的Item
     */
    private View tempView ;
    /**
     * 出现删除button的position
     */
    private int tempPos ;
    /**
     * 删除button
     */
    private Button deleteBtn;
    /**
     * 移除接口
     */
    private RemoveItemListener l;
    /**
     * 能否够滑动
     */
    private boolean isSlide = false;

    public SlideListView(Context context, AttributeSet attrs) {
        super(context, attrs);
        // TODO Auto-generated constructor stub
        init(context);
    }

    private void init(Context context){
        mContext = context;
        AccelerateInterpolator interpolator = new AccelerateInterpolator();
        mScroller = new Scroller(context,interpolator);
        velocityTracker = VelocityTracker.obtain();
        ViewConfiguration config = ViewConfiguration.get(context);
        mTouchSlop = config.getScaledTouchSlop();
        setOnTouchListener(this);
    }

    /**
     * 捕捉用户究竟拖动了哪个view
     * 拦截事件
     */
    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        // TODO Auto-generated method stub
        final int action = ev.getAction();

        if (action == MotionEvent.ACTION_DOWN && ev.getEdgeFlags() != 0) {
            return false;
        }

        if(action == MotionEvent.ACTION_DOWN){

            mLastYvalue = initalYvalue = ev.getY();
            initalXvalue = mLastXvalue = ev.getX();

            if(!mScroller.isFinished()){
                return super.dispatchTouchEvent(ev);
            }


            touchPos  = pointToPosition((int)mLastXvalue, (int)mLastYvalue);

            if(touchPos == AdapterView.INVALID_POSITION){
                return super.dispatchTouchEvent(ev);
            }

            GetTracker(ev);
            dragView = getChildAt(touchPos - getFirstVisiblePosition());
            isSlide = false;
        }else if(action == MotionEvent.ACTION_MOVE){
            mLastXvalue = ev.getX();
            mLastYvalue = ev.getY();
            if(velocityTracker == null){
                GetTracker(ev);
            }
            velocityTracker.computeCurrentVelocity(1000);
            if(Math.abs(velocityTracker.getXVelocity()) > MIN_VELOCITY 
                    ||Math.abs(mLastXvalue - initalXvalue)> mTouchSlop 
                    && Math.abs(mLastYvalue - initalYvalue)  < mTouchSlop){
                    isSlide = true;
            }
        }else if(action == MotionEvent.ACTION_UP){
            CloseTracker();
        }
        return super.dispatchTouchEvent(ev);
    }


    /**
     * 消费事件
     */
    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        // TODO Auto-generated method stub
        if(isSlide){
            switch(ev.getAction()){
            case MotionEvent.ACTION_MOVE:
            {
                mLastXvalue = ev.getX();
                final float moveX = mLastXvalue - initalXvalue;
                if(Math.abs(moveX) > dip2px(mContext, 20))
                {
                    if(moveX < 0 && Math.abs(moveX)<dip2px(mContext, 100))
                    {
                        if(dragView != null){
                            dragView.scrollTo(Math.abs((int)moveX), 0);
                        }
                        return true;
                    }
                }
            }
            break;
            case MotionEvent.ACTION_UP:
            {
                mLastXvalue = ev.getX();
                float scrollDistance = mLastXvalue - initalXvalue;
                if(scrollDistance < 0){
                    if(Math.abs(scrollDistance) < dip2px(mContext, 50)){
                        //滑动回去
                        if(dragView != null){
                            mScroller.startScroll(dragView.getScrollX(), 0, -dragView.getScrollX(), 0, DURATION_TIME);
                            invalidate();
                        }
                    }else if(Math.abs(scrollDistance) > dip2px(mContext, 50) ){
                        //滑动究竟
                        if(dragView != null){
                            mScroller.startScroll(dragView.getScrollX(), 0, (dip2px(mContext, 100) - Math.abs(dragView.getScrollX())), 0,DURATION_TIME);
                            invalidate();
                            tempView = dragView;
                            tempPos = touchPos;
                            setListener();
                        }
                    }
                }
            }
            break;
            }
        }
        return super.onTouchEvent(ev);
    }

    private void GetTracker(MotionEvent ev){
        if(velocityTracker == null){
            velocityTracker  = VelocityTracker.obtain();
        }
        velocityTracker.addMovement(ev);
    }

    private void CloseTracker() {
        if(velocityTracker != null){
            velocityTracker.recycle();
            velocityTracker.clear();
            velocityTracker = null;
        }
    }

    private void setListener(){
        deleteBtn = (Button) tempView.findViewById(R.id.delete);
        deleteBtn.setOnClickListener(this);
    }

    @Override
    public void computeScroll() {
        // TODO Auto-generated method stub
        if(mScroller.computeScrollOffset()){
            dragView.scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
            postInvalidate();
        }
        super.computeScroll();
    }


    public static int dip2px(Context context, float dpValue) {
        final float scale = context.getResources().getDisplayMetrics().density;
        return (int) (dpValue * scale + 0.5f);
    }


    @Override
    public boolean onTouch(View v, MotionEvent event) {
        // TODO Auto-generated method stub
        if(tempView != null){
            dragView = tempView ;
            mScroller.startScroll(dragView.getScrollX(), 0, - dragView.getScrollX(), 0,DURATION_TIME);
            invalidate();
            deleteBtn.setOnClickListener(null);
            deleteBtn = null;
            tempView = null;
            return true;
        }
        return false;
    }

    @Override
    public void onClick(View v) {
        // TODO Auto-generated method stub
        if(l != null){
            l.remove(tempPos);
            dragView = tempView;
            dragView.scrollTo(0, 0);
            tempView = null;
            invalidate();
        }
    }

    public interface RemoveItemListener{
        void remove(int pos);
    }

    public void setOnRemoveItemListener(RemoveItemListener l){
        this.l = l;
    }

}

我认为这两个样例都非常easy。所以没有什么好解释的，我在以下也会给出下载的地址。假设有什么疑惑的，或者什么地方须要改进的请联系我QQ：571829491。

源代码下载

仿微信刷新
仿QQ側滑删除