代码讲解Android Scroller、VelocityTracker

在编写自定义滑动控件时常常会用到Android触摸机制和Scroller及VelocityTracker。Android Touch系统简介（二）:实例详解onInterceptTouchEvent与onTouchEvent的调用过程对Android触摸机制需要用到的函数进行了详细的解释，本文主要介绍两个重要的类：Scroller及VelocityTracker。利用上述知识，最后给出了一个自定义滑动控件的demo，该demo类似于ImageGallery。ImageGallery一般是用GridView来实现的，可以左右滑动。本例子实现的控件直接继承一个ViewGroup，对其回调函数如 onTouchEvent、onInterceptTouchEvent、computeScroll等进行重载。弄懂该代码，对Android touch的认识将会更深一层。

VelocityTracker:用于对触摸点的速度跟踪，方便获取触摸点的速度。
用法：一般在onTouchEvent事件中被调用，先在down事件中获取一个VecolityTracker对象，然后在move或up事件中获取速度，调用流程可如下列所示：

VelocityTracker vTracker = null；
@Override
public boolean onTouchEvent(MotionEvent event){
    int action = event.getAction();
    switch(action){
    case MotionEvent.ACTION_DOWN:
        if(vTracker == null){
            vTracker = VelocityTracker.obtain();
        }else{
            vTracker.clear();
        }
        vTracker.addMovement(event);
        break;
    case MotionEvent.ACTION_MOVE:
        vTracker.addMovement(event);
        //设置单位，1000 表示每秒多少像素（pix/second),1代表每微秒多少像素（pix/millisecond)。
        vTracker.computeCurrentVelocity(1000);
        //从左向右划返回正数，从右向左划返回负数
        System.out.println("the x velocity is "+vTracker.getXVelocity());
        //从上往下划返回正数，从下往上划返回负数
        System.out.println("the y velocity is "+vTracker.getYVelocity());
        break;
    case MotionEvent.ACTION_UP:
    case MotionEvent.ACTION_CANCEL:
        vTracker.recycle();
        break;
    }
    return true;
}

Scroller：用于跟踪控件滑动的轨迹，此类不会移动控件，需要你在View的一个回调函数computerScroll()中使用Scroller对象还获取滑动的数据来控制某个View。

  /**
 * Called by a parent to request that a child update its values for mScrollX
 * and mScrollY if necessary. This will typically be done if the child is
 * animating a scroll using a {@link android.widget.Scroller Scroller}
 * object.
 */
public void computeScroll()
{
}

parentView在绘制式，会调用dispatchDraw(Canvas canvas)，该函数会调用ViewGroup中的每个子view的boolean draw(Canvas canvas, ViewGroup parent, long drawingTime)，用户绘制View，此函数在绘制View的过程中会调用computeScroll（）
下面给出一段代码：

@Override
public void computeScroll() {
    // TODO Auto-generated method stub
    Log.e(TAG, "computeScroll");
    if (mScroller.computeScrollOffset()) { //or !mScroller.isFinished()
        Log.e(TAG, mScroller.getCurrX() + "======" + mScroller.getCurrY());
        scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
        Log.e(TAG, "### getleft is " + getLeft() + " ### getRight is " + getRight());
        postInvalidate();
    }
    else
        Log.i(TAG, "have done the scoller -----");
}

这段代码在滑动view之前先调用mScroller.computeScrollOffset()来判断滑动动画是否已结束。computerScrollerOffset()的源代码如下：

/**
 * Call this when you want to know the new location.  If it returns true,
 * the animation is not yet finished.
 */
public boolean computeScrollOffset() {
    if (mFinished) {
        return false;
    }

    //滑动已经持续的时间
    int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);
    //若在规定时间还未用完，则继续设置新的滑动位置mCurrX和mCurry
    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;
}

ViewGroup.computeScroll()被调用时机：
当我们执行ontouch或invalidate(）或postInvalidate()都会导致这个方法的执行。

我们在开发控件时，常会有这样的需求：当单机某个按钮时，某个图片会在规定的时间内滑出窗口，而不是一下子进入窗口。实现这个功能可以使用Scroller来实现。
下面给出一段代码，该代码控制下一个界面在3秒时间内缓慢进入的效果。

public void moveToRightSide(){
    if (curScreen <= 0) {
        return;
    }
    curScreen-- ;
    Log.i(TAG, "----moveToRightSide---- curScreen " + curScreen);
    mScroller.startScroll((curScreen + 1) * getWidth(), 0, -getWidth(), 0, 3000);
    scrollTo(curScreen * getWidth(), 0);
    invalidate();
}

上述代码用到了一个函数：void android.widget.Scroller.startScroll(int startX, int startY, int dx, int dy, int duration)
当startScroll执行过程中即在duration时间内，computeScrollOffset  方法会一直返回true，但当动画执行完成后会返回返加false.
这个函数的源码如下所示，主要用于设置滑动参数

/**
 * Start scrolling by providing a starting point, the distance to travel,
 * and the duration of the scroll.
 *
 * @param startX Starting horizontal scroll offset in pixels. Positive
 *        numbers will scroll the content to the left.
 * @param startY Starting vertical scroll offset in pixels. Positive numbers
 *        will scroll the content up.
 * @param dx Horizontal distance to travel. Positive numbers will scroll the
 *        content to the left.
 * @param dy Vertical distance to travel. Positive numbers will scroll the
 *        content up.
 * @param duration Duration of the scroll in milliseconds.
 */
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;
}

invalidate()会使得视图重绘，导致parent调用了dispatchDraw(Canvas canvas)，然后递归调用child View的draw()函数，该函数又会调用我们定义的computeScroll(), 而这个函数又会调用mScroller.computeScrollOffset()判断动画是否结束，若没结束则继续重绘直到直到startScroll中设置的时间耗尽mScroller.computeScrollOffset()返回false才停下来。

附上完整的实例代码：

自定义Android可滑动控件源码

运行效果图如下，滑动屏幕会显示不同的图片。