代码解说Android Scroller、VelocityTracker

在编写自己定义滑动控件时经常会用到Android触摸机制和Scroller及VelocityTracker。Android Touch系统简单介绍（二）:实例具体解释onInterceptTouchEvent与onTouchEvent的调用过程对Android触摸机制须要用到的函数进行了具体的解释。本文主要介绍两个重要的类：Scroller及VelocityTracker。利用上述知识，最后给出了一个自己定义滑动控件的demo，该demo类似于ImageGallery。

ImageGallery通常是用GridView来实现的，能够左右滑动。本样例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。
使用方法：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，调用流程可例如以下列所看到的：

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，须要你在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中的每一个子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（）
以下给出一段代码：

@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()来推断滑动动画是否已结束。computerScrollerOffset()的源码例如以下：

/**
 * 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：
当我们运行ontouch或invalidate(）或postInvalidate()都会导致这种方法的运行。

我们在开发控件时。常会有这种需求：当单机某个button时。某个图片会在规定的时间内滑出窗体。而不是一下子进入窗体。实现这个功能能够使用Scroller来实现。
以下给出一段代码，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运行过程中即在duration时间内，computeScrollOffset  方法会一直返回true，但当动画运行完毕后会返回返加false.
这个函数的源代码例如以下所看到的，主要用于设置滑动參数

/**
 * 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()推断动画是否结束。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：

自己定义Android可滑动控件源代码

执行效果图例如以下，滑动屏幕会显示不同的图片。