在編寫自定義滑動控件時常常會用到Android觸摸機制和Scroller及VelocityTracker。Android Touch系統簡介（二）:實例詳解onInterceptTouchEvent與onTouchEvent的調用過程對Android觸摸機制需要用到的函數進行了詳細的解釋，本文主要介紹兩個重要的類：Scroller及VelocityTracker。利用上述知識，最後給出了一個自定義滑動控件的demo，該demo類似於ImageGallery。ImageGallery一般是用GridView來實現的，可以左右滑動。本例子實現的控件直接繼承一個ViewGroup，對其回調函數如 onTouchEvent、onInterceptTouchEvent、computeScroll等進行重載。弄懂該代碼，對Android touch的認識將會更深一層。

VelocityTracker:用於對觸摸點的速度跟蹤，方便獲取觸摸點的速度。
用法：一般在onTouchEvent事件中被調用，先在down事件中獲取一個VecolityTracker對象，然後在move或up事件中獲取速度，調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);  
		//設置單位，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);  
		//從左向右劃返回正數，從右向左劃返回負數
		System.out.println("the x velocity is "+vTracker.getXVelocity());  
		//從上往下劃返回正數，從下往上劃返回負數
		System.out.println("the y velocity is "+vTracker.getYVelocity());  
		break;  
	case MotionEvent.ACTION_UP:  
	case MotionEvent.ACTION_CANCEL:  
		vTracker.recycle();  
		break;  
	}  
	return true;  
}  

Scroller：用於跟蹤控件滑動的軌跡，此類不會移動控件，需要你在View的一個回調函數computerScroll()中使用Scroller對象還獲取滑動的數據來控制某個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在繪制式，會調用dispatchDraw(Canvas canvas)，該函數會調用ViewGroup中的每個子view的boolean draw(Canvas canvas, ViewGroup parent, long drawingTime)，用戶繪制View，此函數在繪制View的過程中會調用computeScroll（）
下面給出一段代碼：

@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 -----");
}

這段代碼在滑動view之前先調用mScroller.computeScrollOffset()來判斷滑動動畫是否已結束。computerScrollerOffset()的源代碼如下：

/**
 * 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;
	}
	
	//滑動已經持續的時間
	int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);
	//若在規定時間還未用完，則繼續設置新的滑動位置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()被調用時機：
當我們執行ontouch或invalidate(）或postInvalidate()都會導致這個方法的執行。

我們在開發控件時，常會有這樣的需求：當單機某個按鈕時，某個圖片會在規定的時間內滑出窗口，而不是一下子進入窗口。實現這個功能可以使用Scroller來實現。
下面給出一段代碼，該代碼控制下一個界面在3秒時間內緩慢進入的效果。

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();
}

上述代碼用到了一個函數：void android.widget.Scroller.startScroll(int startX, int startY, int dx, int dy, int duration)
當startScroll執行過程中即在duration時間內，computeScrollOffset 方法會一直返回true，但當動畫執行完成後會返回返加false.
這個函數的源碼如下所示，主要用於設置滑動參數

/**
 * 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()會使得視圖重繪，導致parent調用了dispatchDraw(Canvas canvas)，然後遞歸調用child View的draw()函數，該函數又會調用我們定義的computeScroll(), 而這個函數又會調用mScroller.computeScrollOffset()判斷動畫是否結束，若沒結束則繼續重繪直到直到startScroll中設置的時間耗盡mScroller.computeScrollOffset()返回false才停下來。

附上完整的實例代碼：

自定義Android可滑動控件源碼

運行效果圖如下，滑動屏幕會顯示不同的圖片。