首先傻蛋先画了个图来再来阐述一下WorkSpace的结构。如下图：
桌面的左右滑动功能主要是在PagedView类中实现的，而WorkSpace是PagedView类的子类，所以会继承PagedView中的方法。当我们的手指点击WorkSpace时，首先就会触发PageView中的onInterceptTouchEvent()方法，会根据相应的条件来判断是否对Touch事件进行拦截，如果onInterceptTouchEvent()方法返回为true，则会对Touch事件进行拦截，PageView类的onTouch方法会进行响应从而得到调用。如果返回false，就分两钟情况：(1)我们是点击在它的子控键上进行滑动时，比如我们是点击在桌面的图标上进行左右滑动的，workspace则会把Touch事件分发给它的子控件。(2)而如果仅仅是点击到桌面的空白出Touch事件就不会发生响应。
在我们手指第一次触摸到屏幕时，首先会对onInterceptTouchEvent中的事件进行判断，如果是按下事件(MotionEvent.ACTION_DOWN), 则会记录按下时的X坐标、Y坐标等等数据，同时改变现在Workspace的状态为滚动状态(OUCH_STATE_SCROLLING)，这时会返回ture，把事件交给onTouchEvent函数来处理，onTouchEvent中同样会对事件类型进行判断，当事件方法为(otionEvent.ACTION_DOWN)的时候，就可以开始显示滚动的指示条了(就是Hotseat上显示第几屏的屏点)。当我们按着屏幕不放进行滑动的时候，又会在onInterceptTouchEvent进行事件拦截，但是现在的事件类型变为了 MotionEvent.ACTION_MOVE，因为是移动的操作，所以会在拦截的时候取消桌面长按的事件的响应，同时转到onTouchEvent中对ACTION_MOVE事件的响应中，判断我们移动了多少距离，使用scrollBy方法来对桌面进行移动，并刷新屏幕。最后我们放开手后会触发onTouchEvent中的MotionEvent.ACTION_UP事件，这时会根据滑动的情况来判断是朝左滑动还是朝右滑动，如果手指只滑动了屏幕宽度的少一半距离，则会弹回原来的页面，滑动多于屏幕宽度的一半则会进行翻页。同时要注意无论在什么情况下触发了WorkSpace滑动的事件，则系统会不断调用computeScroll()方法，我们重写这个方法同时在这个方法中调用刷新界面等操作。
滑动过程中所要注意的主要方法如下，具体见代码注释。
1. //对Touch事件进行拦截主要用于在拦截各种Touch事件时，设置mTouchState的各种状态
2. @Override
3. public boolean onInterceptTouchEvent(MotionEvent ev) {
4./*
5.* This method JUST determines whether we want to intercept the motion.
6.* If we return true, onTouchEvent will be called and we do the actual
7.* scrolling there.
8.* 这个方法仅仅决定了我们是否愿意去对滑动事件进行拦截，如果返回为true,则会调用onTouchEvent我们将会在那里进行事件处理
9.*/
10.//对滑动的速率进行跟踪。
11.
12.acquireVelocityTrackerAndAddMovement(ev);
13.
14.// Skip touch handling if there are no pages to swipe
15.// 如果没有页面，则跳过操作。
16.if (getChildCount() <= 0) return super.onInterceptTouchEvent(ev);
17.
18./*
19.* Shortcut the most recurring case: the user is in the dragging
20.* state and he is moving his finger.We want to intercept this
21.* motion.
22.* shortcut最常见的情况是：用户处于拖动的状态下，同时在移动它的手指，这时候我们需要拦截这个动作。
23.*
24.*/
25.final int action = ev.getAction();
26.//如果是在MOVE的情况下，则进行Touch事件拦截
27.if ((action == MotionEvent.ACTION_MOVE) &&
28.(mTouchState == TOUCH_STATE_SCROLLING)) {
29.return true;
30.}
31.
32.switch (action & MotionEvent.ACTION_MASK) {
33.case MotionEvent.ACTION_MOVE: {
34./*
35.* mIsBeingDragged == false, otherwise the shortcut would have caught it. Check
36.* whether the user has moved far enough from his original down touch.
37.* 如果mIsBeingDragged==false ,否则快捷方式应该捕获到该事件，检查一下用户从它点击的地方位移是否足够
38.*/
39.if (mActivePointerId != INVALID_POINTER) {
40.//根据移动的距离判断是翻页还是移动一段位移，同时设置lastMotionX或者mTouchState这些值。同时取消桌面长按事件。
41.determineScrollingStart(ev);
42.break;
43.}
44.// if mActivePointerId is INVALID_POINTER, then we must have missed an ACTION_DOWN
45.// event. in that case, treat the first occurence of a move event as a ACTION_DOWN
46.// i.e. fall through to the next case (don't break)
47.// (We sometimes miss ACTION_DOWN events in Workspace because it ignores all events
48.// while it's small- this was causing a crash before we checked for INVALID_POINTER)
49.// 如果mActivePointerId 是 INVALID_POINTER，这时候我们应该已经错过了ACTION_DOWN事件。在这种情况下，把
50.// 第一次发生移动的事件当作ACTION——DOWN事件，直接进入下一个情况下。
51.// 我们有时候会错过workspace中的ACTION_DOWN事件，因为在workspace变小的时候会忽略掉所有的事件。
52.}
53.
54.case MotionEvent.ACTION_DOWN: {
55.final float x = ev.getX();
56.final float y = ev.getY();
57.// Remember location of down touch
58.// 记录按下的位置
59.mDownMotionX = x;
60.mLastMotionX = x;
61.mLastMotionY = y;
62.mLastMotionXRemainder = 0;
63.mTotalMotionX = 0;
64.//Return the pointer identifier associated with a particular pointer data index is this event.
65.//The identifier tells you the actual pointer number associated with the data,
66.//accounting for individual pointers going up and down since the start of the current gesture.
67.//返回和这个事件关联的触点数据id，计算单独点的id会上下浮动，因为手势的起始位置挥发声改变。
68.mActivePointerId = ev.getPointerId(0);
69.mAllowLongPress = true;
70.
71./*
72.* If being flinged and user touches the screen, initiate drag;
73.* otherwise don't.mScroller.isFinished should be false when
74.* being flinged.
75.* 如果被拖动同时用户触摸到了屏幕，就开始初始化拖动，否则便不会。
76.* 当拖动完成后mScroller.isFinished就应该设置为false.
77.*
78.*/
79.final int xDist = Math.abs(mScroller.getFinalX() - mScroller.getCurrX());
80.
81.final boolean finishedScrolling = (mScroller.isFinished() || xDist < mTouchSlop);
82.if (finishedScrolling) {
83.//标记为TOUCH_STATE_REST状态
84.mTouchState = TOUCH_STATE_REST;
85.//取消滚动动画
86.mScroller.abortAnimation();
87.} else {
88.//状态为TOUCH_STATE_SCROLLING
89.mTouchState = TOUCH_STATE_SCROLLING;
90.}
91.
92.// check if this can be the beginning of a tap on the side of the pages
93.// to scroll the current page
94.// 检测此事件是不是开始于点击页面的边缘来对当前页面进行滚动。
95.if (mTouchState != TOUCH_STATE_PREV_PAGE && mTouchState != TOUCH_STATE_NEXT_PAGE) {
96.if (getChildCount() > 0) {
97.//根据触点的点位来判断是否点击到上一页，从而更新相应的状态
98.if (hitsPreviousPage(x, y)) {
99.mTouchState = TOUCH_STATE_PREV_PAGE;
100.} else if (hitsNextPage(x, y)) {
101.mTouchState = TOUCH_STATE_NEXT_PAGE;
102.}
103.}
104.}
105.break;
106.}
107.
108.case MotionEvent.ACTION_UP:
109.case MotionEvent.ACTION_CANCEL:
110.//触点不被相应时，所做的动作
111.mTouchState = TOUCH_STATE_REST;
112.mAllowLongPress = false;
113.mActivePointerId = INVALID_POINTER;
114.//释放速率跟踪
115.releaseVelocityTracker();
116.break;
117.
118.case MotionEvent.ACTION_POINTER_UP:
119.onSecondaryPointerUp(ev);
120.releaseVelocityTracker();
121.break;
122.}
123.
124./*
125.* The only time we want to intercept motion events is if we are in the
126.* drag mode.
127.* 我们唯一会去对移动事件进行拦截的情况时我们在拖动模式下
128.*/
129.if(DEBUG) Log.d(TAG, "onInterceptTouchEvent "+(mTouchState != TOUCH_STATE_REST));
130.//只要是mTouchState的状态不为TOUCH_STATE_REST,那么就进行事件拦截
131.return mTouchState != TOUCH_STATE_REST;
132. }
onTouchEvent方法，详细见代码注释：
1. @Override
2. public boolean onTouchEvent(MotionEvent ev) {
3.// Skip touch handling if there are no pages to swipe
4.// 如果没有子页面，就直接跳过
5.if (getChildCount() <= 0) return super.onTouchEvent(ev);
6.
7.acquireVelocityTrackerAndAddMovement(ev);
8.
9.final int action = ev.getAction();
10.
11.switch (action & MotionEvent.ACTION_MASK) {
12.case MotionEvent.ACTION_DOWN:
13./*
14.* If being flinged and user touches, stop the fling. isFinished
15.* will be false if being flinged.
16.* 如果在滑动的过程中下用户又点击桌面，则取消滑动，从而响应当前的点击。
17.* 在滑动的isFinished将返回false.
18.*/
19.if (!mScroller.isFinished()) {
20.mScroller.abortAnimation();
21.}
22.
23.// Remember where the motion event started
24.mDownMotionX = mLastMotionX = ev.getX();
25.mLastMotionXRemainder = 0;
26.mTotalMotionX = 0;
27.mActivePointerId = ev.getPointerId(0);
28.//主要用来显示滚动条，表明要开始滚动了，这里可以进行调整，滚动条时逐渐显示还是立刻显示。
29.if (mTouchState == TOUCH_STATE_SCROLLING) {
30.pageBeginMoving();
31.}
32.break;
33.
34.case MotionEvent.ACTION_MOVE:
35.if (mTouchState == TOUCH_STATE_SCROLLING) {
36.// Scroll to follow the motion event
37.final int pointerIndex = ev.findPointerIndex(mActivePointerId);
38.final float x = ev.getX(pointerIndex);
39.final float deltaX = mLastMotionX + mLastMotionXRemainder - x;
40.//总共移动的距离
41.mTotalMotionX += Math.abs(deltaX);
42.
43.// Only scroll and update mLastMotionX if we have moved some discrete amount.We
44.// keep the remainder because we are actually testing if we've moved from the last
45.// scrolled position (which is discrete).
46.// 如果我们移动了一小段距离，我们则移动和更新mLastMotionX 。我们保存Remainder变量是因为会检测我们
47.
48.//是否是从最后的滚动点位移动的。
49.if (Math.abs(deltaX) >= 1.0f) {
50.mTouchX += deltaX;
51.mSmoothingTime = System.nanoTime() / NANOTIME_DIV;
52.if (!mDeferScrollUpdate) {
53.scrollBy((int) deltaX, 0);
54.if (DEBUG) Log.d(TAG, "onTouchEvent().Scrolling: " + deltaX);
55.} else {
56.invalidate();
57.}
58.mLastMotionX = x;
59.mLastMotionXRemainder = deltaX - (int) deltaX;
60.} else {
61.//Trigger the scrollbars to draw. When invoked this method starts an animation to fade the
62.//scrollbars out after a default delay. If a subclass provides animated scrolling,
63.//the start delay should equal the duration of the scrolling animation.
64.//触发scrollbar进行绘制。 使用这个方法来启动一个动画来使scrollbars经过一段时间淡出。如果子类提供了滚动的动画，则
65.//延迟的时间等于动画滚动的时间。
66.awakenScrollBars();
67.}
68.} else {
69.determineScrollingStart(ev);
70.}
71.break;
72.
73.case MotionEvent.ACTION_UP:
74.if (mTouchState == TOUCH_STATE_SCROLLING) {
75.final int activePointerId = mActivePointerId;
76.final int pointerIndex = ev.findPointerIndex(activePointerId);
77.final float x = ev.getX(pointerIndex);
78.final VelocityTracker velocityTracker = mVelocityTracker;
79.velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
80.int velocityX = (int) velocityTracker.getXVelocity(activePointerId);
81.final int deltaX = (int) (x - mDownMotionX);
82.final int pageWidth = getScaledMeasuredWidth(getPageAt(mCurrentPage));
83.// 屏幕的宽度*0.4f
84.boolean isSignificantMove = Math.abs(deltaX) > pageWidth *
85.SIGNIFICANT_MOVE_THRESHOLD;
86.final int snapVelocity = mSnapVelocity;
87.
88.mTotalMotionX += Math.abs(mLastMotionX + mLastMotionXRemainder - x);
89.
90.boolean isFling = mTotalMotionX > MIN_LENGTH_FOR_FLING &&
91.Math.abs(velocityX) > snapVelocity;
92.
93.// In the case that the page is moved far to one direction and then is flung
94.// in the opposite direction, we use a threshold to determine whether we should
95.// just return to the starting page, or if we should skip one further.
96.// 这钟情况是页面朝一个方向移动了一段距离，然后又弹回去了。我们使用一个阀值来判断是进行翻页还是返回到初始页面
97.boolean returnToOriginalPage = false;
98.if (Math.abs(deltaX) > pageWidth * RETURN_TO_ORIGINAL_PAGE_THRESHOLD &&
99.Math.signum(velocityX) != Math.signum(deltaX) && isFling) {
100.returnToOriginalPage = true;
101.}
102.
103.int finalPage;
104.// We give flings precedence over large moves, which is why we short-circuit our
105.// test for a large move if a fling has been registered. That is, a large
106.// move to the left and fling to the right will register as a fling to the right.
107.//朝右移动
108.if (((isSignificantMove && deltaX > 0 && !isFling) ||
109.(isFling && velocityX > 0)) && mCurrentPage > 0) {
110.finalPage = returnToOriginalPage ? mCurrentPage : mCurrentPage - 1;
111.snapToPageWithVelocity(finalPage, velocityX);
112.//朝左移动
113.} else if (((isSignificantMove && deltaX < 0 && !isFling) ||
114.(isFling && velocityX < 0)) &&
115.mCurrentPage < getChildCount() - 1) {
116.finalPage = returnToOriginalPage ? mCurrentPage : mCurrentPage + 1;
117.snapToPageWithVelocity(finalPage, velocityX);
118.//寻找离屏幕中心最近的页面移动
119.} else {
120.snapToDestination();
121.}
122.}
123.//直接移动到前一页
124.else if (mTouchState == TOUCH_STATE_PREV_PAGE) {
125.// at this point we have not moved beyond the touch slop
126.// (otherwise mTouchState would be TOUCH_STATE_SCROLLING), so
127.// we can just page
128.int nextPage = Math.max(0, mCurrentPage - 1);
129.if (nextPage != mCurrentPage) {
130.snapToPage(nextPage);
131.} else {
132.snapToDestination();
133.}
134.}
135.//直接移动到下一页
136.else if (mTouchState == TOUCH_STATE_NEXT_PAGE) {
137.// at this point we have not moved beyond the touch slop
138.// (otherwise mTouchState would be TOUCH_STATE_SCROLLING), so
139.// we can just page
140.int nextPage = Math.min(getChildCount() - 1, mCurrentPage + 1);
141.if (nextPage != mCurrentPage) {
142.snapToPage(nextPage);
143.} else {
144.snapToDestination();
145.}
146.} else {
147.onUnhandledTap(ev);
148.}
149.mTouchState = TOUCH_STATE_REST;
150.mActivePointerId = INVALID_POINTER;
151.releaseVelocityTracker();
152.break;
153.//对事件不响应
154.case MotionEvent.ACTION_CANCEL:
155.if (mTouchState == TOUCH_STATE_SCROLLING) {
156.snapToDestination();
157.}
158.mTouchState = TOUCH_STATE_REST;
159.mActivePointerId = INVALID_POINTER;
160.releaseVelocityTracker();
161.break;
162.
163.case MotionEvent.ACTION_POINTER_UP:
164.onSecondaryPointerUp(ev);
165.break;
166.}
167.
168.return true;
169. }
最后有个小知识点要搞清楚，不少网友都问到过我。就是scrollTo和scrollBy的区别。我们查看View类的源代码如下所示，mScrollX记录的是当前View针对屏幕坐标在水平方向上的偏移量，而mScrollY则是记录的时当前View针对屏幕在竖值方向上的偏移量。
从以下代码我们可以得知，scrollTo就是把View移动到屏幕的X和Y位置，也就是绝对位置。而scrollBy其实就是调用的 scrollTo，但是参数是当前mScrollX和mScrollY加上X和Y的位置，所以ScrollBy调用的是相对于mScrollX和mScrollY的位置。我们在上面的代码中可以看到当我们手指不放移动屏幕时，就会调用scrollBy来移动一段相对的距离。而当我们手指松开后，会调用 mScroller.startScroll(mUnboundedScrollX, 0, delta, 0, duration); 来产生一段动画来移动到相应的页面，在这个过程中系统回不断调用computeScroll()，我们再使用scrollTo来把View移动到当前Scroller所在的绝对位置。
1. /**
2.* Set the scrolled position of your view. This will cause a call to
3.* www.2cto.com #onScrollChanged(int, int, int, int)} and the view will be
4.* invalidated.
5.* @param x the x position to scroll to
6.* @param y the y position to scroll to
7.*/
8.public void scrollTo(int x, int y) {
9.if (mScrollX != x || mScrollY != y) {
10.int oldX = mScrollX;
11.int oldY = mScrollY;
12.mScrollX = x;
13.mScrollY = y;
14.invalidateParentCaches();
15.onScrollChanged(mScrollX, mScrollY, oldX, oldY);
16.if (!awakenScrollBars()) {
17.invalidate(true);
18.}
19.}
20.}
21./**
22.* Move the scrolled position of your view. This will cause a call to
23.* www.2cto.com #onScrollChanged(int, int, int, int)} and the view will be
24.* invalidated.
25.* @param x the amount of pixels to scroll by horizontally
26.* @param y the amount of pixels to scroll by vertically
27.*/
28.public void scrollBy(int x, int y) {
29.scrollTo(mScrollX + x, mScrollY + y);
30.}
作者：LuoXianXion
