3、WorkSpace结构
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：
- 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。
- 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。
WorkSpace滑动事件：
- MotionEvent.ACTION_DOWN：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(TOUCH_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 (MotionEvent.ACTION_DOWN)的时候，就可以开始显示滚动的指示条了(就是Hotseat上PageIndirector显示第几屏的屏点)。
- MotionEvent.ACTION_MOVE：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。
- MotionEvent.ACTION_UP：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 }
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 * {@link #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 * {@link #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 }
http://mobile.51cto.com/hot-316799.htm
时间： 2024-08-23 04:13:53