Android的Touch系统简介（一

Android的Touch系统简介（一
一、Android touch事件的相关概念

用户的Touch事件被包装成MotionEvent

用户当前的touch事件主要类型有：

ACTION_DOWN: 表示用户开始触摸.

ACTION_MOVE: 表示用户在移动(手指或者其他)

ACTION_UP:表示用户抬起了手指

ACTION_CANCEL:表示手势被取消了,一些关于这个事件类型的讨论见:http://stackoverflow.com/questions/11960861/what-causes-a-motionevent-action-cancel-in-android

ACTION_OUTSIDE: 表示用户触碰超出了正常的UI边界.

ACTION_POINTER_DOWN:有一个非主要的手指按下了.

ACTION_POINTER_UP:一个非主要的手指抬起来了

touch事件的元数据包括：

touch的位置

手指的个数

touch事件的时间

一个touch手势被定义为以ACTION_DOWN开始和以 ACTION_UP结束。

二、Touch事件的处理流程

当用户触摸屏幕时，触发Activity调用dispatchTouchEvent

事件对象会按自顶向下的顺序在View Tree中传递

父View（ViewGroups）会调用dispatchTouchEvent将Event传递给子View

Event在任何时候都可能被拦截

事件流会顺着View链递归向下传递直到被消耗

若某个View想处理touch事件，必须先消耗ACTION_DOWN。考虑到效率，后续的事件将不会向下传递。

若某个事件未被消耗，最后会被Activity的onTouchEvent()消耗

若任何View或ViewGroup设置了OnTouchListener，touch事件将被拦截。

Activity.dispathcTouchEvent()的源码分析：
[java] view plain copy
1. /**
2. * Called to process touch screen events. You can override this to
3. * intercept all touch screen events before they are dispatched to the
4. * window. Be sure to call this implementation for touch screen events
5. * that should be handled normally.
6. *
7. * @param ev The touch screen event.
8. *
9. * @return boolean Return true if this event was consumed.
10. */
11. public boolean dispatchTouchEvent(MotionEvent ev) {
12. if (ev.getAction() == MotionEvent.ACTION_DOWN) {
13. onUserInteraction();
14. }
15. if (getWindow().superDispatchTouchEvent(ev)) {
16. return true;
17. }
18. return onTouchEvent(ev);
19. }
由代码可以看出，对于应用层，该函数在touch事件发生后首先被调用。onUserInteraction()是一个空函数，可被用户重载以进行相关处理。Event随后将被传递到关联到root view的window。若子view消耗了该Event，则返回true，否则Event最后被Activity的onTouchEvent()消耗。

ViewGroup.dispatchTouchEvent()的源码分析如下：
[java] view plain copy
1. public boolean dispatchTouchEvent(MotionEvent ev) {
2. if (mInputEventConsistencyVerifier != null) {
3. mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
4. }
5. boolean handled = false;
6. if (onFilterTouchEventForSecurity(ev)) {
7. final int action = ev.getAction();
8. final int actionMasked = action & MotionEvent.ACTION_MASK;
9. // 处理初始的down事件
10. if (actionMasked == MotionEvent.ACTION_DOWN) {
11. //当新开始一个touch事件时，抛弃先前的touch状态
12. //当app切换，发生ANR或一些其他的touch状态发生时，framework会丢弃或取消先前的touch状态
13. cancelAndClearTouchTargets(ev);
14. resetTouchState();
15. }
16. // 检查是否进行事件拦截
17. final boolean intercepted;
18. if (actionMasked == MotionEvent.ACTION_DOWN
19. || mFirstTouchTarget != null) {
20. final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
21. if (!disallowIntercept) {
22. //回调onInterceptTouchEvent()，返回false表示不拦截touch，否则拦截touch事件。
23. intercepted = onInterceptTouchEvent(ev);
24. ev.setAction(action); // restore action in case it was changed
25. } else {
26. intercepted = false;
27. }
28. } else {
29. //没有touch事件的传递对象，同时touch动作不是初始动作down，所以ViewGroup继续拦截事件
30. intercepted = true;
31. }
32. // 检查cancel事件
33. final boolean canceled = resetCancelNextUpFlag(this)
34. || actionMasked == MotionEvent.ACTION_CANCEL;
35. // 如果有第二个手指touch，更新touch目标列表。touch目标列表是一个View数组
36. final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
37. TouchTarget newTouchTarget = null;
38. boolean alreadyDispatchedToNewTouchTarget = false;
39. if (!canceled && !intercepted) {
40. if (actionMasked == MotionEvent.ACTION_DOWN
41. || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
42. || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
43. final int actionIndex = ev.getActionIndex(); // always 0 for down
44. final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
45. : TouchTarget.ALL_POINTER_IDS;
46. // Clean up earlier touch targets for this pointer id in case they
47. // have become out of sync.
48. removePointersFromTouchTargets(idBitsToAssign);
49. final int childrenCount = mChildrenCount;
50. if (newTouchTarget == null && childrenCount != 0) {
51. final float x = ev.getX(actionIndex);
52. final float y = ev.getY(actionIndex);
53. // 找到一个能接受Event的子View，再对子View的View树进行遍历
54. final View[] children = mChildren;
55. final boolean customOrder = isChildrenDrawingOrderEnabled();
56. //判断每个子View是否是TouchTarget，若是则添加到TouchTarget链表中
57. for (int i = childrenCount - 1; i >= 0; i--) {
58. final int childIndex = customOrder ?
59. getChildDrawingOrder(childrenCount, i) : i;
60. final View child = children[childIndex];
61. if (!canViewReceivePointerEvents(child)
62. || !isTransformedTouchPointInView(x, y, child, null)) {
63. continue;
64. }
65. newTouchTarget = getTouchTarget(child);
66. if (newTouchTarget != null) {
67. // 若子View处于touch目标中，同时已经接收了touch事件，则为器增加新的touch点
68. newTouchTarget.pointerIdBits |= idBitsToAssign;
69. break;
70. }
71. resetCancelNextUpFlag(child);
72. //把MotionEvent的点坐标转换到子View的坐标系中，为ViewGroup创建一个新TouchTarget，TouchTarget包含了子View
73. if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
74. // Child wants to receive touch within its bounds.
75. mLastTouchDownTime = ev.getDownTime();
76. mLastTouchDownIndex = childIndex;
77. mLastTouchDownX = ev.getX();
78. mLastTouchDownY = ev.getY();
79. newTouchTarget = addTouchTarget(child, idBitsToAssign);
80. alreadyDispatchedToNewTouchTarget = true;
81. break;
82. }
83. }
84. }
85. if (newTouchTarget == null && mFirstTouchTarget != null) {
86. // 没有发现接收event的子View，把Touch点赋给最早添加到TouchTarget链中的对象
87. newTouchTarget = mFirstTouchTarget;
88. while (newTouchTarget.next != null) {
89. newTouchTarget = newTouchTarget.next;
90. }
91. newTouchTarget.pointerIdBits |= idBitsToAssign;
92. }
93. }
94. }
95. // 传递给touch目标
96. if (mFirstTouchTarget == null) {
97. // 若没有Touch目标，则把自己当成一个View，调用
98. handled = dispatchTransformedTouchEvent(ev, canceled, null,
99. TouchTarget.ALL_POINTER_IDS);
100. } else {
101. // Dispatch to touch targets, excluding the new touch target if we already
102. // dispatched to it. Cancel touch targets if necessary.
103. TouchTarget predecessor = null;
104. TouchTarget target = mFirstTouchTarget;
105. while (target != null) {
106. final TouchTarget next = target.next;
107. //若已被处理，则忽略。
108. if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
109. handled = true;
110. } else {
111. final boolean cancelChild = resetCancelNextUpFlag(target.child)
112. || intercepted;
113. //传递给子View处理
114. if (dispatchTransformedTouchEvent(ev, cancelChild,
115. target.child, target.pointerIdBits)) {
116. handled = true;
117. }
118. if (cancelChild) {
119. if (predecessor == null) {
120. mFirstTouchTarget = next;
121. } else {
122. predecessor.next = next;
123. }
124. target.recycle();
125. target = next;
126. continue;
127. }
128. }
129. predecessor = target;
130. target = next;
131. }
132. }
133. // 若在触摸点发生了up或cancel，则更新TouchTarget链表
134. if (canceled
135. || actionMasked == MotionEvent.ACTION_UP
136. || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
137. resetTouchState();
138. } else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {
139. final int actionIndex = ev.getActionIndex();
140. final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
141. removePointersFromTouchTargets(idBitsToRemove);
142. }
143. }
144. if (!handled && mInputEventConsistencyVerifier != null) {
145. mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
146. }
147. return handled;
148. }
ViewGroup中将TouchEvent传递给子View的函数为dispatchTransformedTouchEvent()，源代码如下：
[java] view plain copy
1. /**
2. * Transforms a motion event into the coordinate space of a particular child view,
3. * filters out irrelevant pointer ids, and overrides its action if necessary.
4. * If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.
5. */
6. private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
7. View child, int desiredPointerIdBits) {
8. final boolean handled;
9. // Canceling motions is a special case. We don't need to perform any transformations
10. // or filtering. The important part is the action, not the contents.
11. // cancel动作是个特列，无需坐标转换或过滤。
12. final int oldAction = event.getAction();
13. if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
14. event.setAction(MotionEvent.ACTION_CANCEL);
15. if (child == null) {
16. handled = super.dispatchTouchEvent(event);
17. } else {
18. handled = child.dispatchTouchEvent(event);
19. }
20. event.setAction(oldAction);
21. return handled;
22. }
23. // 计算将被传递的点的数量。
24. final int oldPointerIdBits = event.getPointerIdBits();
25. final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;
27. // Motion事件没有对应点，则丢弃这个Motion
28. if (newPointerIdBits == 0) {
29. return false;
30. }
32. /*若点的数量一致则无需进行不相关的点坐标转换，调用子View的dispatchTouchEvent*/
33. // If the number of pointers is the same and we don't need to perform any fancy
34. // irreversible transformations, then we can reuse the motion event for this
35. // dispatch as long as we are careful to revert any changes we make.
36. // Otherwise we need to make a copy.
37. /*该变量用于保存坐标转换后的MoetionEvent*/
38. final MotionEvent transformedEvent;
39. if (newPointerIdBits == oldPointerIdBits) {
40. if (child == null || child.hasIdentityMatrix()) {
41. if (child == null) {
42. handled = super.dispatchTouchEvent(event);
43. } else {
44. final float offsetX = mScrollX - child.mLeft;
45. final float offsetY = mScrollY - child.mTop;
46. /*直接对MotionEvent进行坐标变换，将MotionEvent传递下去*/
47. event.offsetLocation(offsetX, offsetY);
48. handled = child.dispatchTouchEvent(event);
49. /*回复MotionEvent*/
50. event.offsetLocation(-offsetX, -offsetY);
51. }
52. return handled;
53. }
54. transformedEvent = MotionEvent.obtain(event);
55. } else {
56. transformedEvent = event.split(newPointerIdBits);
57. }
58. // Perform any necessary transformations and dispatch.
59. if (child == null) {
60. /*调用父类即View的dispatchTouchEvent方法，该方法会调用onTouchEvent*/
61. handled = super.dispatchTouchEvent(transformedEvent);
62. } else {
63. final float offsetX = mScrollX - child.mLeft;
64. final float offsetY = mScrollY - child.mTop;
65. transformedEvent.offsetLocation(offsetX, offsetY);
66. if (! child.hasIdentityMatrix()) {
67. transformedEvent.transform(child.getInverseMatrix());
68. }
69. /*传递给子View处理*/
70. handled = child.dispatchTouchEvent(transformedEvent);
71. }
72. // Done.
73. transformedEvent.recycle();
74. return handled;
75. }
View对象的dispatchTouchEvent代码如下：
[java] view plain copy
1. /**
2. * Pass the touch screen motion event down to the target view, or this
3. * view if it is the target.
4. *
5. * @param event The motion event to be dispatched.
6. * @return True if the event was handled by the view, false otherwise.
7. */
8. public boolean dispatchTouchEvent(MotionEvent event) {
9. if (mInputEventConsistencyVerifier != null) {
10. mInputEventConsistencyVerifier.onTouchEvent(event, 0);
11. }
12. if (onFilterTouchEventForSecurity(event)) {
13. //noinspection SimplifiableIfStatement
14. ListenerInfo li = mListenerInfo;
15. /*先调用listener接口*/
16. if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
17. && li.mOnTouchListener.onTouch(this, event)) {
18. return true;
19. }
20. /*若MotionEvent未被消耗，则调用View的onTouchEvent *
21. * ViewGroup中没有定义onTouchEvent，故做后调用View中的onTouchEvent*/
22. if (onTouchEvent(event)) {
23. return true;
24. }
25. }
26. if (mInputEventConsistencyVerifier != null) {
27. mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
28. }
29. return false;
30. }
小结：

onInterceptTouchEvent:

onInterceptTouchEvent是在ViewGroup里面定义的，被ViewGroup.dispatchTouchEvent()调用，用于拦截所有的touch事件。默认返回false，表示不拦截touch事件，ViewGroup.dispatchTouchEvent()会调用子View的dispatchTouchEvent,将touch事件传递到子View中。若子View的dispatchTouchEvent 返回false，则ViewGroup的onTouchEvent会被调用；若子View的dispatchTouchEvent 返回true，表示消耗了手势事件，ViewGroup的onTouchEvent则不会被调用。若ViewGroup.onInterceptTouchEvent()返回true，表示Touch事件被拦截，ViewGroup. dispatchTransformedTouchEvent()函数将被调用，该函数会调用super.dispatchTouchEvent(event)，即View的dispatchEvent()，该函数首先会调用View.OnTouchListener.onTouch().若listener未消耗Touch事件，则会调用View.onTouchEvent().

onTouchEvent:

view中定义的方法onTouchEvent默认返回true,表示消耗了一个touch事件，ViewGroup中定义的onTouchEvent默认返回false，表示不处理Touch手势事件。手势事件类型包括ACTION_DOWN,ACTION_MOVE,ACTION_UP,ACTION_CANCEL等事件。

本节及后续都是参考了一篇国外讲义，下载地址：http://download.csdn.net/detail/bigconvience/7376431
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原文地址：https://www.cnblogs.com/Free-Thinker/p/5504175.html