本文介绍了Android Touch事件的处理流程，包括ACTION_DOWN、ACTION_MOVE等事件类型，以及dispatchTouchEvent和onTouchEvent在View与ViewGroup中的作用。讲解了如何在View Tree中传递和拦截Touch事件，重点阐述了onInterceptTouchEvent和onTouchEvent的功能及其在处理手势事件中的角色。
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。
/**
* Called to process touch screen events. You can override this to
* intercept all touch screen events before they are dispatched to the
* window. Be sure to call this implementation for touch screen events
* that should be handled normally.
*
* @param ev The touch screen event.
*
* @return boolean Return true if this event was consumed.
*/
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}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：
public boolean dispatchTouchEvent(MotionEvent ev) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
boolean handled = false;
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;
// 处理初始的down事件
if (actionMasked == MotionEvent.ACTION_DOWN) {
//当新开始一个touch事件时，抛弃先前的touch状态
//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
cancelAndClearTouchTargets(ev);
resetTouchState();
}
// 检查是否进行事件拦截
final boolean intercepted;
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {
//回调onInterceptTouchEvent()，返回false表示不拦截touch，否则拦截touch事件。
intercepted = onInterceptTouchEvent(ev);
ev.setAction(action); // restore action in case it was changed
} else {
intercepted = false;
}
} else {
//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
intercepted = true;
}
// 检查cancel事件
final boolean canceled = resetCancelNextUpFlag(this)
|| actionMasked == MotionEvent.ACTION_CANCEL;
// 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
final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
TouchTarget newTouchTarget = null;
boolean alreadyDispatchedToNewTouchTarget = false;
if (!canceled && !intercepted) {
if (actionMasked == MotionEvent.ACTION_DOWN
|| (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
final int actionIndex = ev.getActionIndex(); // always 0 for down
final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
: TouchTarget.ALL_POINTER_IDS;
// Clean up earlier touch targets for this pointer id in case they
// have become out of sync.
removePointersFromTouchTargets(idBitsToAssign);
final int childrenCount = mChildrenCount;
if (newTouchTarget == null && childrenCount != 0) {
final float x = ev.getX(actionIndex);
final float y = ev.getY(actionIndex);
// 找到一个能接受Event的子View，再对子View的View树进行遍历
final View[] children = mChildren;
final boolean customOrder = isChildrenDrawingOrderEnabled();
//判断每个子View是否是TouchTarget，若是则添加到TouchTarget链表中
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = customOrder ?
getChildDrawingOrder(childrenCount, i) : i;
final View child = children[childIndex];
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
continue;
}
newTouchTarget = getTouchTarget(child);
if (newTouchTarget != null) {
// 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
newTouchTarget.pointerIdBits |= idBitsToAssign;
break;
}
resetCancelNextUpFlag(child);
//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
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
// Child wants to receive touch within its bounds.
mLastTouchDownTime = ev.getDownTime();
mLastTouchDownIndex = childIndex;
mLastTouchDownX = ev.getX();
mLastTouchDownY = ev.getY();
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;
}
}
}
if (newTouchTarget == null && mFirstTouchTarget != null) {
// 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
newTouchTarget = mFirstTouchTarget;
while (newTouchTarget.next != null) {
newTouchTarget = newTouchTarget.next;
}
newTouchTarget.pointerIdBits |= idBitsToAssign;
}
}
}
// 传递给touch目标
if (mFirstTouchTarget == null) {
// 若没有Touch目标，则把自己当成一个View，调用
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);
} else {
// Dispatch to touch targets, excluding the new touch target if we already
// dispatched to it. Cancel touch targets if necessary.
TouchTarget predecessor = null;
TouchTarget target = mFirstTouchTarget;
while (target != null) {
final TouchTarget next = target.next;
//若已被处理，则忽略。
if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
handled = true;
} else {
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
//传递给子View处理
if (dispatchTransformedTouchEvent(ev, cancelChild,
target.child, target.pointerIdBits)) {
handled = true;
}
if (cancelChild) {
if (predecessor == null) {
mFirstTouchTarget = next;
} else {
predecessor.next = next;
}
target.recycle();
target = next;
continue;
}
}
predecessor = target;
target = next;
}
}
// 若在触摸点发生了up或cancel，则更新TouchTarget链表
if (canceled
|| actionMasked == MotionEvent.ACTION_UP
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
resetTouchState();
} else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {
final int actionIndex = ev.getActionIndex();
final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
removePointersFromTouchTargets(idBitsToRemove);
}
}
if (!handled && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
}
return handled;
}ViewGroup中将TouchEvent传递给子View的函数为dispatchTransformedTouchEvent()，源代码如下：
/**
* Transforms a motion event into the coordinate space of a particular child view,
* filters out irrelevant pointer ids, and overrides its action if necessary.
* If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.
*/
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
final boolean handled;
// Canceling motions is a special case. We don't need to perform any transformations
// or filtering. The important part is the action, not the contents.
// cancel动作是个特列，无需坐标转换或过滤。
final int oldAction = event.getAction();
if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
event.setAction(MotionEvent.ACTION_CANCEL);
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
handled = child.dispatchTouchEvent(event);
}
event.setAction(oldAction);
return handled;
}
// 计算将被传递的点的数量。
final int oldPointerIdBits = event.getPointerIdBits();
final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;
// Motion事件没有对应点，则丢弃这个Motion
if (newPointerIdBits == 0) {
return false;
}
/*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*/
// If the number of pointers is the same and we don't need to perform any fancy
// irreversible transformations, then we can reuse the motion event for this
// dispatch as long as we are careful to revert any changes we make.
// Otherwise we need to make a copy.
/*该变量用于保存坐标转换后的MoetionEvent*/
final MotionEvent transformedEvent;
if (newPointerIdBits == oldPointerIdBits) {
if (child == null || child.hasIdentityMatrix()) {
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
/*直接对MotionEvent进行坐标变换，将MotionEvent传递下去*/
event.offsetLocation(offsetX, offsetY);
handled = child.dispatchTouchEvent(event);
/*回复MotionEvent*/
event.offsetLocation(-offsetX, -offsetY);
}
return handled;
}
transformedEvent = MotionEvent.obtain(event);
} else {
transformedEvent = event.split(newPointerIdBits);
}
// Perform any necessary transformations and dispatch.
if (child == null) {
/*调用父类即View的dispatchTouchEvent方法，该方法会调用onTouchEvent*/
handled = super.dispatchTouchEvent(transformedEvent);
} else {
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
transformedEvent.offsetLocation(offsetX, offsetY);
if (! child.hasIdentityMatrix()) {
transformedEvent.transform(child.getInverseMatrix());
}
/*传递给子View处理*/
handled = child.dispatchTouchEvent(transformedEvent);
}
// Done.
transformedEvent.recycle();
return handled;
}
/**
* Pass the touch screen motion event down to the target view, or this
* view if it is the target.
*
* @param event The motion event to be dispatched.
* @return True if the event was handled by the view, false otherwise.
*/
public boolean dispatchTouchEvent(MotionEvent event) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
if (onFilterTouchEventForSecurity(event)) {
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
/*先调用listener接口*/
if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
return true;
}
/*若MotionEvent未被消耗，则调用View的onTouchEvent *
* ViewGroup中没有定义onTouchEvent，故做后调用View中的onTouchEvent*/
if (onTouchEvent(event)) {
return true;
}
}
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
return false;
}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.youkuaiyun.com/detail/bigconvience/7376431
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