android 触摸事件分析
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背景知识：
- 触摸屏可以有多个触控点
- android中管理触控点通过一个数组来管理，涉及到index和id两个变量，
index表示在数组中的下标，id表示这个触控点(pointer)的id,pointer对应的index子不同的MotionEvent中是可以变化的，
但是它的id是不会变的。
在不同的控件类型上，touch事件的传递方式会不一样。
- 普通View的touch事件处理过程：
1.view消耗touch事件的地方要有两个一个是OnTouchListener,另一个则是onTouchEvent,OnTouchListener 比onTouchEvent优先级高
2.只要view是CLICKABLE或则LONG_CLICKABLE 或者CONTEXT_CLICKABLE ,无论是否 disable,它在onTouchEvent阶段默认都会消耗这个事件
- ViewGroup的touch事件处理思路：
ViewGroup的touch事件下面几个方面是要考虑的
1.一般的事件序列的过程是ACTION_DOWN,ACTION_MOVE.................,最后ACTION_UP
2.确定是否是ACTION_DOWN事件
3.确定是否intercept
4.确定是否要cancel
源代码代码中mFirstTouchTarget 变量很关键，它保存着处理当前事件序列的view,在ACTION_DOWN的时候确定将新的view加入到这个列表中来，
当在ACTION_MOVE过程中确定当前ViewGroup需要intercept了，那么应该清理掉mFirstTouchTarget中所有的view，因为这个事件序列之后的的event都不会派发给这些view了。
总之,ViewGroup处理touch时间的需要遵守下面几条
1.事件是从父view传递到子view的，如果子view不处理，那么父view再来处理
2.如果ViewGroup的某个子view处理了ACTION_DOWN或则 ACTION_POINTER_DOWN（View.OnTouchEvent默认是返回ture的，也就是处理该事件序列的）, 那么这个事件序列后续的事件都传给这个view处理，除非View.dispatchTouchEvent返回false或者 当前ViewGroup确定需要intercept了。
3.ViewGroup的onInterceptTouchEvent 这个函数有机会在将事件传递给子view之前获得调用的机会，以确定是否需要intercept
下面是对ViewGroup.dispatchTouchEvent代码一段分析
public boolean dispatchTouchEvent(MotionEvent ev) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
// If the event targets the accessibility focused view and this is it, start
// normal event dispatch. Maybe a descendant is what will handle the click.
//ev 是否设置 FLAG_TARGET_ACCESSIBILITY_FOCUS这个
if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
ev.setTargetAccessibilityFocus(false);
}
boolean handled = false;
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;
// Handle an initial down.
if (actionMasked == MotionEvent.ACTION_DOWN) {
// Throw away all previous state when starting a new touch gesture.
// The framework may have dropped the up or cancel event for the previous gesture
// due to an app switch, ANR, or some other state change.
/*
* ACTION_DOWN重新设置状态，给mFirstTouchTarget中的target发生cancel事件，
* 清空mFirstTouchTarget列表
* 清空mGroupFlags的FLAG_DISALLOW_INTERCEPT标志
* mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
*/
cancelAndClearTouchTargets(ev);
resetTouchState();
}
// Check for interception.
final boolean intercepted;
/*在 ACTION_DOWN 和 mFirstTouchTarget不为空的时候就会去intercept
* 在ACTION_MOVE事件的时候，虽然有子view处理事件序列了，
* 但是viewgroup还是有机会插手事件序列
* */
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
//是否不允许intercept
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {
//允许intercept
intercepted = onInterceptTouchEvent(ev);
ev.setAction(action); // restore action in case it was changed
} else {
intercepted = false;
}
} else {
//这种情况比较少
// There are no touch targets and this action is not an initial down
// so this view group continues to intercept touches.
intercepted = true;
}
// If intercepted, start normal event dispatch. Also if there is already
// a view that is handling the gesture, do normal event dispatch.
if (intercepted || mFirstTouchTarget != null) {
//取消FLAG_TARGET_ACCESSIBILITY_FOCUS
ev.setTargetAccessibilityFocus(false);
}
// Check for cancelation.
//取消PFLAG_CANCEL_NEXT_UP_EVENT标记，返回之前是否已经 PFLAG_CANCEL_NEXT_UP_EVENT
final boolean canceled = resetCancelNextUpFlag(this)
|| actionMasked == MotionEvent.ACTION_CANCEL;
// Update list of touch targets for pointer down, if needed.
final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
TouchTarget newTouchTarget = null;
boolean alreadyDispatchedToNewTouchTarget = false;
//canceled==false && interepted==false
if (!canceled && !intercepted) {
//没有intercepted也没有cancel掉,转发到子view中去
// If the event is targeting accessiiblity focus we give it to the
// view that has accessibility focus and if it does not handle it
// we clear the flag and dispatch the event to all children as usual.
// We are looking up the accessibility focused host to avoid keeping
// state since these events are very rare.
View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
? findChildWithAccessibilityFocus() : null;
//ACTION_POINTER_DOWN，会有多个触控点
if (actionMasked == MotionEvent.ACTION_DOWN
|| (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
//获取触发这个事件的索引
final int actionIndex = ev.getActionIndex(); // always 0 for down
//根据索引查找到在ev中id
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);
// Find a child that can receive the event.
// Scan children from front to back.
//根据Z-order得到拍好序的children view list
final ArrayList<View> preorderedList = buildOrderedChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
for (int i = childrenCount - 1; i >= 0; i--) {
//child 在绘制次序中的次序
final int childIndex = customOrder
? getChildDrawingOrder(childrenCount, i) : i;
final View child = (preorderedList == null)
? children[childIndex] : preorderedList.get(childIndex);
// If there is a view that has accessibility focus we want it
// to get the event first and if not handled we will perform a
// normal dispatch. We may do a double iteration but this is
// safer given the timeframe.
if (childWithAccessibilityFocus != null) {
if (childWithAccessibilityFocus != child) {
continue;
}
childWithAccessibilityFocus = null;
i = childrenCount - 1;
}
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
//child是否已经在mFirstTouchTarget列表中，多点触控时，几个点都在一个view上
newTouchTarget = getTouchTarget(child);
if (newTouchTarget != null) {
// Child is already receiving touch within its bounds.
// Give it the new pointer in addition to the ones it is handling.
newTouchTarget.pointerIdBits |= idBitsToAssign;
break;
}
resetCancelNextUpFlag(child);
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
//将事件投递到子view成功
// Child wants to receive touch within its bounds.
mLastTouchDownTime = ev.getDownTime();
if (preorderedList != null) {
// childIndex points into presorted list, find original index
for (int j = 0; j < childrenCount; j++) {
if (children[childIndex] == mChildren[j]) {
mLastTouchDownIndex = j;
break;
}
}
} else {
mLastTouchDownIndex = childIndex;
}
mLastTouchDownX = ev.getX();
mLastTouchDownY = ev.getY();
//加入到mFirstTouchTarget队头
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;
}
// The accessibility focus didn‘t handle the event, so clear
// the flag and do a normal dispatch to all children.
ev.setTargetAccessibilityFocus(false);
}
if (preorderedList != null) preorderedList.clear();
}
//如果都没有没找到，那么用mFirstTouchTarget队列中尾巴上的那个target来处理
if (newTouchTarget == null && mFirstTouchTarget != null) {
// Did not find a child to receive the event.
// Assign the pointer to the least recently added target.
newTouchTarget = mFirstTouchTarget;
while (newTouchTarget.next != null) {
newTouchTarget = newTouchTarget.next;
}
newTouchTarget.pointerIdBits |= idBitsToAssign;
}
}
}
// Dispatch to touch targets.
//如果是ACTION_MOVE事件，上面的这个if 是没有不会执行的，直接到这个if了
if (mFirstTouchTarget == null) {
/*这种状况会是 ACTION_MOVE事件,并且这个时候是 intercepted==true的状况
*这个会转发到View.dispatchTouchEvent() ,这View类中会调用OnTouchListener,和onTouchEvent
*/
// No touch targets so treat this as an ordinary view.
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;
if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
//新加入的 target，跳过
handled = true;
} else {
/*
* 如果之前设置calcel标记或者intercepted了
* 如果当前viewgroup 确定intercepted了，
* 那么给mFirstTouchTarget中的所有view发送ACTION_MOVE事件，
* 并且移除mFirstTouchTarget中所有的target
*
* 如果没有intercepted那么照原事件发送
*
*/
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
//如果intecepted了，那么会给之前的target发送一个cancel事件
if (dispatchTransformedTouchEvent(ev, cancelChild,
target.child, target.pointerIdBits)) {
handled = true;
}
//移除这个target
if (cancelChild) {
if (predecessor == null) {
//第一个节点
mFirstTouchTarget = next;
} else {
predecessor.next = next;
}
target.recycle();
target = next;
//这个continue很关键，它会保持predecessor==null的状况，最后会出现mFirstTouchTarget==null的状况
continue;
}
}
predecessor = target;
target = next;
}
}
// Update list of touch targets for pointer up or cancel, if needed.
if (canceled
|| actionMasked == MotionEvent.ACTION_UP
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
//取消或者所有手指都起开
resetTouchState();
} else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {
//值起开一个手指
final int actionIndex = ev.getActionIndex();
final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
removePointersFromTouchTargets(idBitsToRemove);
}
}
if (!handled && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
}
return handled;
}
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