ps：本文系为转载文章，阅读原文可读性会更好，文章末尾有原文链接
ps：源码是基于 android api 27 来分析的
前面写了好几篇 View 事件的分发，但更多的偏向于总结结论并写 demo 来演示验证结论；这一篇我们来详细的分析 View 事件分发中的 ViewGroup 中的 dispatchTouchEvent 方法源码。
首先我们先找到 View 事件分发的源头，那就是 Activity 中的 dispatchTouchEvent方法：
public boolean dispatchTouchEvent(MotionEvent ev) {
//1、
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
//2、
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
//3、
return onTouchEvent(ev);}
注释1 是 down 事件，onUserInteraction 是空的方法，不必理会，注释2 如果 if 语句的值是 false，那么就执行注释3 的返回语句，也就是 Activity 的 onTouchEvent 方法，即 Activity 处理触摸事件；那么注释2 是什么呢？注释2 是事件的分发，它首先分发给 Window，getWindow 方法就是获取一个 Window，Window 的实现类是 PhoneWindow，所以我们看看 PhoneWindow 的 superDispatchTouchEvent 方法；
@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
//4、
return mDecor.superDispatchTouchEvent(event);}
看注释4，mDecor 是一个 DecorView 类对象，父类是 FrameLayout，FrameLayout 继承于 ViewGroup；DecorView 是我们经常用到 setContentView 方法时的底层根布局，我们往下查看 DecorView 的 superDispatchTouchEvent 方法；
public boolean superDispatchTouchEvent(MotionEvent event) {
//5、
return super.dispatchTouchEvent(event);}
我们看注释5，最终调用了父类的 dispatchTouchEvent 方法，这时候终于见到我们的主角 ViewGroup 了，ViewGroup 的 dispatchTouchEvent 方法如下所示：
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
//6、
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.
if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
ev.setTargetAccessibilityFocus(false);
}
//7、
boolean handled = false;
//8、
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
//9、
final int actionMasked = action & MotionEvent.ACTION_MASK;
//10、
// 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.
cancelAndClearTouchTargets(ev);
resetTouchState();
}
//11、
// Check for interception.
final boolean intercepted;
//12、
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
//13、
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {
//14、
intercepted = onInterceptTouchEvent(ev);
//15、
ev.setAction(action); // restore action in case it was changed
} else {
intercepted = false;
}
//16、
} else {
// 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) {
ev.setTargetAccessibilityFocus(false);
}
//17、
// Check for cancelation.
final boolean canceled = resetCancelNextUpFlag(this)
|| actionMasked == MotionEvent.ACTION_CANCEL;
//18、
// Update list of touch targets for pointer down, if needed.
final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
//19、
TouchTarget newTouchTarget = null;
//20、
boolean alreadyDispatchedToNewTouchTarget = false;
//21、
if (!canceled && !intercepted) {
// 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;
//21A、
if (actionMasked == MotionEvent.ACTION_DOWN
|| (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
//22、
final int actionIndex = ev.getActionIndex(); // always 0 for down
//23、
final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
: TouchTarget.ALL_POINTER_IDS;
//24、
// Clean up earlier touch targets for this pointer id in case they
// have become out of sync.
removePointersFromTouchTargets(idBitsToAssign);
final int childrenCount = mChildrenCount;
//25、
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.
final ArrayList preorderedList = buildTouchDispatchChildList();
//26、
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
//27、
final View[] children = mChildren;
for (int i = childrenCount - 1; i >= 0; i--) {
//28、
final int childIndex = getAndVerifyPreorderedIndex(
childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(
preorderedList, children, 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;
}
//29、
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
//30、
newTouchTarget = getTouchTarget(child);
//31、
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;
}
//32、
resetCancelNextUpFlag(child);
//33、
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
// 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();
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();
}
//34、
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;
}
}
}
//35、
// Dispatch to touch targets.
if (mFirstTouchTarget == null) {
//36、
// No touch targets so treat this as an ordinary view.
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);
//37、
} else {
//38、
// 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) {
handled = true;
} else {
//39、
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
//40、
if (dispatchTransformedTouchEvent(ev, cancelChild,
target.child, target.pointerIdBits)) {
handled = true;
}
//41、
if (cancelChild) {
//42、
if (predecessor == null) {
mFirstTouchTarget = next;
//43、
} else {
predecessor.next = next;
}
target.recycle();
target = next;
continue;
}
}
//44、
predecessor = target;
target = next;
}
}
//45、
// Update list of touch targets for pointer up or cancel, if needed.
if (canceled
|| actionMasked == MotionEvent.ACTION_UP
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
resetTouchState();
//46、
} else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {
final int actionIndex = ev.getActionIndex();
final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
removePointersFromTouchTargets(idBitsToRemove);
}
}
//47、
if (!handled && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
}
return handled; }
为了方便阅读，我们把代码一段一段的拿出来分析，有些不重要的代码我会进行省略。
//6、
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
......
//7、
boolean handled = false;
注释6 它表示的是验证事件是否连续；注释7 表示的是这个变量用来记录这个事件是否被处理过。
//8、
if (onFilterTouchEventForSecurity(ev)) {
......
}
注释8 这里是为了过滤掉一些不合理的事件，比如说当前的 View 的窗口被遮挡了，比如再详细一点弹出一个 Dialog；如果没有被挡住，那么就执行 if 括号里面的语句。
//9、
final int actionMasked = action & MotionEvent.ACTION_MASK;
//10、
// 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.
cancelAndClearTouchTargets(ev);
resetTouchState();
}
注释9 表示重置前面为0 ，只留下后八位，用于判断相等时候，可以提高性能；注释10 表示判断是不是 down 事件，如果是的话，就要做初始化操作，cancelAndClearTouchTargets 方法会调用 resetCancelNextUpFlag 方法，而 resetCancelNextUpFlag 方法是为了清空 mPrivateFlags 的 PFLAG_CANCEL_NEXT_UP_EVEN 标记，作用是将下一个事件变成 Cancel；resetTouchState 方法清空了 mGroupFlags 的 FLAG_DISALLOW_INTERCEPT 标记,如果设置了FLAG_DISALLOW_INTERCEPT，ViewGroup 对触摸事件进行拦截；resetTouchState 方法调用了 clearTouchTargets 方法，目的是为了清空 mFirstTouchTarget 链表，并设置 mFirstTouchTarget 为 null，mFirstTouchTarget 是"接受触摸事件的 View "所组成的单链表。
//11、
// Check for interception.
final boolean intercepted;
//12、
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
//13、
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {
//14、
intercepted = onInterceptTouchEvent(ev);
//15、
ev.setAction(action); // restore action in case it was changed
} else {
intercepted = false;
}
//16、
} else {
// There are no touch targets and this action is not an initial down
// so this view group continues to intercept touches.
intercepted = true;
}
注释11 表示检查是否拦截；注释12 表示如果为 down 事件，或者 mFirstTouchTarget 不为null（那么事件直接给到自己），先不拦截；注释13 表示查看是否设置了禁止拦截的标记，即子元素是否调用了 getParent().requestDisallowInterceptTouchEvent(true) 语句；注释14 表示 ViewGroup 的 onInterceptTouchEvent 方法是否要拦截事件，它默认不执行拦截，除非它的子类（注意是子类不是子元素）重写它的 onInterceptTouchEvent 方法并返回 true；注释15 表示重新恢复 Action，以免 Action 在上面的步骤被改变了；注释16 表示事件已经初始化过了，而且没有子 View 被分配处理，这就说明了 ViewGroup 已经拦截了过这个事件了，那么 ViewGroup 第二次就不会是否需要拦截了的。
//17、
// Check for cancelation.
final boolean canceled = resetCancelNextUpFlag(this)
|| actionMasked == MotionEvent.ACTION_CANCEL;
//18、
// Update list of touch targets for pointer down, if needed.
final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
//19、
TouchTarget newTouchTarget = null;
//20、
boolean alreadyDispatchedToNewTouchTarget = false;
注释17 表示查看 viewFlag 是否被标记了 PFLAG_CANCEL_NEXT_UP_EVENT，那么下一步应该是 Cancel 事件，或者假设当前的 Action 为取消，那么当前事件就是取消了；注释18 表示当前的 ViewGroup 是不是支持把 MotionEvent 传送到不同的 View 当中，比如我们把两个手指放到了屏幕上，是否要将第二个手指的事件传送下面去；注释19 表示新的触摸对象；注释20 表示是否把事件分配给了新的触摸。
//21、
if (!canceled && !intercepted) {
//21A、
if (actionMasked == MotionEvent.ACTION_DOWN
|| (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
//22、
final int actionIndex = ev.getActionIndex(); // always 0 for down
//23、
final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
: TouchTarget.ALL_POINTER_IDS;
//24、
// Clean up earlier touch targets for this pointer id in case they
// have become out of sync.
removePointersFromTouchTargets(idBitsToAssign);
......
}
}
注释21 表示如果事件不是取消事件，也没有进行拦截；注释21A 表示如果是个新的 down 事件，或者是有新的触摸点，又或者是光标来回移动事件；注释22 表示这个事件的索引，即第几个事件，如果是 up 事件就是1；注释23 表示获取分配的 id 的 bit 数量；注释24 表示清除 Targets 中相应的 pointer 中的 ids，避免它们的目标变得不同步。