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网上看到过很多人写的事件分发机制解析, 感觉表述都不是很清楚, 也可能没有看到写得好的文章, 所以自己重新看了一遍源码, 来彻底搞清楚 Android 事件分发机制.
触摸事件有哪些以及怎么从 Activity 传递到 DecorView 大家可以上网查下，几个重要方法的基本调用顺序, 这些很容易搜到, 我们重点关注事件从 ViewGroup 到 View 的事件具体的执行过程.
1 Android 事件分发涉及到的方法主要有
dispatchTouchEvent()事件分发
onInterceptTouchEvent() 拦截事件
onTouchEvent() 处理事件
requestDisallowInterceptTouchEvent() 请求不要拦截事件
2 测试类介绍
BigGroup 类型 ViewGroup
SmallGroup 类型 ViewGroup
TestView 类型 View*(clickable 默认 false)
3 默认情况下的事件分发流程 (View.onTouchEvent() 返回 super.onTouchEvent())
log 如下所示：
BigGroup: dispatchTouchEvent() called with: ev = [ACTION_DOWN]
BigGroup: onInterceptTouchEvent() called with: ev = [ACTION_DOWN]
SmallGroup: dispatchTouchEvent() called with: ev = [ACTION_DOWN]
SmallGroup: onInterceptTouchEvent() called with: ev = [ACTION_DOWN]
TestView: dispatchTouchEvent() called with: ev = [ACTION_DOWN]
TestView: onTouchEvent() called with: event = [ACTION_DOWN]
SmallGroup: onTouchEvent() called with: event = [ACTION_DOWN]
BigGroup: onTouchEvent() called with: event = [ACTION_DOWN]
我们看到不管怎么点击滑动，都只会触发 DOWN 事件的流程，我们来看下这是为什么，首先我们来看下 ViewGroup 的 dispatchTouchEvent()
///////////ViewGroup
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
... 省略部分代码
boolean handled = false;
if (onFilterTouchEventForSecurity(ev)) {final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;
// 如果是 DOWN 事件, 清除标记, 恢复一些标记状态
if (actionMasked == MotionEvent.ACTION_DOWN) {cancelAndClearTouchTargets(ev);
resetTouchState();}
// Check for interception.
// 判断是否拦截
final boolean intercepted;
// 触发 DOWN 事件或者 mFirstTouchTarget 不是 Null, 才会走 if 里面的逻辑
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {intercepted = onInterceptTouchEvent(ev);
ev.setAction(action); // restore action in case it was changed
} else {intercepted = false;}
} else {
// There are no touch targets and this action is not an initial down
// so this view group continues to intercept touches.
// 不是 DOWN 事件 mFirstTouchTarget 为空 拦截后续事件
intercepted = true;
}
... 省略部分代码
// Check for cancelation.
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;
// 事件没有被取消或者没有被拦截执行 if 中逻辑
if (!canceled && !intercepted) {View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
? findChildWithAccessibilityFocus() : null;
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);
// Find a child that can receive the event.
// Scan children from front to back.
final ArrayList<View> preorderedList = buildTouchDispatchChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
if (childWithAccessibilityFocus != null) {if (childWithAccessibilityFocus != child) {continue;}
childWithAccessibilityFocus = null;
i = childrenCount - 1;
}
// 判断 child 是否可以响应事件 是否点击在了 View 的范围之内
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {ev.setTargetAccessibilityFocus(false);
continue;
}
// 获取上次响应事件的 View,DOWN 事件时候 newTouchTarget 为 Null
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);
// 很重要的一个方法, 在下面我们会看这个方法做了什么
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();
// 在这个方法中把 child 赋值给了 mFirstTouchTarget
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;
}
ev.setTargetAccessibilityFocus(false);
}
if (preorderedList != null) preorderedList.clear();}
...
}
}
// Dispatch to touch targets.
if (mFirstTouchTarget == null) {
// 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) {handled = true;} else {final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
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;
}
}
...
}
...
return handled;
}
// 最重要的几行代码的逻辑
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
final boolean handled;
... 省略部分代码
// Perform any necessary transformations and dispatch.
//child 为空调用父 View 的 dispatchTouchEvent
if (child == null) {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());
}
//child 不为空调用 child 的 dispatchTouchEvent 方法
handled = child.dispatchTouchEvent(transformedEvent);
}
// Done.
transformedEvent.recycle();
return handled;
}
// 默认返回 false
public boolean onInterceptTouchEvent(MotionEvent ev) {if (ev.isFromSource(InputDevice.SOURCE_MOUSE)
&& ev.getAction() == MotionEvent.ACTION_DOWN
&& ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
&& isOnScrollbarThumb(ev.getX(), ev.getY())) {return true;}
return false;
}
//////////View
public boolean dispatchTouchEvent(MotionEvent event) {
...
boolean result = false;
...
if (onFilterTouchEventForSecurity(event)) {if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {result = true;}
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
// 我们看到有 mOnTouchListener, 并且 onTouch 返回 true, 就没有 onTouchEvent 啥事了
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {result = true;}
if (!result && onTouchEvent(event)) {result = true;}
}
...
return result;
}
public boolean onTouchEvent(MotionEvent event) {final float x = event.getX();
final float y = event.getY();
final int viewFlags = mViewFlags;
final int action = event.getAction();
final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE
|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;
...
if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {switch (action) {
case MotionEvent.ACTION_UP:
...
break;
case MotionEvent.ACTION_DOWN:
...
break;
case MotionEvent.ACTION_CANCEL:
...
break;
}
return true;
}
return false;
}
我们梳理下 DOWN 事件的执行流程:
1 触发 BigGroup 的 dispatchTouchEvent()方法
2 给 intercepted 赋值,disallowIntercept 默认为 false, 这是就会触发 onInterceptTouchEvent() 方法,onInterceptTouchEvent 默认返回 false.intercepted 赋值为 false
3 接下来走到 if (!canceled && !intercepted)的逻辑里面去了, 注意注意 intercepted 为 true 的话这个方法直接就跳过去了. 在这段逻辑里遍历所有的子 View, 接下来进入淘汰机制,child 不能获得焦点, 下一个, 然后看看 View 不可见, 下一个, 没有点到 child 范围之内, 下一个. 剩下来的娃会执行 dispatchTransformedTouchEvent 方法
4 接下来 dispatchTransformedTouchEvent 中调用了 child 的 dispatchTouchEvent 方法
5 执行 dispatchTouchEvent 的 child 就是我们的 SmallGroup, 执行逻辑同 2 -4
6 这次执行 dispatchTouchEvent 的 child 就是我们的 TestView 了
7 TestView dispatchTouchEvent 中首先判断了有没有 onTouchListener, 判断 onTouch 方法的返回值. 我们肯定没写的了, 接下来会执行 onTouchEvent
8 在 TestView onTouchEvent 中我们看到如果这个 View 可点击就返回 true, 不可点击就返回 false.TestView 不可点击, 所以返回了 false
9 触发了连锁反应,TestView dispatchTouchEvent 返回 false
10SmallGroup dispatchTransformedTouchEvent 返回了 false, mFirstTouchTarget 为 null
11SmallGroup 再次执行 dispatchTransformedTouchEvent child 参数为 null, 执行 super.dispatchTouchEvent(),ViewGroup 的 super 最后就是 View, 所以会执行逻辑 7 -9, 不同的事主角事 SmallGroup
12BigGroup dispatchTransformedTouchEvent 返回了 false, mFirstTouchTarget 为 null, 重复步骤 11, 主角 BigGroup,DOWN 事件结束
mFirstTouchTarget 为 null 会有什么影响呢? 后续事件中 intercepted 为 true, 执行 dispatchTransformedTouchEvent child 参数为 null, 执行 super.dispatchTouchEvent(), 事件直接传递不下来了, 后续事件都接收不到了
所以 log 只能看到有关于 DOWN 事件的．
