android 事件分发机制
1.android 点击事件
当用户触摸屏幕时(View 或 ViewGroup派生的控件)，将产生点击事件(Touch事件)
一共包含四种事件类型
事件类型 具体动作
MotionEvent.ACTION_DOWN 按下View（所有事件的开始）
MotionEvent.ACTION_UP 抬起View（与DOWN对应）
MotionEvent.ACTION_MOVE 滑动View
MotionEvent.ACTION_CANCEL 结束事件（非人为原因）
一般情况下，事件列都是以DOWN事件开始、UP事件结束，中间有无数的MOVE事件，当一个点击事件（MotionEvent ）产生后，
系统需把这个事件传递给一个具体的 View 去处理
2.事件的传递
事件传递顺序: activity ——>ViewGroup——>View
事件分发过程由哪些方法协作完成？
dispatchTouchEvent() 、onInterceptTouchEvent()和onTouchEvent()
dispatchTouchEvent():分发(传递)事件,当点击事件可以传递时，这个方法就会被调用
onTouchEvent():处理点击事件,在dispatchTouchEvent()内部调用
onInterceptTouchEvent():判断是否拦截某个事件,在ViewGroup的dispatchTouchEvent()内部调用
下面我们逐个来分析各事件的传递
2.1.Activity的事件传递
当一个点击事件发生时，事件最先传到Activity的dispatchTouchEvent()进行事件分发,源码入下:
/**
* 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);
}
分析2：getWindow().superDispatchTouchEvent(ev)
说明：
a. getWindow() = 获取Window类的对象
b. Window类是抽象类，其唯一实现类 = PhoneWindow类；即此处的Window类对象 = PhoneWindow类对象
c. Window类的superDispatchTouchEvent() = 1个抽象方法，由子类PhoneWindow类实现
DecorView类是PhoneWindow类的一个内部类
b. DecorView继承自FrameLayout，是所有界面的父类，FrameLayout是ViewGroup的子类，故DecorView的间接父类 = ViewGroup
这就实现了事件从activity 传递到viewGroup,我们看看源码：(截取部分代码)
2.2.ViewGroup事件分发
我们看源码中核心分发方法dispatchTouchEvent
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
...
// Check for interception.
final boolean intercepted;
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {
// 重点分析1：ViewGroup每次事件分发时，都需调用onInterceptTouchEvent()询问是否拦截事件
// 判断值1：disallowIntercept = 是否禁用事件拦截的功能(默认是false)，可通过调用requestDisallowInterceptTouchEvent（）修改
// 判断值2： onInterceptTouchEvent(ev)
// a. 若在onInterceptTouchEvent()中返回false（即不拦截事件），intercepted值为true，从而进入到条件判断的内部
// b. 若在onInterceptTouchEvent()中返回true（即拦截事件），intercepted值为false，从而跳出了这个条件判断
// c. 关于onInterceptTouchEvent() ->>分析1
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.
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);
}
// 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;
//重点分析2：只有当canceled 、intercepted都为false 时，事件传递才能进行下去，接下来遍历所有子view
找到被触摸或者点击的子view
if (!canceled && !intercepted) {
...
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 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 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;
}
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);
//重点分析3：将事件继续传递到子view 中，
// 若该控件可点击，那么点击时，dispatchTouchEvent的返回值必定是true，因此会导致条件判断成立
alreadyDispatchedToNewTouchTarget 赋值为true,并且直接跳出遍历
// 于是给ViewGroup的dispatchTouchEvent（）直接返回了true，即直接跳出
// 即把ViewGroup的点击事件拦截掉
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();
}
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.
if (mFirstTouchTarget == null) {
// No touch targets so treat this as an ordinary view.
//没有找到目标view,则viewGroup自己处理该事件，调用onTouch——>onTouchEvent()
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;
}
2.3.View的事件分发过程
接下来我们看view的分发
public boolean dispatchTouchEvent(MotionEvent event) {
...
if (onFilterTouchEventForSecurity(event)) {
if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
result = true;
}
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
/**重点分析1：*/
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
if (!result && onTouchEvent(event)) {
result = true;
}
}
if (!result && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
// Clean up after nested scrolls if this is the end of a gesture;
// also cancel it if we tried an ACTION_DOWN but we didn't want the rest
// of the gesture.
if (actionMasked == MotionEvent.ACTION_UP ||
actionMasked == MotionEvent.ACTION_CANCEL ||
(actionMasked == MotionEvent.ACTION_DOWN && !result)) {
stopNestedScroll();
}
return result;
}
重点分析1：
条件1：mOnTouchListener != null，说明：mOnTouchListener变量在View.setOnTouchListener（）方法里赋值，也就是说
如果有设置view 的触摸事件回调，那么，view 会优先执行onTouch方法
条件2：(mViewFlags & ENABLED_MASK) == ENABLED，该条件是判断当前点击的控件是否enable，由于很多View默认enable，
故该条件恒定为true
条件3：mOnTouchListener.onTouch(this, event),说明：即 回调控件注册Touch事件时的onTouch（）；需手动复写设置,
以button 为例，
button.setOnTouchListener(new OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
return false;
}
});
若在onTouch()返回true，就会让上述三个条件全部成立，从而使得View.dispatchTouchEvent（）直接返回true，事件分发结束
若在onTouch()返回false，就会使得上述三个条件不全部成立，从而使得View.dispatchTouchEvent（）中跳出If，执行onTouchEvent(event)
继续来看onTouchEvent()事件处理
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 ((viewFlags & ENABLED_MASK) == DISABLED) {
if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
// A disabled view that is clickable still consumes the touch
// events, it just doesn't respond to them.
return clickable;
}
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}
//若该控件可点击，则进入switch判断中
if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
switch (action) {
//若当前的事件是抬起View（主要分析）
case MotionEvent.ACTION_UP:
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
if ((viewFlags & TOOLTIP) == TOOLTIP) {
handleTooltipUp();
}
if (!clickable) {
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
break;
}
boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
// take focus if we don't have it already and we should in
// touch mode.
boolean focusTaken = false;
if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
focusTaken = requestFocus();
}
if (prepressed) {
// The button is being released before we actually
// showed it as pressed. Make it show the pressed
// state now (before scheduling the click) to ensure
// the user sees it.
setPressed(true, x, y);
}
if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
// This is a tap, so remove the longpress check
removeLongPressCallback();
// Only perform take click actions if we were in the pressed state
if (!focusTaken) {
// Use a Runnable and post this rather than calling
// performClick directly. This lets other visual state
// of the view update before click actions start.
if (mPerformClick == null) {
mPerformClick = new PerformClick();
}
if (!post(mPerformClick)) {
performClick();
}
}
}
if (mUnsetPressedState == null) {
mUnsetPressedState = new UnsetPressedState();
}
if (prepressed) {
postDelayed(mUnsetPressedState,
ViewConfiguration.getPressedStateDuration());
} else if (!post(mUnsetPressedState)) {
// If the post failed, unpress right now
mUnsetPressedState.run();
}
removeTapCallback();
}
mIgnoreNextUpEvent = false;
break;
case MotionEvent.ACTION_DOWN:
if (event.getSource() == InputDevice.SOURCE_TOUCHSCREEN) {
mPrivateFlags3 |= PFLAG3_FINGER_DOWN;
}
mHasPerformedLongPress = false;
if (!clickable) {
checkForLongClick(0, x, y);
break;
}
if (performButtonActionOnTouchDown(event)) {
break;
}
// Walk up the hierarchy to determine if we're inside a scrolling container.
boolean isInScrollingContainer = isInScrollingContainer();
// For views inside a scrolling container, delay the pressed feedback for
// a short period in case this is a scroll.
if (isInScrollingContainer) {
mPrivateFlags |= PFLAG_PREPRESSED;
if (mPendingCheckForTap == null) {
mPendingCheckForTap = new CheckForTap();
}
mPendingCheckForTap.x = event.getX();
mPendingCheckForTap.y = event.getY();
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
} else {
// Not inside a scrolling container, so show the feedback right away
setPressed(true, x, y);
checkForLongClick(0, x, y);
}
break;
case MotionEvent.ACTION_CANCEL:
if (clickable) {
setPressed(false);
}
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
break;
case MotionEvent.ACTION_MOVE:
if (clickable) {
drawableHotspotChanged(x, y);
}
// Be lenient about moving outside of buttons
if (!pointInView(x, y, mTouchSlop)) {
// Outside button
// Remove any future long press/tap checks
removeTapCallback();
removeLongPressCallback();
if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
}
break;
}
// 若该控件可点击，就一定返回true
return true;
}
// 若该控件不可点击，就一定返回false
return false;
}
当view 可点击切且手势为抬手，进过一些判断，会进入执行performClick()方法，这里会判断是否有设置点击事件setOnclickListener()，
如果有，执行onClick(),并返回true
分析到这里，我们可以发现，view的 onTouch()的执行 先于 onClick()
总结：
1.dispatchTouchEvent() 逐上至下调用 Activity——>viewGroup——>View
2.onTouchEvent() 逐下至上级返回 view——>ViewGroup——>Activity
3.View的滑动冲突
滑动冲突有三种解决方式
- 外部拦截法
- 内部拦截法
3.1 外部拦截法
点击事件都先进过父容器的拦截处理，如果父容器需要此事件，就拦截，不需要则不拦截，外部拦截方法需要重写父容器的onInterceptTouchEvent方法，在内部做相应的拦截即可，类似这样的伪代码：
public boolean onInterceptTouchEvent(MotionEvent event){
boolean intercepted = false;
int x = (int)event.getX();
int y = (int)event.getY();
switch(event.getAction()){
case MotionEvent.ACTION_DOWN:{
intercepted = false;
break;
}
case MotionEvent.ACTION_MOVE:{
if(父容器需要当前点击事件)
{
intercepted = true;
}else {
intercepted = false;
}
break;
}
case MotionEvent.ACTION_UP:{
intercepted = false;
break;
}
default:
break;
}
mLastXIntercept = x;
mLastYIntercept = y;
return intercepted;
}
3.2 内部拦截法
内部拦截法指父容器不拦截任何事件，所有事件都传给子元素，如果子元素需要则消耗掉，不需要则交还给父容器处理，，需配合requestDisallowInterceptTouchEvent()方法才能正常工作，我们需要重写子元素的
dispatchTouchEvent()方法
public boolean dispatchTouchEvent(MotionEvent event){
int x = (int)event.getX();
int y = (int)event.getY();
switch(event.getAction()){
case MotionEvent.ACTION_DOWN:{
parent.requestDisallowInterceptTouchEvent(true);
break;
}
case MotionEvent.ACTION_MOVE:{
int deltaX = x-mLastX;
int deltaY = y-mLastY;
if(父容器需要当前点击事件)
{
parent.requestDisallowInterceptTouchEvent(false);
}
break;
}
case MotionEvent.ACTION_UP:{
break;
}
default:
break;
}
mLastX = x;
mLastY = y;
return super.dispatchTouchEvent(event);
}
除了子元素需要处理外，父元素也需要默认拦截除了ACTION_DOWN以外的其他事件，这样当子元素调用
parent.requestDisallowInterceptTouchEvent(false)方法时，父元素才能继续拦截所需的事件。
父元素修改如下：
public Boolean onInterceptTouchEvent(MotionEvent event){
int action = event.getAction();
if(action == MotionEvent.ACTION_DOWN){
renturn false;
}else {
return true;
}
}
文章有部分参考自：https://www.jianshu.com/p/38015afcdb58