触摸事件分发机制的实例分析(一)
我们知道android中的事件分发有三个重要方法
onIntercept
ondispatchtouchevent
ontouchevent
他们之间的关系可以用下面的伪代码来阐述：
public boolean dispatchTouchEvent(MotionEvent e){
boolean consume = false;
if(onInterceptTouchEvent(ev)) {
consume = onToucheEvent(ev);
}else{
consume = child.dispatchTouchEvent(ev);
}
return consume;
}可以这样理解，当ViewGroup的onInterceptTouchEvent返回true，就表示要拦截这个事件，接着事件交由ViewGroup
处理，即它的OnTouchEvent方法会被调用；如果这个ViewGroup的onInterceptTouchEvent返回false就表示它不拦截这个事件，这时当前事件就传递给它的子元素来处理，接着子元素的dispatchTouchEvent就会被调用。如此往复直到事件被最终处理。
ontouch和onclick之间 的关系 ？
当一个View需要处理事件时，如果设置了OnTouchListener，那么OnTouchListener中的OnTouch就会被调用。这时，如果OnTouch如果返回false，当前View的onTouchEvent就会被调用；如果返回true，那么OnTouchEvent就不会被调用。只有OnTouch被调用后，我们通常设置的OnClick方法才会被调用。
事件滑动冲突例子
事件分发在应用层面的应用只是滑动冲突的解决。
这里用viewpager嵌套listview的例子来讲解。
其实用viewpager来讲事件滑动冲突并不是特别好，因为viewpager在内部已经帮我们解决了滑动冲突。
如果这个viewpager在实际操作可以是具体的一个自定LinearLayout或者其他什么的，原理是一样的。
我们为了掩饰滑动冲突的解决思路，在viewpager里面嵌套listview，并且在viewpager的onInterceptTouchEvent方法里面返回true，表示viewpager父布局拦截了事件，这时候里面的listview就不能上下滑动了。如果在viewpager的onInterceptTouchEvent方法里面返回false，表示viewpager不拦截事件，那么这时候viewpager又不能左右滑动了，这就是滑动冲突。
为什么会这个样子？
这就需要分析ViewGroup的dispatchTouchEvent源码。（点击事件分发会先执行Activity的dispatchTouchEvent，然后执行到ViewGrou的dispatchTouchEvent方法）
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);
}
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.
cancelAndClearTouchTargets(ev);
resetTouchState();
}
// Check for interception.
final boolean intercepted;
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.
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;
if (!canceled && !intercepted) {
// If the event is targeting accessibility 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;
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 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);
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.
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;
}
}
// Update list of touch targets for pointer up or cancel, if needed.
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;
} 这个方法的代码很长，但是不用每个细节都关注。主要的作用是完成事件的分发。
当ACTION_DOWN事件的时候，代码里会执行cancelAndClearTouchTargets和resetTouchState方法。其中，resetTouchState用来重置一些状态。
intercepted以及cenceled都是为false的时候，也就是说，既不取消事件，也不拦截事件的时候，就开始倒序遍历子View。
对于子ViewGroup的子view根据Z轴坐标进行排序，分别将ACTION_DOWN以及后续事件交由他们去处理。所以最外面的View会最先收到事件，这也就是我们平时记的，ACTION_DOWN等触摸事件是先由最外面的View一层层向根ViewGroup传递的道理是一样的。
调用dispatchTransformedTouchEvent方法里面调用childview的dispatchTouchEvent（）方法，从而完成事件由ViewGroup到View的一个分发。
那么，滑动冲突如何去解决？
1.外部拦截法
外部拦截的思路比内部拦截要简单，父容器针对不同的事件选择是否进行拦截。
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;//必须不能拦截，否则后续的ACTION_MOME和ACTION_UP事件都会拦截。
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;
mLastXIntercept = y;
return intercepted;
} 2.内部拦截法
内部拦截的话可以直接看我这个例子demo了
具体就是看这个demo
Github地址
它主要的逻辑就是重写内部类的dispatchTouchEvent方法。
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
int x = (int) ev.getX();
int y = (int) ev.getY();
switch (ev.getAction()){
case MotionEvent.ACTION_DOWN: {
getParent().requestDisallowInterceptTouchEvent(true);
break;
}
case MotionEvent.ACTION_MOVE:{
int deltax = x - mLastX;
int deltay = y - mLastY;
if(Math.abs(deltax) > Math.abs(deltay) ){
getParent().requestDisallowInterceptTouchEvent(false);
}
break;
}
case MotionEvent.ACTION_UP:
break;
}
return super.dispatchTouchEvent(ev);
}这里有个难点，就是仅仅这样做还是不够的，就是说还要在父布局里面的onInterceptTouchEvent里面增加一个处理，就是不拦截ACTION_DOWN事件，为什么不能拦截这个事件呢，因为这个事件比较特殊，不受FLAG_DISALLOW_INTERCEPT这个标志位的控制，所以一旦父容器拦截了ACTION_DOWN的事件，那么后续所有事件都无法传递到子布局里面去，所以内部拦截就无法起作用了。