我們知道android中的事件分發有三個重要方法
onIntercept
ondispatchtouchevent
ontouchevent
他們之間的關係可以用下面的僞代碼來闡述：
public boolean dispatchTouchEvent(MotionEvent e){
boolean consume = false;
if(onInterceptTouchEvent(ev)) {
consume = onToucheEvent(ev);
}else{
consume = child.dispatchTouchEvent(ev);
}
return consume;
}
可以這樣理解，當ViewGroup的onInterceptTouchEvent返回true，就表示要攔截這個事件，接着事件交由ViewGroup
處理，即它的OnTouchEvent方法會被調用；如果這個ViewGroup的onInterceptTouchEvent返回false就表示它不攔截這個事件，這時當前事件就傳遞給它的子元素來處理，接着子元素的dispatchTouchEvent就會被調用。如此往復直到事件被最終處理。
ontouch和onclick之間 的關係 ？
當一個View需要處理事件時，如果設置了OnTouchListener，那麼OnTouchListener中的OnTouch就會被調用。這時，如果OnTouch如果返回false，當前View的onTouchEvent就會被調用；如果返回true，那麼OnTouchEvent就不會被調用。只有OnTouch被調用後，我們通常設置的OnClick方法纔會被調用。
事件滑動衝突例子
事件分發在應用層面的應用只是滑動衝突的解決。
這裏用viewpager嵌套listview的例子來講解。
其實用viewpager來講事件滑動衝突並不是特別好，因爲viewpager在內部已經幫我們解決了滑動衝突。
如果這個viewpager在實際操作可以是具體的一個自定LinearLayout或者其他什麼的，原理是一樣的。
我們爲了掩飾滑動衝突的解決思路，在viewpager裏面嵌套listview，並且在viewpager的onInterceptTouchEvent方法裏面返回true，表示viewpager父佈局攔截了事件，這時候裏面的listview就不能上下滑動了。如果在viewpager的onInterceptTouchEvent方法裏面返回false，表示viewpager不攔截事件，那麼這時候viewpager又不能左右滑動了，這就是滑動衝突。
爲什麼會這個樣子？
這就需要分析ViewGroup的dispatchTouchEvent源碼。（點擊事件分發會先執行Activity的dispatchTouchEvent，然後執行到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<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 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;
}
這個方法的代碼很長，但是不用每個細節都關注。主要的作用是完成事件的分發。
當ACTION_DOWN事件的時候，代碼裏會執行cancelAndClearTouchTargets和resetTouchState方法。其中，resetTouchState用來重置一些狀態。
intercepted以及cenceled都是爲false的時候，也就是說，既不取消事件，也不攔截事件的時候，就開始倒序遍歷子View。
對於子ViewGroup的子view根據Z軸座標進行排序，分別將ACTION_DOWN以及後續事件交由他們去處理。所以最外面的View會最先收到事件，這也就是我們平時記的，ACTION_DOWN等觸摸事件是先由最外面的View一層層向根ViewGroup傳遞的道理是一樣的。
調用dispatchTransformedTouchEvent方法裏面調用childview的dispatchTouchEvent（）方法，從而完成事件由ViewGroup到View的一個分發。
那麼，滑動衝突如何去解決？
1.外部攔截法
外部攔截的思路比內部攔截要簡單，父容器針對不同的事件選擇是否進行攔截。
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;//必須不能攔截，否則後續的ACTION_MOME和ACTION_UP事件都會攔截。
break;
case MotionEvent.ACTION_MOVE:
if (父容器需要當前點擊事件) {
intercepted = true;
} else {
intercepted = false;
}
break;
case MotionEvent.ACTION_UP:
intercepted = false;
break;
default:
break;
}
mLastXIntercept = x;
mLastXIntercept = y;
return intercepted;
}
2.內部攔截法
內部攔截的話可以直接看我這個例子demo了
具體就是看這個demo
它主要的邏輯就是重寫內部類的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);
}
這裏有個難點，就是僅僅這樣做還是不夠的，就是說還要在父佈局裏面的onInterceptTouchEvent裏面增加一個處理，就是不攔截ACTION_DOWN事件，爲什麼不能攔截這個事件呢，因爲這個事件比較特殊，不受FLAG_DISALLOW_INTERCEPT這個標誌位的控制，所以一旦父容器攔截了ACTION_DOWN的事件，那麼後續所有事件都無法傳遞到子佈局裏面去，所以內部攔截就無法起作用了。