从源码角度谈就ViewGroup的onInterceptTouchEvent方法的调用流程。
- 当ViewGroup的childView的onTouchEvent方法返回true时表示该childView可以接受MotionEvent事件, 此时在ViewGroup上的有MotionEvent时间发生时,每次都会调用OnInterceptTouchEvent方法。
当ViewGroup的childView的onTouchEvent方法返回false时表示该childView不接受MotionEvent事件, 如果ViewGroup的onTouchEvent方法返回true表示ViewGroup可以接受MotionEvent方法,此后onInterceptTouchEvent方法不会再调用,直接调用ViewGroup 的onTouchEvent方法。
一个MotionEvent事件由一个ACTION_DOWN事件,若干个ACTION_MOVE事件,一个ACTION_UP事件组成。当一个MotionEvent事件发生时,ViewGroup接到MotionEvent的上述的事件后,ViewGroup调用自己的dispatchTouchEvent方法来向其childView以及自身来分配MotionEvent的事件。来看ViewGroup的dispatchTouchEvent方法:
1 | public boolean dispatchTouchEvent(MotionEvent ev) { if (mInputEventConsistencyVerifier != null) { mInputEventConsistencyVerifier.onTouchEvent(ev, 1); } boolean handled = false; if (onFilterTouchEventForSecurity(ev)) { final int action = ev.getAction(); final int actionMasked = action & MotionEvent.ACTION_MASK; // Handle an initial down. /**ACTION_DOWN表示新的事件到来,取消并且清楚之前保留的TouchTarget,并且重置touchState, *TouchTarget表示接受MotionEvent的childView。 **/ 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; /* *下面的判断表示当ACTION_DOWN事件到来时,需要调用OnInterceptTouchEvent方法。同时如果 *mFirstTouchTarget不为null时需要调用OnInterceptTouchEvent方法,mFirstTouchTarget不为null *表示childView可以接受MotionEvent事件,即childView中有onTouchEvent返回true的。当ACTION_DOWN事 *件到来时,mFirstTouchEvent必然为null。当MotionEvent后面的ACTION_MOVE和ACTION_UP事件到来时, *mFirstTouchTarget为null(childView都不接受MotionEvent事件),就会进入else语句,不会再调用 *onInterceptTouchEvent方法。 */ if (actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) { //disallowIntercept可以让childView通过调用requestDisallowInterceptTouchEvent方法来设置 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; } // 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; /* *当上面的intercepted为false,并且canceled为false时,就要向childview传递MotionEvent事件了。 * *当ACTION_DOWN事件到来时,再上面调用了onInterceptTouchEvent方法,该方法默认返回false,当该方法 *返回false,即intercepted为false(resetCancelNextUpFlag(this)大多情况下放回false,所以 *canceled肯定为false,因为是ACTION_DOWN),if语句满足。 */ if (!canceled && !intercepted) { /* *当ACTION_DOWN或者ACTION_MOVE或者ACTION_POINTER_DOWN同时ViewGroup可以讲一个事件分给多个 *childView时,进入if语句。 */ 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); /* *下面找出MotionEvent对应的那个childView */ 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 = buildOrderedChildList(); final boolean customOrder = preorderedList == null && isChildrenDrawingOrderEnabled(); final View[] children = mChildren; for (int i = childrenCount - 1; i >= 0; i--) { final int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i; final View child = (preorderedList == null) ? children[childIndex] : preorderedList.get(childIndex); /* *当该childView不是可见,或者childView的区域不包含MotionEvent的位置,则继续遍历 */ if (!canViewReceivePointerEvents(child) || !isTransformedTouchPointInView(x, y, child, null)) { continue; } //获取该childView对应的touchTarget newTouchTarget = getTouchTarget(child); //在ACTION_DOWN阶段,newTouchTarget肯定为null,这里在ACTION_POINTER_DOWN阶段进入if条件 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); /* *dispatchTransformedTouchEvent方法中会调用child的dispatchTouchEvent方法。 *如果child不是ViewGroup,则child的dispatchTouchEvent调用了OnTouchListener *的onTouch方法,如果onTouch返回false,调用onTouchEvent方法。 *如果dispatchTransformedTouchEven返回true,说明child接受Touch事件。 */ 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(); /* *调用addTouchTarget后,mFirstTouchTarget就不为null了。 */ newTouchTarget = addTouchTarget(child, idBitsToAssign); alreadyDispatchedToNewTouchTarget = true; break; } } 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; } } } /* *没有child可以接受MotionEvent, 让自己来。 */ // 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; //下面的if语句表示在ACTION_DOWN阶段已经处理过了就不再处理 if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) { handled = true; } else { /* *下面将ACTION_MOVE事件和ACTION_UP事件对之前接受ACTION_DOWN事件的childView进行传递。 */ 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; } |