Android开发总结笔记 ViewGroup的事件分发机制 3-10

参考:/article/1503767.html

事实上，当点击某个组件。会先出发它所在的ViewGroup的dispatchTouchEvent方法，然后才分发到View的

所以，来看一下ViewGroup的dispatchTouchEvent方法

public boolean dispatchTouchEvent(MotionEvent ev) {

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 accessiiblity 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 = 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);

// 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;

}

[/code]

分步解析

第一步:15-21行，对ACTION_DOWN进行处理

在这里会对ACTION_DOWN进行一些初始化的操作

cancelAndClearTouchTargets(ev); //里面有一个重要的操作，将mFirstTouchTarget设为null

resetTouchState();// 重置Touch状态标识

[/code]

第二步:23-43行，

这段代码中决定了intercepted的值，而这个值是用来标记ViewGroup是否拦截Touch事件的传递的。

/ 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;

}

[/code]

一开始判断是否新起一轮Touch事件，不是的话直接拦截

disallowIntercept这个值默认是false，

可以通过ViewGroup提供的public方法requestDisallowInterceptTouchEvent方法来设置

onInterceptTouchEvent方法默认是false，可以通过重写返回true

第三步:43-45，检查cancel

final boolean canceled = resetCancelNextUpFlag(this)

|| actionMasked == MotionEvent.ACTION_CANCEL;

[/code]

通过标记和action检查cancel

第五步:47-函数结束，事件分发

在54行可以看到一个split变量，默认是true，作用是是否把事件分发给多个子View

在ViewGroup中提供了一个public方法设置setMotionEventSplittingEnabled

50行的

if (!canceled && !intercepted)

[/code]

事件如果不是ACTION_CANCEL并且标志位intercepted为false则会进入

到这里。。思路大概也差不多了。详细的，还是看开头那个文章把。。。