ViewGroup 事件分发

@Override
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;
//判断当前view是否被覆盖
    if (onFilterTouchEventForSecurity(ev)) {
        final int action = ev.getAction();
        final int actionMasked = action & MotionEvent.ACTION_MASK;
 
        // Handle an initial down.
//down事件是一切事件的开端，所以首先进行down事件一系列初始化操作
        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.
//清除以往的Touch状态，开始新的手势，这里面将mFirstTouchTarget置空，也就是清除之前目标组件
            cancelAndClearTouchTargets(ev);
//重置touch状态标识
            resetTouchState();
        }
 
        // Check for interception.
//检查是否拦截事件
        final boolean intercepted;
//如果当前事件为down事件并且mFirstTouchTarget不为空，即找到目标组件，就进方法，否则拦截事件
        if (actionMasked == MotionEvent.ACTION_DOWN
                || mFirstTouchTarget != null) {
            final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
//这个返回为true根据是否调用parent.requestDisllowInterceptTouchEvent方法并设置false,就交给viewgroup的
onInterceptTouchEvent方法执行，设置true就不拦截该事件
            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.
//设置split标记作用是将事件分发给多个子view
        final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
        TouchTarget newTouchTarget = null;
        boolean alreadyDispatchedToNewTouchTarget = false;
//当前事件不为cancel并且没有拦截事件
        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.
//这里拿到子view的集合preorderedList，然后从后往前遍历集合，由于addview方法中后添加的子view会由于Android UI的刷新
机制而显示在上方，所以从后往前遍历,后面添加的view也会先响应事件
                    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 (childWithAccessibilityFocus != null) {
                            if (childWithAccessibilityFocus != child) {
                                continue;
                            }
                            childWithAccessibilityFocus = null;
                            i = childrenCount - 1;
                        }
 
                        if (!canViewReceivePointerEvents(child)
                                || !isTransformedTouchPointInView(x, y, child, null)) {
                            ev.setTargetAccessibilityFocus(false);
                            continue;
                        }
//getTouchTarget方法是用来从target链中查找某一个taget,存在就返回，否则返回null,也就是接收Touch事件的子view
                        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;
                        }
//重置当前子view取消事件后下一个up事件标记
                        resetCancelNextUpFlag(child);
//dispatchTransformedTouchEvent方法返回为true表示view消费了该事件，并且开始下一个事件
                        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();
//新的touch标识组件添加到链的后端
                            newTouchTarget = addTouchTarget(child, idBitsToAssign);
//赋值为true
                            alreadyDispatchedToNewTouchTarget = true;
//break是判断哪个子view接收到touch，就跳出循坏
                            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) {
                    newTouchTarget = mFirstTouchTarget;
                    while (newTouchTarget.next != null) {
                        newTouchTarget = newTouchTarget.next;
                    }
                    newTouchTarget.pointerIdBits |= idBitsToAssign;
                }
            }
        }
 
//如果mFirsttouchEvent返回空，表示事件未被消费，down事件未被消费，后面up，move事件也就无法进入处理
//没有找到子组件或者touch被拦截，也就交给viewgroup处理，子view没有处理，就会交给子view上层viewgroup处理。false事件才会回溯，交给viewgroup处理事件
        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;
//表示当前事件被消费了，dispatchTouchevent就返回true，后面事件才会进来
                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;
}
public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {
//是否禁用拦截标记
    if (disallowIntercept == ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0)) {
        // We're already in this state, assume our ancestors are too
        return;
    }
//如果为true就不允许拦截事件，intercept返回false,否则调用viewgroup的onInterceptTouchEvent方法
    if (disallowIntercept) {
        mGroupFlags |= FLAG_DISALLOW_INTERCEPT;
    } else {
        mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
    }
 
    // Pass it up to our parent
    if (mParent != null) {
        mParent.requestDisallowInterceptTouchEvent(disallowIntercept);
    }
}
private void cancelAndClearTouchTargets(MotionEvent event) {
    if (mFirstTouchTarget != null) {
        boolean syntheticEvent = false;
        if (event == null) {
            final long now = SystemClock.uptimeMillis();
            event = MotionEvent.obtain(now, now,
                    MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0);
            event.setSource(InputDevice.SOURCE_TOUCHSCREEN);
            syntheticEvent = true;
        }
 
        for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {
            resetCancelNextUpFlag(target.child);
            dispatchTransformedTouchEvent(event, true, target.child, target.pointerIdBits);
        }
        //clearTouchTargets方法中将第一个手势状态置空
        clearTouchTargets();
        if (syntheticEvent) {
            event.recycle();
        }
    }
}
private void clearTouchTargets() {
    TouchTarget target = mFirstTouchTarget;
    if (target != null) {
        do {
            TouchTarget next = target.next;
            target.recycle();
            target = next;
        } while (target != null);
        mFirstTouchTarget = null;
    }
}
public boolean onInterceptTouchEvent(MotionEvent ev) {
//判断控件是否可点击，点击在控件范围内，是否是down事件，返回true
    if (ev.isFromSource(InputDevice.SOURCE_MOUSE)
            && ev.getAction() == MotionEvent.ACTION_DOWN
            && ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
            && isOnScrollbarThumb(ev.getX(), ev.getY())) {
        return true;
    }
    return false;
}
//这个方法中最需要关注第三个变量child，如果子view为空，就交给viewgroup的dispatchTouchevent进行处理
//否则交给子view的dispatchTouchEvent进行处理，这个子view可以是继承自view或者viewGroup
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
        View child, int desiredPointerIdBits) {
    final boolean handled;
 
    // Canceling motions is a special case.  We don't need to perform any transformations
    // or filtering.  The important part is the action, not the contents.
    final int oldAction = event.getAction();
    if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
        event.setAction(MotionEvent.ACTION_CANCEL);
        if (child == null) {
            handled = super.dispatchTouchEvent(event);
        } else {
            handled = child.dispatchTouchEvent(event);
        }
        event.setAction(oldAction);
        return handled;
    }
    final int oldPointerIdBits = event.getPointerIdBits();
    final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;
    if (newPointerIdBits == 0) {
        return false;
    }
    final MotionEvent transformedEvent;
    if (newPointerIdBits == oldPointerIdBits) {
        if (child == null || child.hasIdentityMatrix()) {
            if (child == null) {
                handled = super.dispatchTouchEvent(event);
            } else {
                final float offsetX = mScrollX - child.mLeft;
                final float offsetY = mScrollY - child.mTop;
                event.offsetLocation(offsetX, offsetY);
 
                handled = child.dispatchTouchEvent(event);
 
                event.offsetLocation(-offsetX, -offsetY);
            }
            return handled;
        }
        transformedEvent = MotionEvent.obtain(event);
    } else {
        transformedEvent = event.split(newPointerIdBits);
    }
 
    // Perform any necessary transformations and dispatch.
    if (child == null) {
        handled = super.dispatchTouchEvent(transformedEvent);
    } else {
        final float offsetX = mScrollX - child.mLeft;
        final float offsetY = mScrollY - child.mTop;
        transformedEvent.offsetLocation(offsetX, offsetY);
        if (! child.hasIdentityMatrix()) {
            transformedEvent.transform(child.getInverseMatrix());
        }
 
        handled = child.dispatchTouchEvent(transformedEvent);
    }
 
    // Done.
    transformedEvent.recycle();
    return handled;
}