Andriod高级开发工程师-事件分发机制(dispatchTouchEvent源码)

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上篇文章,我们从日志层面分析了View的事件传递机制,dispatchTouchEvent、onInterceptTouchEvent、onTouchEvent等的执行逻辑和顺序。

今天逐步分析源码来讲解事件分发逻辑的实现

dispatchTouchEvent

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
    ......

    // 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.
    ......

    boolean handled = false;
    if (onFilterTouchEventForSecurity(ev)) {
        final int action = ev.getAction();
        final int actionMasked = action & MotionEvent.ACTION_MASK;

        // Handle an initial down.
        // ACTION_DOWN会进行初始化操作
        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.
            cancelAndClearTouchTargets(ev);
            resetTouchState();
        }
        ......
    }
    ......
    return handled;
}

先判断ACTION_DOWN进行部分初始化操作

  • cancelAndClearTouchTargets() 取消清除所有点击节点
  • resetTouchState() 重置点击状态
/**
 * Resets all touch state in preparation for a new cycle.
 */
private void resetTouchState() {
    clearTouchTargets();
    resetCancelNextUpFlag(this);
    // 重点关注该变量,是FLAG_DISALLOW_INTERCEPT的取非
    mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
    mNestedScrollAxes = SCROLL_AXIS_NONE;
}

继续分析下面的代码,即是否执行onInterceptTouchEvent()

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
    ......

    // 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.
    ......

    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) {
            // ACTION_DOWN的情况,因为前面在resetTouchState()中设置    mGroupFlags=~FLAG_DISALLOW_INTERCEPT,
            // 所以disallowIntercept的值一定是false,即一定会执行onInterceptTouchEvent()
            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;
        }
        ......
    }
    ......
    return handled;
}

onInterceptTouchEvent

是否执行onInterceptTouchEvent,是通过disallowIntercept变量来判断的

disallowIntercept变量

// ACTION_DOWN的情况,因为前面在resetTouchState()中设置 mGroupFlags=~FLAG_DISALLOW_INTERCEPT,
// 所以disallowIntercept的值一定是false,即一定会执行onInterceptTouchEvent()
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;
}

requestDisallowInterceptTouchEvent

我们知道该方法用来在子View中,调用getParent().requestDisallowInterceptTouchEvent(true)来阻止父ViewGroup拦截除了ACTION_DOWN事件外的所有事件。

根本原因就在上面提到的代码,ViewGroup的dispatchTouchEvent()在ACTION_DOWN时,disallowIntercept每次都会重新设置成false。

又因为ViewGroup的dispatchTouchEvent()中disallowIntercept每次都会重新计算,所以requestDisallowInterceptTouchEvent方法只能在子View的onTouchEvent()中每次调用来设置。

结论1

ViewGroup的onInterceptTouchEvent在ACTION_DOWN一定会执行

requestDisallowInterceptTouchEvent()只能在子View的onTouchEvent()中调用,来阻止父ViewGroup拦截除了ACTION_DOWN事件外的所有事件。

requestDisallowInterceptTouchEvent()要想可以使用,必须父ViewGroup在onInterceptTouchEvent的ACTION_DOWN时,返回false,即不父ViewGroup不处理ACTION_DOWN事件,而执行子View的onTouchEvent()。

onTouchEvent

下面分析下如何执行的onTouchEvent

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
    ......

    boolean handled = false;
    if (onFilterTouchEventForSecurity(ev)) {
        final int action = ev.getAction();
        final int actionMasked = action & MotionEvent.ACTION_MASK;

        // Handle an initial down.
        ......

        // Check for interception.
        final boolean intercepted;
        ......

        // 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;
                    
                    // 倒序遍历子View
                    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;
                        }

                        // 如果子View不可见或正在动画
                        // 或点击区域不在子View上
                        // 则跳出循环,执行循环下次操作
                        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);
                        // 执行分发TouchEvent
                        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();
                            // 设置mFirstTouchTarget为子View
                            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;
                }
            }
        }

        ......

        // Update list of touch targets for pointer up or cancel, if needed.
       ......
    }

    ......
    return handled;
}

如果ViewGroup没有拦截且没有cancel,则倒序遍历子View,执行dispatchTransformedTouchEvent。

如果子类处理了onTouchEvent,则会调用addTouchTarget来设置mFirstTouchTarget为子View。否则,如果没有子类处理onTouchEvent,则mFirstTouchTarget为null。

/**
 * Adds a touch target for specified child to the beginning of the list.
 * Assumes the target child is not already present.
 */
private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
    final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
    target.next = mFirstTouchTarget;
    mFirstTouchTarget = target;
    return target;
}

执行ViewGroup的onTouchEvent

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
    ......

    boolean handled = false;
    if (onFilterTouchEventForSecurity(ev)) {
        final int action = ev.getAction();
        final int actionMasked = action & MotionEvent.ACTION_MASK;

        // Handle an initial down.
        ......

        // Check for interception.
        final boolean intercepted;
        ......

        // 前面说过,在ACTION_DOWN会先遍历子View,如果处理了onTouchEvent则mFirstTouchTarget为子View,否则为null。
        // 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 {
                    // 处理子View ACTION_DOWN后续的事件
                    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 ......
    }

    ......
    return handled;
}

前面说过,在ACTION_DOWN会先遍历子View,如果处理了onTouchEvent则mFirstTouchTarget为子View,否则为null,则执行父ViewGroup的onTouchEvent。

具体在dispatchTransformedTouchEvent来调用onTouchEvent()

可见,只要有子View,就调用子View的onTouchEvent()

/**
 * Transforms a motion event into the coordinate space of a particular child view,
 * filters out irrelevant pointer ids, and overrides its action if necessary.
 * If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.
 */
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;
    }

    // Calculate the number of pointers to deliver.
    final int oldPointerIdBits = event.getPointerIdBits();
    final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;

    // If for some reason we ended up in an inconsistent state where it looks like we
    // might produce a motion event with no pointers in it, then drop the event.
    if (newPointerIdBits == 0) {
        return false;
    }

    // If the number of pointers is the same and we don't need to perform any fancy
    // irreversible transformations, then we can reuse the motion event for this
    // dispatch as long as we are careful to revert any changes we make.
    // Otherwise we need to make a copy.
    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;
}

结论2

dispatchTouchEvent会先判断ViewGroup的onInterceptTouchEvent(),如果拦截,intercepted为true,则不处理后面子View的onTouchEvent,所以mFirstTouchTarget为null。否则然后在ACTION_DOWN先遍历子View的onTouchEvent(),如果子类不处理,即返回false,则mFirstTouchTarget为null,则以后只执行父ViewGroup的onTouchEvent(),不会再执行子View的任何方法,且也不会执行父ViewGroup的onInterceptTouchEvent()。

ViewGroup的onInterceptTouchEvent(),如果拦截,即返回true,intercepted为true,则不处理后面的View的onTouchEvent,所以mFirstTouchTarget为null。只执行ViewGroup的onTouchEvent。

ACTION_DOWN子类处理onTouch及返回true,则后续会执行View的onTouchEvent和ViewGroup的onInterceptTouchEvent,不会执行ViewGroup的onTouchEvent。

ACTION_DOWN子类处理onTouch及返回false,则后续只会执行ViewGroup的onTouchEvent,不会执行ViewGroup的onInterceptTouchEvent和View的onTouchEvent。