事件分发源码解析

Android开发艺术探索

Activity分发

事件最先传递给Activity，Activity通过dispatchTouchEvent分发出去

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public boolean dispatchTouchEvent(MotionEvent ev) {
    if (ev.getAction() == MotionEvent.ACTION_DOWN) {
        onUserInteraction();
    }
    if (getWindow().superDispatchTouchEvent(ev)) {
        return true;
    }
    return onTouchEvent(ev);
}

Activity将事件交由window分发。若返回true，表明事件已被处理，否则调用Activity自己的onTouchEvent().Window是个抽象类，PhoneWindow是其唯一的实现类。

Window分发

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@Override
    public boolean superDispatchTouchEvent(MotionEvent event) {
        return mDecor.superDispatchTouchEvent(event);
    }

Window分发给了顶层DecorView。

顶级View的分发

DecorView 是FrameLayout，会将事件分发给ViewGroup

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public class DecorView extends FrameLayout implements RootViewSurfaceTaker, WindowCallbacks {

    public boolean superDispatchTouchEvent(MotionEvent event) {
        return super.dispatchTouchEvent(event);//Invoke ViewGroup's dispatchTouchEvent()
    }
}

对ViewGroup 的dispatchTouchEvent逐段分析

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

ViewGroup会在ACTION_DOWN到来时做重置操作resetTouchState()重置FLAG_DISALLOW_INTERCEPT. 因而当子View设置了FLAG_DISALLOW_INTERCEPT，ViewGroup仅可以拦截到ACTION_DOWN点击事件,ViewGroup会在两种情况决定拦截事件actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null。 mFirstTouchTarget是 First touch target in the linked list of touch targets. ;TouchTarget是一个View的链表，用来记录TouchView事件涉及的View链表。而mFirstTouchTarget是记录首次Touch事件涉及的View。那就好理解啦。如果触摸的那个位置没有子View则mFirstTouchTarget为null，那么ViewGroup处理当前事件。且同一序列的其它事件亦交由它处理。因为actionMasked == MotionEvent.ACTION_DOWN.
由此可得出ViewGroup在拦截事件后，后续的点击事件会默认交由它处理，不再调用onInterceptTouchEvent。
故 onInterceptTouchEvent并非每次都会调用，而dispatchTouchEvent会
当ViewGroup不拦截时，事件向下分发给子View

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if (!canceled && !intercepted) {
...
                    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();
                    }
					...
			｝

遍历所有子View，判断其是否能够收到点击事件——1. 子View能否收到点击事件 2. 点击事件的坐标是否落在子View的区域内。在如下的dispatchTransformedTouchEvent中实际调用的就是子View的dispatchEvent

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/**
 * 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;
    }

若子View的dispatchTouchEvent返回true，mFirstTouchEvent会被赋值，然后break。

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 newTouchTarget = addTouchTarget(child, idBitsToAssign);
                                alreadyDispatchedToNewTouchTarget = true;
                                break;
private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
        final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
        target.next = mFirstTouchTarget;
        mFirstTouchTarget = target;
        return target;
    }

当该事件最终未被子View合适地的处理有两种情况。

  1. ViewGroup没有子View
  2. 子View的dispatchTouchEvent()返回了false(常见)
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         if (mFirstTouchTarget == null) {
             // No touch targets so treat this as an ordinary view.
             handled = dispatchTransformedTouchEvent(ev, canceled, null,
                     TouchTarget.ALL_POINTER_IDS);
         }
...
    if (child == null) {
             handled = super.dispatchTouchEvent(event);

将事件重新交由父元素View处理，且同一事件序列的后续事件不会再交给它处理。下面看View是如何处理

View对点击事件的处理

这里View不包含ViewGroup

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public boolean dispatchTouchEvent(MotionEvent event) {
  ...
    boolean result = false;

    if (onFilterTouchEventForSecurity(event)) {
        if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
            result = true;
        }
        //noinspection SimplifiableIfStatement
        ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnTouchListener != null
                && (mViewFlags & ENABLED_MASK) == ENABLED
                && li.mOnTouchListener.onTouch(this, event)) {
            result = true;
        }

        if (!result && onTouchEvent(event)) {
            result = true;
        }
    }
 ...
    return result;
}

实现判断onTouchListener中的onTouch（）若返回为true,就不会调用onTouchEvent()，这样可以方便在Activity等外界直接setOnTouchListener().进入onTouchEvent()

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    public boolean onTouchEvent(MotionEvent event) {
...
        final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE
                || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
                || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;

        if ((viewFlags & ENABLED_MASK) == DISABLED) {
            if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
                setPressed(false);
            }
            mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
            // A disabled view that is clickable still consumes the touch
            // events, it just doesn't respond to them.
            return clickable;
        ｝
		...
		}

注释解释了在View不可用状态下，但View是可点击的，View仍旧会消耗这个事件。

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final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE
        || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
        || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;

if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
    switch (action) {
        case MotionEvent.ACTION_UP:
            mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
            if ((viewFlags & TOOLTIP) == TOOLTIP) {
                handleTooltipUp();
            }
            if (!clickable) {
                removeTapCallback();
                removeLongPressCallback();
                mInContextButtonPress = false;
                mHasPerformedLongPress = false;
                mIgnoreNextUpEvent = false;
                break;
            }
            boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
            if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
                // take focus if we don't have it already and we should in
                // touch mode.
                boolean focusTaken = false;
                if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
                    focusTaken = requestFocus();
                }

                if (prepressed) {
                    // The button is being released before we actually
                    // showed it as pressed.  Make it show the pressed
                    // state now (before scheduling the click) to ensure
                    // the user sees it.
                    setPressed(true, x, y);
                }

                if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
                    // This is a tap, so remove the longpress check
                    removeLongPressCallback();

                    // Only perform take click actions if we were in the pressed state
                    if (!focusTaken) {
                        // Use a Runnable and post this rather than calling
                        // performClick directly. This lets other visual state
                        // of the view update before click actions start.
                        if (mPerformClick == null) {
                            mPerformClick = new PerformClick();
                        }
                        if (!post(mPerformClick)) {
                            performClick();
                        }
                    }
                }
            break;              
    }
    return true;
}

只有子View的CLICKABLE、LONG_CLICKABLE，CONTEXT_CLICKABLE，TOOLTIP有一个为true它就会返回true消耗该事件。当ACTION_UP时会调用performClick().

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public boolean performClick() {
    final boolean result;
    final ListenerInfo li = mListenerInfo;
    if (li != null && li.mOnClickListener != null) {
        playSoundEffect(SoundEffectConstants.CLICK);
        li.mOnClickListener.onClick(this);
        result = true;
    } else {
        result = false;
    }

    sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);

    notifyEnterOrExitForAutoFillIfNeeded(true);

    return result;
}

在performClick()若onClickListener不为空，则调用onClick()

本文标题:事件分发源码解析

文章作者:

发布时间:2019-02-04, 00:00:00

最后更新:2025-03-02, 10:44:42

原始链接:http://yoursite.com/2019/02/04/%E4%BA%8B%E4%BB%B6%E5%88%86%E5%8F%91%E6%BA%90%E7%A0%81%E8%A7%A3%E6%9E%90/

许可协议: "署名-非商用-相同方式共享 4.0" 转载请保留原文链接及作者。

文章目录
  1. 1. Activity分发
  2. 2. Window分发
  3. 3. 顶级View的分发
  4. 4. View对点击事件的处理
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