Android事件分发机制

Posted by Gityuan on September 19, 2015

本文源码来自andorid sdk 22,不同版本会有细微差别,但核心机制是一致的

一.概述

事件分发有多种类型, 本文主要介绍Touch相关的事件分发.

  • 整个事件分发流程中,会有大量MotionEvent对象,该对象用于记录所有与移动相关的事件,比如手指触摸屏幕事件。
  • 一次完整的MotionEvent事件,是从用户触摸屏幕到离开屏幕。整个过程的动作序列:ACTION_DOWN(1次) -> ACTION_MOVE(N次) -> ACTION_UP(1次),
  • 多点触摸,每一个触摸点Pointer会有一个id和index。对于多指操作,通过pointerindex来获取指定Pointer的触屏位置。比如,对于单点操作时获取x坐标通过getX(),而多点操作获取x坐标通过getX(pointerindex)

对于View,ViewGroup,Activity都能处理Touch事件, 它们之间处理的先后顺序和方法有所不同.

1.1 View

  • View是所有视图对象的父类,实现了动画相关的接口Drawable.Callback, 按键相关的接口KeyEvent.Callback, 交互相关的接口AccessibilityEventSource。比如Button继承自View。
  • TouchEvent事件处理相关的方法:
    • dispatchTouchEvent(MotionEvent event)
    • onTouchEvent(MotionEvent event)

1.2 ViewGroup

  • ViewGroup,是一个abstract类,一组View的集合,可以包含View和ViewGroup,是所有布局的父类或间接父类。继承了View,实现了ViewParent(用于与父视图交互的接口), ViewManager(用于添加、删除、更新子视图到Activity的接口)。比如常用的LinearLayout,RelativeLayout都是继承自ViewGroup。
  • TouchEvent事件处理相关的方法:
    • dispatchTouchEvent(MotionEvent event)
    • onInterceptTouchEvent(MotionEvent ev)
    • onTouchEvent(MotionEvent event)

1.3 Activity

  • Activity是Android四大基本组件之一,当手指触摸到屏幕时,屏幕硬件一行行不断地扫描每个像素点,获取到触摸事件后,从底层产生中断上报。再通过native层调用Java层InputEventReceiver中的dispatchInputEvent方法。经过层层调用,交由Activity的dispatchTouchEvent方法来处理。
  • TouchEvent事件处理相关的方法:
    • dispatchTouchEvent(MotionEvent event)
    • onTouchEvent(MotionEvent event)

二. 分发原理

Input系统—进程交互文章的小节[3.3]已介绍事件分发过程的前期工作. 当UI主线程收到底层上报的input事件,便会调用InputEventReceiver.dispachInputEvent方法.

input_event_dispatcher

2.1 DecorView.dispatchTouchEvent

[-> PhoneWindow.java ::DecorView]

public boolean dispatchTouchEvent(MotionEvent ev) {
    final Callback cb = getCallback();
    // [见小节2.2]
    return cb != null && !isDestroyed() && mFeatureId < 0 ? cb.dispatchTouchEvent(ev)
            : super.dispatchTouchEvent(ev);
}

此处cb是指Window的内部接口Callback. 对于Activity实现了Window.Callback接口. 故接下来调用Activity类.

2.2 Activity.dispatchTouchEvent

[-> Activity.java]

public boolean dispatchTouchEvent(MotionEvent ev) {
    if (ev.getAction() == MotionEvent.ACTION_DOWN) {
        //第一次按下操作时,用户希望能与设备进行交互,可通过实现该方法
        onUserInteraction();
    }

    //获取当前Activity的顶层窗口是PhoneWindow [见小节2.3]
    if (getWindow().superDispatchTouchEvent(ev)) {
        return true;
    }
    //当没有任何view处理时,交由activity的onTouchEvent处理
    return onTouchEvent(ev); // [见小节2.2.1]
}

如果重写Activity的该方法,则会在分发事件之前拦截所有的触摸事件. 另外此处getWindow()返回的是Activity的mWindow成员变量, 该变量赋值过程是在Activity.attach()方法, 可知其类型为PhoneWindow.

2.2.1 Activity.onTouchEvent

[-> Activity.java]

public boolean onTouchEvent(MotionEvent event) {
    //当窗口需要关闭时,消费掉当前event
    if (mWindow.shouldCloseOnTouch(this, event)) {
        finish();
        return true;
    }

    return false;
}

2.3 superDispatchTouchEvent

[-> PhoneWindow.java]

public boolean superDispatchTouchEvent(KeyEvent event) {
    return mDecor.superDispatcTouchEvent(event); // [见小节2.4]
}

PhoneWindow的最顶View是DecorView,再交由DecorView处理。而DecorView的父类的父类是ViewGroup,接着调用 ViewGroup.dispatchTouchEvent()方法。为了精简篇幅,有些中间函数调用不涉及关键逻辑,可能会直接跳过。

2.4 ViewGroup.dispatchTouchEvent

public boolean dispatchTouchEvent(MotionEvent ev) {
    ...
    boolean handled = false;
    //根据隐私策略而来决定是否过滤本次触摸事件,
    if (onFilterTouchEventForSecurity(ev)) {  // [见小节2.4.1]
        final int action = ev.getAction();
        final int actionMasked = action & MotionEvent.ACTION_MASK;

        if (actionMasked == MotionEvent.ACTION_DOWN) {
            // 发生ACTION_DOWN事件, 则取消并清除之前所有的触摸targets
            cancelAndClearTouchTargets(ev);
            resetTouchState(); // 重置触摸状态
        }

        // 发生ACTION_DOWN事件或者已经发生过ACTION_DOWN;才进入此区域,主要功能是拦截器
        //只有发生过ACTION_DOWN事件,则mFirstTouchTarget != null;
        final boolean intercepted;
        if (actionMasked == MotionEvent.ACTION_DOWN
                || mFirstTouchTarget != null) {
            //可通过调用requestDisallowInterceptTouchEvent,不让父View拦截事件
            final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
            //判断是否允许调用拦截器
            if (!disallowIntercept) {
                //调用拦截方法
                intercepted = onInterceptTouchEvent(ev);  // [见小节2.4.2]
                ev.setAction(action);
            } else {
                intercepted = false;
            }
        } else {
            // 当没有触摸targets,且不是down事件时,开始持续拦截触摸。
            intercepted = true;
        }
        ...


        //不取消事件,同时不拦截事件, 并且是Down事件才进入该区域
        if (!canceled && !intercepted) {
            //把事件分发给所有的子视图,寻找可以获取焦点的视图。
            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(); // down事件等于0
                final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
                        : TouchTarget.ALL_POINTER_IDS;

                removePointersFromTouchTargets(idBitsToAssign); //清空早先的触摸对象

                final int childrenCount = mChildrenCount;
                //第一次down事件,同时子视图不会空时
                if (newTouchTarget == null && childrenCount != 0) {
                    final float x = ev.getX(actionIndex);
                    final float y = ev.getY(actionIndex);
                    // 从视图最上层到下层,获取所有能接收该事件的子视图
                    final ArrayList<View> preorderedList = buildOrderedChildList(); // [见小节2.4.3]
                    final boolean customOrder = preorderedList == null
                            && isChildrenDrawingOrderEnabled();
                    final View[] children = mChildren;

                    /* 从最底层的父视图开始遍历,
                    ** 找寻newTouchTarget,并赋予view与 pointerIdBits;
                    ** 如果已经存在找寻newTouchTarget,说明正在接收触摸事件,则跳出循环。
                    */
                    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);

                        // 如果当前视图无法获取用户焦点,则跳过本次循环
                        if (childWithAccessibilityFocus != null) {
                            if (childWithAccessibilityFocus != child) {
                                continue;
                            }
                            childWithAccessibilityFocus = null;
                            i = childrenCount - 1;
                        }
                        //如果view不可见,或者触摸的坐标点不在view的范围内,则跳过本次循环
                        if (!canViewReceivePointerEvents(child)
                                || !isTransformedTouchPointInView(x, y, child, null)) {
                            ev.setTargetAccessibilityFocus(false);
                            continue;
                        }

                        newTouchTarget = getTouchTarget(child);
                        // 已经开始接收触摸事件,并退出整个循环。
                        if (newTouchTarget != null) {
                            newTouchTarget.pointerIdBits |= idBitsToAssign;
                            break;
                        }

                        //重置取消或抬起标志位
                        //如果触摸位置在child的区域内,则把事件分发给子View或ViewGroup
                        if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) { // [见小节2.4.4]
                            // 获取TouchDown的时间点
                            mLastTouchDownTime = ev.getDownTime();
                            // 获取TouchDown的Index
                            if (preorderedList != null) {
                                for (int j = 0; j < childrenCount; j++) {
                                    if (children[childIndex] == mChildren[j]) {
                                        mLastTouchDownIndex = j;
                                        break;
                                    }
                                }
                            } else {
                                mLastTouchDownIndex = childIndex;
                            }

                            //获取TouchDown的x,y坐标
                            mLastTouchDownX = ev.getX();
                            mLastTouchDownY = ev.getY();
                            //添加TouchTarget,则mFirstTouchTarget != null。 [见小节2.4.5]
                            newTouchTarget = addTouchTarget(child, idBitsToAssign);
                            //表示以及分发给NewTouchTarget
                            alreadyDispatchedToNewTouchTarget = true;
                            break;
                        }
                        ev.setTargetAccessibilityFocus(false);
                    }
                    // 清除视图列表
                    if (preorderedList != null) preorderedList.clear();
                }

                if (newTouchTarget == null && mFirstTouchTarget != null) {
                    //将mFirstTouchTarget的链表最后的touchTarget赋给newTouchTarget
                    newTouchTarget = mFirstTouchTarget;
                    while (newTouchTarget.next != null) {
                        newTouchTarget = newTouchTarget.next;
                    }
                    newTouchTarget.pointerIdBits |= idBitsToAssign;
                }
            }
        }

        // mFirstTouchTarget赋值是在通过addTouchTarget方法获取的;
        // 只有处理ACTION_DOWN事件,才会进入addTouchTarget方法。
        // 这也正是当View没有消费ACTION_DOWN事件,则不会接收其他MOVE,UP等事件的原因
        if (mFirstTouchTarget == null) {
            //没有触摸target,则由当前ViewGroup来处理
            handled = dispatchTransformedTouchEvent(ev, canceled, null,
                    TouchTarget.ALL_POINTER_IDS);
        } else {
            //如果View消费ACTION_DOWN事件,那么MOVE,UP等事件相继开始执行
            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;
            }
        }

        //当发生抬起或取消事件,更新触摸targets
        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);
        }
    } //此处大括号,是if (onFilterTouchEventForSecurity(ev))的结尾

    //通知verifier由于当前时间未处理,那么该事件其余的都将被忽略
    if (!handled && mInputEventConsistencyVerifier != null) {
        mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
    }
    return handled;
}

2.4.1 onFilterTouchEventForSecurity

public boolean onFilterTouchEventForSecurity(MotionEvent event) {
    if ((mViewFlags & FILTER_TOUCHES_WHEN_OBSCURED) != 0
            && (event.getFlags() & MotionEvent.FLAG_WINDOW_IS_OBSCURED) != 0) {
        //隐私包含,则丢弃该事件
        return false;
    }
    return true;
}

根据隐私策略来过滤触摸事件。当返回true,表示继续分发事件;当返回flase,表示该事件应该被过滤掉,不再进行任何分发。

2.4.2 onInterceptTouchEvent

public boolean onInterceptTouchEvent(MotionEvent ev) {
    return false;
}
  • 当返回true,表示该事件被当前视图拦截;
  • 当返回false,继续执行事件分发。

2.4.3 buildOrderedChildList

ArrayList<View> buildOrderedChildList() {
    final int count = mChildrenCount;
    if (count <= 1 || !hasChildWithZ()) return null;

    if (mPreSortedChildren == null) {
        mPreSortedChildren = new ArrayList<View>(count);
    } else {
        mPreSortedChildren.ensureCapacity(count);
    }

    final boolean useCustomOrder = isChildrenDrawingOrderEnabled();
    for (int i = 0; i < mChildrenCount; i++) {
        // 添加下一个子视图到列表
        int childIndex = useCustomOrder ? getChildDrawingOrder(mChildrenCount, i) : i;
        View nextChild = mChildren[childIndex];
        float currentZ = nextChild.getZ(); //获取Z轴

        int insertIndex = i;
        //按Z轴,从小到大排序所有的子视图
        while (insertIndex > 0 && mPreSortedChildren.get(insertIndex - 1).getZ() > currentZ) {
            insertIndex--;
        }
        mPreSortedChildren.add(insertIndex, nextChild);
    }
    return mPreSortedChildren;
}

获取一个视图组的先序列表,通过虚拟的Z轴来排序。

public float getZ() {
    return getElevation() + getTranslationZ();
}

getZ()用于获取Z轴坐标。屏幕只有x,y坐标,而Z是虚拟的,可通过setElevation(),setTranslationZ()或者setZ()方法来修改Z轴的坐标值。

2.4.4 dispatchTransformedTouchEvent

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
        View child, int desiredPointerIdBits) {
    final boolean handled;

    // 发生取消操作时,不再执行后续的任何操作
    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;

    //由于某些原因,发生不一致的操作,那么将抛弃该事件
    if (newPointerIdBits == 0) {
        return false;
    }

    //分发的主要区域
    final MotionEvent transformedEvent;
    //判断预期的pointer id与事件的pointer id是否相等
    if (newPointerIdBits == oldPointerIdBits) {
        if (child == null || child.hasIdentityMatrix()) {
            if (child == null) {
                //不存在子视图时,ViewGroup调用View.dispatchTouchEvent分发事件,再调用ViewGroup.onTouchEvent来处理事件
                handled = super.dispatchTouchEvent(event);  // [见小节2.4]
            } else {
                final float offsetX = mScrollX - child.mLeft;
                final float offsetY = mScrollY - child.mTop;
                event.offsetLocation(offsetX, offsetY);
                //将触摸事件分发给子ViewGroup或View;
                //如果是ViewGroup,则调用代码(2.1);
                //如果是View,则调用代码(3.1);
                handled = child.dispatchTouchEvent(event);

                event.offsetLocation(-offsetX, -offsetY); //调整该事件的位置
            }
            return handled;
        }
        transformedEvent = MotionEvent.obtain(event); //拷贝该事件,来创建一个新的MotionEvent
    } else {
        //分离事件,获取包含newPointerIdBits的MotionEvent
        transformedEvent = event.split(newPointerIdBits);
    }

    if (child == null) {
        //不存在子视图时,ViewGroup调用View.dispatchTouchEvent分发事件,再调用ViewGroup.onTouchEvent来处理事件
        handled = super.dispatchTouchEvent(transformedEvent);  // [见小节2.4]
    } else {
        final float offsetX = mScrollX - child.mLeft;
        final float offsetY = mScrollY - child.mTop;
        transformedEvent.offsetLocation(offsetX, offsetY);
        if (! child.hasIdentityMatrix()) {
            //将该视图的矩阵进行转换
            transformedEvent.transform(child.getInverseMatrix());
        }
        //将触摸事件分发给子ViewGroup或View;
        /如果是ViewGroup,则 [见小节2.4]; 如果是View,则[见小节2.5];
        handled = child.dispatchTouchEvent(transformedEvent);
    }

    //回收transformedEvent
    transformedEvent.recycle();
    return handled;
}

该方法是ViewGroup真正处理事件的地方,分发子View来处理事件,过滤掉不相干的pointer ids。当子视图为null时,MotionEvent将会发送给该ViewGroup。最终调用View.dispatchTouchEvent方法来分发事件。

2.4.5 addTouchTarget

private TouchTarget addTouchTarget(View child, int pointerIdBits) {
    TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
    target.next = mFirstTouchTarget;
    mFirstTouchTarget = target;
    return target;
}

调用该方法,获取了TouchTarget,同时mFirstTouchTarget不再为null。

2.5 View.dispatchTouchEvent

[-> View.java]

public boolean dispatchTouchEvent(MotionEvent event) {
    ...
    
    final int actionMasked = event.getActionMasked();
    if (actionMasked == MotionEvent.ACTION_DOWN) {
        //在Down事件之前,如果存在滚动操作则停止。不存在则不进行操作
        stopNestedScroll();
    }

    // mOnTouchListener.onTouch优先于onTouchEvent。
    if (onFilterTouchEventForSecurity(event)) {
        //当存在OnTouchListener,且视图状态为ENABLED时,调用onTouch()方法
        ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnTouchListener != null
                && (mViewFlags & ENABLED_MASK) == ENABLED
                && li.mOnTouchListener.onTouch(this, event)) {
            result = true; //如果已经消费事件,则返回True
        }
        //如果OnTouch()没有消费Touch事件则调用OnTouchEvent()
        if (!result && onTouchEvent(event)) { // [见小节2.5.1]
            result = true; //如果已经消费事件,则返回True
        }
    }

    if (!result && mInputEventConsistencyVerifier != null) {
        mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
    }

    // 处理取消或抬起操作
    if (actionMasked == MotionEvent.ACTION_UP ||
            actionMasked == MotionEvent.ACTION_CANCEL ||
            (actionMasked == MotionEvent.ACTION_DOWN && !result)) {
        stopNestedScroll();
    }

    return result;
}
  1. 先由OnTouchListener的OnTouch()来处理事件,当返回True,则消费该事件,否则进入2。
  2. onTouchEvent处理事件,的那个返回True时,消费该事件。否则不会处理

2.5.1 View.onTouchEvent

public boolean onTouchEvent(MotionEvent event) {
    final float x = event.getX();
    final float y = event.getY();
    final int viewFlags = mViewFlags;

    // 当View状态为DISABLED,如果可点击或可长按,则返回True,即消费事件
    if ((viewFlags & ENABLED_MASK) == DISABLED) {
        if (event.getAction() == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
            setPressed(false);
        }
        return (((viewFlags & CLICKABLE) == CLICKABLE ||
                (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE));
    }

    if (mTouchDelegate != null) {
        if (mTouchDelegate.onTouchEvent(event)) {
            return true;
        }
    }

    //当View状态为ENABLED,如果可点击或可长按,则返回True,即消费事件;
    //与前面的的结合,可得出结论:只要view是可点击或可长按,则消费该事件.
    if (((viewFlags & CLICKABLE) == CLICKABLE ||
            (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_UP:
                boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
                if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
                    boolean focusTaken = false;
                    if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
                        focusTaken = requestFocus();
                    }

                    if (prepressed) {
                        setPressed(true, x, y);
                   }

                    if (!mHasPerformedLongPress) {
                        //这是Tap操作,移除长按回调方法
                        removeLongPressCallback();

                        if (!focusTaken) {
                            if (mPerformClick == null) {
                                mPerformClick = new PerformClick();
                            }
                            //调用View.OnClickListener
                            if (!post(mPerformClick)) {
                                performClick();
                            }
                        }
                    }

                    if (mUnsetPressedState == null) {
                        mUnsetPressedState = new UnsetPressedState();
                    }

                    if (prepressed) {
                        postDelayed(mUnsetPressedState,
                                ViewConfiguration.getPressedStateDuration());
                    } else if (!post(mUnsetPressedState)) {
                        mUnsetPressedState.run();
                    }

                    removeTapCallback();
                }
                break;

            case MotionEvent.ACTION_DOWN:
                mHasPerformedLongPress = false;

                if (performButtonActionOnTouchDown(event)) {
                    break;
                }

                //获取是否处于可滚动的视图内
                boolean isInScrollingContainer = isInScrollingContainer();

                if (isInScrollingContainer) {
                    mPrivateFlags |= PFLAG_PREPRESSED;
                    if (mPendingCheckForTap == null) {
                        mPendingCheckForTap = new CheckForTap();
                    }
                    mPendingCheckForTap.x = event.getX();
                    mPendingCheckForTap.y = event.getY();
                    //当处于可滚动视图内,则延迟TAP_TIMEOUT,再反馈按压状态,用来判断用户是否想要滚动。默认延时为100ms
                    postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
                } else {
                    //当不再滚动视图内,则立刻反馈按压状态
                    setPressed(true, x, y);
                    checkForLongClick(0); //检测是否是长按
                }
                break;

            case MotionEvent.ACTION_CANCEL:
                setPressed(false);
                removeTapCallback();
                removeLongPressCallback();
                break;

            case MotionEvent.ACTION_MOVE:
                drawableHotspotChanged(x, y);

                if (!pointInView(x, y, mTouchSlop)) {
                    removeTapCallback();
                    if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
                        removeLongPressCallback();
                        setPressed(false);
                    }
                }
                break;
        }

        return true;
    }
    return false;
}

三. 总结

事件分发流程图:

touch

  1. onInterceptTouchEvent返回值true表示事件拦截, onTouch/onTouchEvent 返回值true表示事件消费。
  2. 触摸事件先交由Activity.dispatchTouchEvent。再一层层往下分发,当中间的ViewGroup都不拦截时,进入最底层的View后,开始由最底层的OnTouchEvent来处理,如果一直不消费,则最后返回到Activity.OnTouchEvent
  3. ViewGroup才有onInterceptTouchEvent拦截方法。在分发过程中,中间任何一层ViewGroup都可以直接拦截,则不再往下分发,而是交由发生拦截操作的ViewGroup的OnTouchEvent来处理。
  4. 子View可调用requestDisallowInterceptTouchEvent方法,来设置disallowIntercept=true,从而阻止父ViewGroup的onInterceptTouchEvent拦截操作。
  5. OnTouchEvent由下往上冒泡时,当中间任何一层的OnTouchEvent消费该事件,则不再往上传递,表示事件已处理。
  6. 如果View没有消费ACTION_DOWN事件,则之后的ACTION_MOVE等事件都不会再接收。
  7. 只要View.onTouchEvent是可点击或可长按,则消费该事件.
  8. onTouch优先于onTouchEvent执行,上面流程图中省略,onTouch的位置在onTouchEvent前面。当onTouch返回true,则不执行onTouchEvent,否则会执行onTouchEvent。onTouch只有View设置了OnTouchListener,且是enable的才执行该方法。

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