事件分发拦截消费源码解读

事件源头

touch事件（用MotionEvent表示）发生时，事件最先传递给当前的Activity, 由Activity的dispatchTouchEvent进行分发：
android7.0.0_r4\androidn\frameworks\base\core\java\android\app\Activity.java

    /**     * Called to process touch screen events.  You can override this to     * intercept all touch screen events before they are dispatched to the     * window.  Be sure to call this implementation for touch screen events     * that should be handled normally.     *     * @param ev The touch screen event.     *     * @return boolean Return true if this event was consumed.     */    public boolean dispatchTouchEvent(MotionEvent ev) {        if (ev.getAction() == MotionEvent.ACTION_DOWN) {            onUserInteraction(); // 空实现        }        if (getWindow().superDispatchTouchEvent(ev)) { //getWindow()返回的是PhoneWindow对象             return true;        }        return onTouchEvent(ev); // 如果没有View处理则回调Activity.onTouchEvent();    }

android7.0.0_r4\androidn\frameworks\base\core\java\com\android\internal\policy\PhoneWindow.java

    @Override    public boolean superDispatchTouchEvent(MotionEvent event) {        // mDecor(DecorView类型对象）is the top-level view        // of the window, containing the window decor.        // mDecor.superDispatchTouchEvent(event)实际调用的        // 是super.dispatchTouchEvent(event);        return mDecor.superDispatchTouchEvent(event);            }

根据上面类图，mDecor.superDispatchTouchEvent(event)最终调用的是ViewGrop.dispatchTouchEvent(event)
android7.0.0_r4\androidn\frameworks\base\core\java\android\view\ViewGroup.java

    @Override    public boolean dispatchTouchEvent(MotionEvent ev) {        ....        boolean handled = false;        // onFilterTouchEventForSecurity 应用安全策略过滤touch event,比如窗口模糊或被遮掩（此时        // 返回false）,正常返回true;        if (onFilterTouchEventForSecurity(ev)) {            final int action = ev.getAction();            // MotionEvent.ACTION_MASK = 0xff            final int actionMasked = action & MotionEvent.ACTION_MASK;            // stage 1 ---> 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);  // 注意回调mFirstTouchTarget = null;                resetTouchState(); // 注意回调mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;                        }            // stage 2 ---> Check for interception.            final boolean intercepted;            if (actionMasked == MotionEvent.ACTION_DOWN                    || mFirstTouchTarget != null) { // DOWN事件或者DOWN事件被它的child view处理                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; // 调用requestDisableInterceptTouchEvent方法后则该boolean为true                if (!disallowIntercept) {                    // onInterceptTouchEvent 返回true则该MotionEvent不传给child view,                     // 而被这个ViewGrop onTouchEvent消费                    intercepted = onInterceptTouchEvent(ev);                    ev.setAction(action); // restore action in case it was changed                } else {                    intercepted = false;                }            } else { // 不是DOWN事件并且DOWN事件没被它的child view拦截消费，继续拦截该事件                // 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);            }            // stage 3 ---> Check for cancelation.            final boolean canceled = resetCancelNextUpFlag(this)                    || actionMasked == MotionEvent.ACTION_CANCEL;            // stage 4 ---> Update list of touch targets for pointer down, if needed.            final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;  // split常为true            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) { // 只处理down和hover_move(值7，move值2)事件                    final int actionIndex = ev.getActionIndex(); // always 0 for down                    final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)                            : TouchTarget.ALL_POINTER_IDS; // idBitsToAssign常为1                    // Clean up earlier touch targets for this pointer id in case they                    // have become out of sync.                    removePointersFromTouchTargets(idBitsToAssign);                    final int childrenCount = mChildrenCount; // 当前ViewGroup的child view个数                    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.                        // 下面这句根据该ViewGroup的child views的Z序是否为0，为0返回空(最顶层的child view                        // 的Z序不为0)，不为0返回child views list;                        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); // customOrder为false返回i                            final View child = getAndVerifyPreorderedView(                                    preorderedList, children, childIndex);// preorderedList不为null返回它list里面的子项，否则返回mChildren[]数组的子项                            // 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) {// childWithAccessibilityFocus常null                                if (childWithAccessibilityFocus != child) {                                    continue;                                }                                childWithAccessibilityFocus = null;                                i = childrenCount - 1;                            }                            // 检查child控件是否正在动画或者点击区域是否在child控件区域内                            if (!canViewReceivePointerEvents(child)                                    || !isTransformedTouchPointInView(x, y, child, null)) {                                ev.setTargetAccessibilityFocus(false);                                continue;                            }                            // 从mFirstTouchTarget及其链表中找TouchTarget对象的view成员是否和child对象一致，找到则返回该TouchTarget,否则返回null                            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);// mFirstTouchTarget = newTouchTarget;                                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);                        } // end of for (int i = childrenCount - 1; i >= 0; i--) {                        if (preorderedList != null) preorderedList.clear();                    } // end of if (newTouchTarget == null && childrenCount != 0) {                    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;                    }                } // end of if (actionMasked == MotionEvent.ACTION_DOWN....)            } // end of if (!canceled && !intercepted)            // stage 5 ---> 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;                }            }            // stage 6 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);            }        } // end of onFilterTouchEventForSecurity        if (!handled && mInputEventConsistencyVerifier != null) {            mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);        }        return handled;    }

这个函数的代码太长了，没关系，我们从头理起就更好理解了，这个头是什么呢？我们知道touch事件最先是down事件,后面可能跟着一丢move，然后是up,所以我们的头就是down事件，我们带着下面的问题看代码:
问题1： ViewGroup拦截down事件，down事件怎么分发消费？
问题2：ViewGroup没有拦截down事件，down事件又怎么分发消费？

---

问题1 ViewGroup拦截down事件

stage 1: mFirstTouchTarget将置null，mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
stage 2: disallowIntercept返回false, boolean intercepted的值取决于onInterceptTouchEvent函数的返回值，假如该函数返回true，也即拦截down事件；

• stage 3: boolean canceled将返回false；
• stage 4: 由于intercepted为true导致if (!canceled && !intercepted)返回false，所以stage 4不走；
• stage 5: 由于mFirstTouchTarget在stage 1时被置null，所以执行handled = dispatchTransformedTouchEvent(ev, canceled, null, TouchTarget.ALL_POINTER_IDS)，从下面的代码看最终会回调handled = super.dispatchTouchEvent(event)这句代码，由于ViewGroup继承View类，所以最终调用View.dispatchTouchEvent, 看代码如果该root viewGroup属性enable并且通过setOnTouchListener设置了Touch Listener, 会回调该Listener的onTouch()接口，如果onTouch返回false才会调用onTouchEvent方法，否则不执行onTouchEvent方法。

    /**     * 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) { // 这个函数很重要        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) { // 常相等            if (child == null || child.hasIdentityMatrix()) { // child不为null时hasIdentityMatrix()常为true                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); // 分发看child是否拦截和消费(注意View.java没有拦截接口)                    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;    }

android7.0.0_r4\androidn\frameworks\base\core\java\android\view\View.java

    /**     * Pass the touch screen motion event down to the target view, or this     * view if it is the target.     *     * @param event The motion event to be dispatched.     * @return True if the event was handled by the view, false otherwise.     */    public boolean dispatchTouchEvent(MotionEvent event) {        // If the event should be handled by accessibility focus first.        if (event.isTargetAccessibilityFocus()) {            // We don't have focus or no virtual descendant has it, do not handle the event.            if (!isAccessibilityFocusedViewOrHost()) {                return false;            }            // We have focus and got the event, then use normal event dispatch.            event.setTargetAccessibilityFocus(false);        }        boolean result = false;        if (mInputEventConsistencyVerifier != null) {            mInputEventConsistencyVerifier.onTouchEvent(event, 0);        }        final int actionMasked = event.getActionMasked();        if (actionMasked == MotionEvent.ACTION_DOWN) {            // Defensive cleanup for new gesture            stopNestedScroll();        }        if (onFilterTouchEventForSecurity(event)) {            if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {                result = true;            }            //noinspection SimplifiableIfStatement            ListenerInfo li = mListenerInfo;            // 属性ENABLE并且设置了OnTouchListener接口，执行该接口onTouch回调，            // 并根据该回调返回值决定是否走onTouchEvent方法            if (li != null && li.mOnTouchListener != null                    && (mViewFlags & ENABLED_MASK) == ENABLED                    && li.mOnTouchListener.onTouch(this, event)) { // 注意onTouch返回值对result的影响                result = true;            }            if (!result && onTouchEvent(event)) { // 注意onTouchEvent返回值对result的影响                result = true;            }        }        if (!result && mInputEventConsistencyVerifier != null) {            mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);        }        // Clean up after nested scrolls if this is the end of a gesture;        // also cancel it if we tried an ACTION_DOWN but we didn't want the rest        // of the gesture.        if (actionMasked == MotionEvent.ACTION_UP ||                actionMasked == MotionEvent.ACTION_CANCEL ||                (actionMasked == MotionEvent.ACTION_DOWN && !result)) {            stopNestedScroll();        }        return result; // 如果onTouch和onTouchEvent返回false，则result为false    }

总结问题1：ViewGroup拦截down事件，down事件不会传给它的child view，而是被自己消费，如果该ViewGroup拦截down事件前设置了OnTouchListener监听接口则调用监听接口的onTouch回调，如果onTouch回调未消费该down事件（返回false）则down事件传给OnTouchEvent方法消费，如果onTouchEvent还未消费（返回false），则传给该ViewGroup的父ViewGroup消费。

---

问题2 ViewGroup不拦截down事件

stage 1: 将mFirstTouchTarget置null，mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
stage 2: disallowIntercept返回false, boolean intercepted的值取决于onInterceptTouchEvent函数的返回值，假如该函数返回false，也即不拦截down事件；

• stage 3: boolean canceled将返回false；
• stage 4: 由于intercepted为false导致if (!canceled && !intercepted)为true，所以进入stage 4,其伪代码如下图；
  
• stage 5: 如果stage 4没有消费down事件，则mFirstTouchTarget为null,该ViewGroup自己消费down事件。

总结问题2：ViewGroup拦截down事件不拦截down事件，down事件传给它的child view，如果child view没有拦截和消费down事件，则由该ViewGroup消费。

---

后续move/up事件

理清了问题1和问题2，也即理清了down事件（action值0）后面的move(action值2)和up(action值1)怎么分发和消费也可根据代码理清了，下面只针对问题3进行说明(备注cancel事件action值是3)：
问题3： 如果down事件被ViewGroup的child view消费了，而后该ViewGroup拦截了后续的move/up事件，消费了down事件的child view会消费move/up事件吗？
为了说明这个问题，写了个demo，布局和日志代码以及日志结果截图如下：

<?xml version="1.0" encoding="utf-8"?><com.hjq.view.EventBusParentLinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:id="@+id/linearlayout_parenteventbus"    android:layout_width="match_parent"    android:layout_height="match_parent"    android:orientation="vertical">    <com.hjq.view.EventBusLinearLayout xmlns:app="http://schemas.android.com/apk/res-auto"        android:id="@+id/linearlayout_eventbus"        android:layout_width="match_parent"        android:layout_height="wrap_content"        android:orientation="vertical">        <android.support.design.widget.AppBarLayout            android:layout_width="match_parent"            android:layout_height="wrap_content"            android:theme="@style/AppTheme.AppBarOverlay">            <android.support.v7.widget.Toolbar                android:id="@+id/toolbar"                android:layout_width="match_parent"                android:layout_height="?attr/actionBarSize"                android:background="?attr/colorPrimary"                app:popupTheme="@style/AppTheme.PopupOverlay" />        </android.support.design.widget.AppBarLayout>        <com.hjq.view.EventBusButton            android:id="@+id/btn_eventbus"            android:layout_width="match_parent"            android:layout_height="wrap_content"            android:text="Test"></com.hjq.view.EventBusButton>    </com.hjq.view.EventBusLinearLayout></com.hjq.view.EventBusParentLinearLayout>

public class EventBusLinearLayout extends LinearLayout {    private static final String TAG = "EventBusLinearLayout";    .....    @Override    public boolean dispatchTouchEvent(MotionEvent ev) {        boolean result = super.dispatchTouchEvent(ev);        Log.i(TAG, "dispatchTouchEvent(" + ev.getAction() + "): " + result);        return result;    }    @Override    public boolean onInterceptTouchEvent(MotionEvent ev) {        // boolean result = super.onInterceptTouchEvent(ev);        boolean result = false;        switch (ev.getAction()) {            case MotionEvent.ACTION_DOWN:                result = super.onInterceptTouchEvent(ev);                break;            case MotionEvent.ACTION_MOVE:                result = true;                break;        }        Log.i(TAG, "onInterceptTouchEvent(" + ev.getAction() + "): " + result);        return result;    }    @Override    public boolean onTouchEvent(MotionEvent event) {        // boolean result = super.onTouchEvent(event);        boolean result = false;        Log.i(TAG, "onTouchEvent(" + event.getAction() + "): " + result);        return result;    }}

public class EventBusButton extends Button {    private static final String TAG = "EventBusButton";    ....    @Override    public boolean dispatchTouchEvent(MotionEvent ev) {        boolean result = super.dispatchTouchEvent(ev);        Log.i(TAG, "dispatchTouchEvent(" + ev.getAction() + "): " + result);        return result;    }    @Override    public boolean onTouchEvent(MotionEvent event) {        boolean result = super.onTouchEvent(event);        // boolean result = false;        Log.i(TAG, "onTouchEvent(" + event.getAction() + "): " + result);        return result;    }}

07-12 17:38:41.861 7331-7331/com.hjq.contentproviderclient I/EventBusLinearLayout: onInterceptTouchEvent(0): false07-12 17:38:41.863 7331-7331/com.hjq.contentproviderclient I/EventBusButton: onTouchEvent(0): true07-12 17:38:41.863 7331-7331/com.hjq.contentproviderclient I/EventBusButton: dispatchTouchEvent(0): true07-12 17:38:41.863 7331-7331/com.hjq.contentproviderclient I/EventBusLinearLayout: dispatchTouchEvent(0): true07-12 17:38:41.863 7331-7331/com.hjq.contentproviderclient W/EventBusDemoActivity: dispatchTouchEvent(0): true07-12 17:38:41.880 7331-7331/com.hjq.contentproviderclient I/EventBusLinearLayout: onInterceptTouchEvent(2): true07-12 17:38:41.884 7331-7331/com.hjq.contentproviderclient I/EventBusButton: onTouchEvent(3): true07-12 17:38:41.884 7331-7331/com.hjq.contentproviderclient I/EventBusButton: dispatchTouchEvent(3): true07-12 17:38:41.884 7331-7331/com.hjq.contentproviderclient I/EventBusLinearLayout: dispatchTouchEvent(2): true07-12 17:38:41.884 7331-7331/com.hjq.contentproviderclient W/EventBusDemoActivity: dispatchTouchEvent(2): true07-12 17:38:41.897 7331-7331/com.hjq.contentproviderclient I/EventBusLinearLayout: onTouchEvent(2): false07-12 17:38:41.897 7331-7331/com.hjq.contentproviderclient I/EventBusLinearLayout: dispatchTouchEvent(2): false07-12 17:38:41.897 7331-7331/com.hjq.contentproviderclient W/EventBusDemoActivity: dispatchTouchEvent(2): false07-12 17:38:41.898 7331-7331/com.hjq.contentproviderclient I/EventBusLinearLayout: onTouchEvent(1): false07-12 17:38:41.898 7331-7331/com.hjq.contentproviderclient I/EventBusLinearLayout: dispatchTouchEvent(1): false07-12 17:38:41.898 7331-7331/com.hjq.contentproviderclient W/EventBusDemoActivity: dispatchTouchEvent(1): false

总结问题3：move/up事件不会分发给消费了down事件的child view，该child view会消费cancel事件，对于拦截move事件的ViewGroup会处理后续的move/up事件。