Android事件分发机制源码畅游解析(ViewGroup篇)

CSDN怎么搞的，还是我操作问题！！！点了“写新文章”，结果编辑界面还是上一篇文章的内容，就没注意，清除开始写新的，结果还真真修改了以前的文章！纠结要不要重写，就问了下客服，说没办法，要么试着百度下原标题或网址，看有没有快照什么的。我擦擦..
后来不抱希望的搜索了下，百度没找到，google找到了，不过是另一个网站转载了，好吧，就不怪你侵权了哈哈…（偷偷告诉你，被人转载了还有点儿小高兴^_^）

事件分发机制view篇已经完结，哪些说的不对的地方多多指出。建议先了解上篇 Android事件分发机制源码畅游解析(View篇) ，因为最终执行的还是view中的内容。
本篇继续基于APILevel 25(7.1.1) 源码，理解为先、各版本代码可能有差别，核心部分是一致的。

1、找出事件分发的流程、循序

好了，接力开始，本文接着解析事件分发机制源码，上篇文章已经说了view中事件是从dispatchTouchEvent开始传递的，其实我们知道，一个界面至少有Activity、ViewGroup、View。那么我们就会有个疑问，触摸事件只是传给链条上某一个吗？要么是到达了ViewGroup，要么是View？
我们不妨胡乱猜测一下：比如直接到button，button直接会触发onclick，可是当view是一个textview时，点击事件怎么办，contain它的viewgroup不可能不要事件吧。那会不会是从View->ViewGroup向上传递事件呢？
行了，先别乱猜了，我们测试一下，基于上篇，先自定义一个MyLinearLayout（看起来下面的代码有点儿多，其实只用大概瞟一眼写了什么就行…）

1-1、MyLinearLayout

public class MyLinearLayout extends LinearLayout {    private static final String TAG = MyLinearLayout.class.getSimpleName();    public MyLinearLayout(Context context, @Nullable AttributeSet attrs) {        super(context, attrs);    }    @Override    public boolean dispatchTouchEvent(MotionEvent ev) {        int action = ev.getAction();        switch (action) {            case MotionEvent.ACTION_DOWN:                Log.i(TAG, "dispatchTouchEvent ACTION_DOWN");                break;            case MotionEvent.ACTION_MOVE:                Log.i(TAG, "dispatchTouchEvent ACTION_MOVE");                break;            case MotionEvent.ACTION_UP:                Log.i(TAG, "dispatchTouchEvent ACTION_UP");                break;            default:                break;        }        return super.dispatchTouchEvent(ev);    }    @Override    public boolean onTouchEvent(MotionEvent event) {        int action = event.getAction();        switch (action) {            case MotionEvent.ACTION_DOWN:                Log.i(TAG, "onTouchEvent ACTION_DOWN");                break;            case MotionEvent.ACTION_MOVE:                Log.i(TAG, "onTouchEvent ACTION_MOVE");                break;            case MotionEvent.ACTION_UP:                Log.i(TAG, "onTouchEvent ACTION_UP");                break;            default:                break;        }        return super.onTouchEvent(event);    }    @Override    public boolean onInterceptTouchEvent(MotionEvent ev) {        int action = ev.getAction();        switch (action) {            case MotionEvent.ACTION_DOWN:                Log.i(TAG, "onInterceptTouchEvent ACTION_DOWN");                break;            case MotionEvent.ACTION_MOVE:                Log.i(TAG, "onInterceptTouchEvent ACTION_MOVE");                break;            case MotionEvent.ACTION_UP:                Log.i(TAG, "onInterceptTouchEvent ACTION_UP");                break;            default:                break;        }        return super.onInterceptTouchEvent(ev);    }    @Override    public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {        Log.i(TAG, "requestDisallowInterceptTouchEvent ");        super.requestDisallowInterceptTouchEvent(disallowIntercept);    }}

1-2、button代码

@SuppressLint("AppCompatCustomView")public class MyButton extends Button {    private static final String TAG = MyButton.class.getSimpleName();    public MyButton(Context context, AttributeSet attrs) {        super(context, attrs);    }    @Override    public boolean onTouchEvent(MotionEvent event) {        int action = event.getAction();        switch (action) {            case MotionEvent.ACTION_DOWN:                Log.e(TAG, "onTouchEvent ACTION_DOWN");                break;            case MotionEvent.ACTION_MOVE:                Log.e(TAG, "onTouchEvent ACTION_MOVE");                break;            case MotionEvent.ACTION_UP:                Log.e(TAG, "onTouchEvent ACTION_UP");                break;        }        return super.onTouchEvent(event);    }    @Override    public boolean dispatchTouchEvent(MotionEvent event) {        int action = event.getAction();        switch (action) {            case MotionEvent.ACTION_DOWN:                Log.e(TAG, "dispatchTouchEvent ACTION_DOWN");                break;            case MotionEvent.ACTION_MOVE:                Log.e(TAG, "dispatchTouchEvent ACTION_MOVE");                break;            case MotionEvent.ACTION_UP:                Log.e(TAG, "dispatchTouchEvent ACTION_UP");                break;        }        return super.dispatchTouchEvent(event);    }}

1-3、MainActivity布局文件

<?xml version="1.0" encoding="utf-8"?><com.hds.viewevent.MyLinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:tools="http://schemas.android.com/tools"    android:id="@+id/main_layout"    android:layout_width="match_parent"    android:layout_height="match_parent"    tools:context="com.hds.viewevent.MainActivity">    <com.hds.viewevent.MyButton        android:id="@+id/button"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:text="Button" /></com.hds.viewevent.MyLinearLayout>

1-4、MainActivity代码

public class MainActivity extends AppCompatActivity {    private static final String TAG = MainActivity.class.getSimpleName();    private ImageView imageView;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        Button button = (Button) findViewById(R.id.button);        button.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                Log.e("button", "onclick");            }        });        button.setOnTouchListener(new View.OnTouchListener() {            @Override            public boolean onTouch(View v, MotionEvent event) {                Log.e("button", "onTouch");                return false;            }        });        MyLinearLayout myLinearLayout = (MyLinearLayout) findViewById(R.id.main_layout);        myLinearLayout.setOnTouchListener(new View.OnTouchListener() {            @Override            public boolean onTouch(View v, MotionEvent event) {                Log.i("MyLinearLayout", "onTouch");                return false;            }        });        myLinearLayout.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                Log.i("MyLinearLayout", "onclick");            }        });    }    @Override    public boolean dispatchTouchEvent(MotionEvent ev) {        int action = ev.getAction();        switch (action) {            case MotionEvent.ACTION_DOWN:                Log.d(TAG, "dispatchTouchEvent ACTION_DOWN");                break;            case MotionEvent.ACTION_MOVE:                Log.d(TAG, "dispatchTouchEvent ACTION_MOVE");                break;            case MotionEvent.ACTION_UP:                Log.d(TAG, "dispatchTouchEvent ACTION_UP");                break;            default:                break;        }        return super.dispatchTouchEvent(ev);    }    @Override    public boolean onTouchEvent(MotionEvent event) {        int action = event.getAction();        switch (action) {            case MotionEvent.ACTION_DOWN:                Log.d(TAG, "onTouchEvent ACTION_DOWN");                break;            case MotionEvent.ACTION_MOVE:                Log.d(TAG, "onTouchEvent ACTION_MOVE");                break;            case MotionEvent.ACTION_UP:                Log.d(TAG, "onTouchEvent ACTION_UP");                break;            default:                break;        }        return super.onTouchEvent(event);    }}

1-5、点击button测试
点了几次，点了一次完美的，^_^
看到了吧，事件是从Activity->ViewGroup->View的流程

2、先从ViewGroup说起

有人问了，为什么不直接从上到下呢。从上到下也可以，不过我们是源码解析，那样这篇文章将会无比巨长，可能到中途已经忘了开始是什么。我们将事件顺序反过来，从View->ViewGroup->Activity，就好比一个问题根源的寻找过程。
就来先讨论ViewGroup，根据上图的顺序，先来看dispatchTouchEvent

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;    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;        }        // 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);        }        //1.ACTION_CANCEL时,canceled=true是必然的。        //2.如果当前ViewGroup正处于detach状态，那么mPrivateFlags 的PFLAG_CANCEL_NEXT_UP_EVENT被置位。        //resetCancelNextUpFlag(this)也会为return true。        //3.非1、2的情况下，canceled = false;        // Check for cancelation.        final boolean canceled = resetCancelNextUpFlag(this)                || actionMasked == MotionEvent.ACTION_CANCEL;        // 查看mGroupFlags中的FLAG_SPLIT_MOTION_EVENTS位设置情况，看ViewGroup是否可以支持事件适时传给多个子view        // 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 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.                    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;                        }                        //child的mPrivateFlags中的PFLAG_CANCEL_NEXT_UP_EVENT位归0重置。                        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();                }                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;                }            }        }        // Dispatch to touch targets.        if (mFirstTouchTarget == null) {            // No touch targets so treat this as an ordinary view.            // 没有子View接收事件，ViewGroup就不能高看自己了，作为普通view处理事件吧。            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;            }        }        // 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;}

1、第18-24 部分，当事件是MotionEvent.ACTION_DOWN，做一些初始化的东西，如注释的英文所示：当开始一个touch操作时，清除之前所有的状态。可能由于switch, ANR, or some other state change，framework可能已经丢掉了之前操作的up和cancel事件

// 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();}

cancelAndClearTouchTargets()方法，取消和清除所有的touch target

private void cancelAndClearTouchTargets(MotionEvent event) {    if (mFirstTouchTarget != null) {        boolean syntheticEvent = false;        //当DetachedFromWindow时event = null        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) {            //view.mPrivateFlags的PFLAG_CANCEL_NEXT_UP_EVENT位设置为0            resetCancelNextUpFlag(target.child);            //第二个参数是true，意味传到onTouchEvent时，MotionEvent.Action_CANCEL，事件进入取消的case中,后面会详细分析            dispatchTransformedTouchEvent(event, true, target.child, target.pointerIdBits);        }        //进入看，将TouchTarget链条中的都recycle掉，目的是每一个target中的child=null;最重要的是初始化mFirstTouchTarget = null;        clearTouchTargets();        if (syntheticEvent) {            event.recycle();        }    }}

resetTouchState()取消一个touch状态，FLAG_DISALLOW_INTERCEPT位归0等。
2、第28-41 段部分，只要是 MotionEvent.ACTION_DOWN就可以进入，FLAG_DISALLOW_INTERCEPT是requestDisallowInterceptTouchEvent可动态设置的标识位，最终影响是否能走到onInterceptTouchEvent。一般disallowIntercept都是false，会走入onInterceptTouchEvent

//如果去看源码，注释无比巨长，其实就是当return true时，事件将不会传到子view中，大家可以根据本篇开头的demo中改成true试试。

public boolean onInterceptTouchEvent(MotionEvent ev) {    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;}

3、第89 行，preorderedList 得到有顺序的view，根据layout文件的先后顺序和addView的先后顺序。最先被加入的是0,接着是1,2,3…，后面循环取出倒序是因为我们肯定想要点中的是最靠近用户的一层view。
4、第111 行，判断处于循环中的view是否可以接收（是否处于visiable、animation中）事件；事件发生地(x/y)是否在view中。
5、第117 行，newTouchTarget = getTouchTarget(child);查看child是否在TouchTarget的链条中，在返回target，不再返回null。

    /**     * Gets the touch target for specified child view.     * Returns null if not found.     */    private TouchTarget getTouchTarget(@NonNull View child) {        for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {            if (target.child == child) {                return target;            }        }        return null;    }

6、第126 行，if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign))重点来看一下，哎呀，源代码好长。主要流程就是当child=null时，事件会走到ViewGroup的父类View.dispatchTouchEvent中；否则会都到child.dispatchTouchEvent的。然后的就跟上一篇（View篇）一样。
所以如果126行if判断成功，即有view处理了此事件。那么addTouchTarget就会执行，target.child = child，mFirstTouchTarget = target;mFirstTouchTarget赋值。

//根据情况将事件分发dispatchTouchEvent，所以有可能是分发给child，有可能是自我调用了。

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,        View child, int desiredPointerIdBits) {    final boolean handled;    //cancel = true的时候，分发的结果就是在view的onTouchEvent中取消状态、恢复初始状态的操作。    // 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;}

如果恰好是点击了一个类button，那么根据上面分析mFirstTouchTarget ！=null，那么在ACTION_DOWN之后的事件（如MOVE/UP）就会在第28 行判断走到else，结果就是intercepted = true;
又导致第62 行不能进入，此时61 行alreadyDispatchedToNewTouchTarget = false。
仔细分析你会发现最终会走到184 行，好嘛，还是target.child = child，child.dispatchTouchEvent。这个child还是在上一步addTouchTarget时候已接收ACTION_DOWN事件时保存的。

3、总结流程

上面说了那么多，说的短吧就不叫源码解析，说的细吧，说着说着都不知道飞到哪儿去了。其实运行下开头的demo，理解下事件的流程，然后再分析，会有更深入的理解。毕竟实践和直观现场才最能代入剧情。伪代码如下

ViewGroup.java...public boolean dispatchTouchEvent(MotionEvent ev) {    ...    if(onInterceptTouchEvent(ev)){        super.dispatchTouchEvent(event);//自身    } else {        if(child){//有可以接收事件的child            child.dispatchTouchEvent(event);        }else{            super.dispatchTouchEvent(event);//自身        }    }    ...}