Android事件分发机制

事件分发流程相关

一个事件发生后，首先从Acrtivity开始传递，然后一层一层往下传，从上往下调用dispatchTouchEvent方法传递事件：
Activity——>PhoneWindow——>DecorView——>ViewGroup——>…——>View

如果事件传递给最下层的View还没有被消费，就会按照反方向回传给Activity，从下往上调用onTouchEvent方法，最后会到Activity的onTouchEvent()函数，如果Activity也没有消费处理事件，这个事件就会被抛弃：
View——>…——>ViewGroup——>DecorView——>PhoneWindow——>Activity

当点击了一个布局里面的一个控件，会调用布局的dispatchTouchEvent(MotionEvent ev)方法，然后布局的dispatchTouchEvent(MotionEvent ev)方法中会找到被点击的控件，再调用控件的dispatchTouchEvent(MotionEvent ev)方法

在ViewGroup的dispatchTouchEvent()方法可以通过onInterceptTouchEvent()方法对事件传递进行拦截，onInterceptTouchEvent方法返回true代表不允许事件继续向子View传递，返回false代表不对事件进行拦截，默认返回false。

mOnTouchListener.onTouch()方法是在View的dispatchTouchEvent()方法中执行的，并且是先于View的onTouchEvent()方法执行的，如果 mOnTouchListener.onTouch()方法里返回了true，dispatchTouchEvent()方法就会直接返回true，不会再继续往下执行。

用伪代码表示ViewGroup的事件分发过程

    public boolean dispatchTouchEvent(MotionEvent ev) {
        boolean consume = false;//事件是否被消费
        if (onInterceptTouchEvent(ev)){//调用onInterceptTouchEvent判断是否拦截事件
            consume = onTouchEvent(ev);//如果拦截则调用自身的onTouchEvent方法消费掉事件
        }else{
            consume = child.dispatchTouchEvent(ev);//不拦截则调用子View的dispatchTouchEvent方法继续分发事件
        }
        return consume;//返回值表示事件是否被消费，true表示事件终止，false表示调用父View的onTouchEvent方法
    }

相关源码分析 ViewGroup有onInterceptTouchEvent()方法，View没有onInterceptTouchEvent()方法，onInterceptTouchEvent()在ViewGroup()的dispatchTouchEvent()方法中被调用。 dispatchTouchEvent()是分发事件，onTouchEvent()是处理事件，onInterceptTouchEvent()是拦截事件，onTouchEvent()方法只会在View的dispatchTouchEvent()方法中被调用。 View的dispatchTouchEvent()方法：

    
public boolean dispatchTouchEvent(MotionEvent event) {
    if (mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED &&
            mOnTouchListener.onTouch(this, event)) {
        return true;
    }
    return onTouchEvent(event);
}

onTouchEvent()方法只会在View的dispatchTouchEvent()方法中被调用。
按钮的点击事件是在onTouchEvent()方法中的perfORMClick()方法中被调用的。

ViewGroup的dispatchTouchEvent()方法是重写过的，比View的dispatchTouchEvent()方法复杂很多，会通过一个for循环判断各个子View是否是目标View，ViewGroup的的dispatchTouchEvent()方法：

    @Override
    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);
            }
            // 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 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 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();
                    }
                    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.
                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;
    }

分析这个方法的代码，首先会调用onInterceptTouchEvent()方法判断是否要拦截本次事件：

  intercepted = onInterceptTouchEvent(ev);

如果没有拦截掉事件，才会作相应的操作。
继续分析代码，这段代码非常关键：

            // 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);
            } 

如果没有找到目标子View处理这次事件，就调用dispatchTransformedTouchEvent()方法处理，看下dispatchTransformedTouchEvent()方法源码：

    
    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;
    }

里面这句非常关键:

 if (child == null) {
 	 handled = super.dispatchTouchEvent(event);
 } 

如果child为null，则调用super.dispatchTouchEvent(event)处理本次事件，即如果没有子View消费掉本次事件则调用ViewGroup的父类View中的dispatchTouchEvent()方法来消费本次事件。

5.onTouch()方法是OnTouchListener里的方法

    
    public interface OnTouchListener {
        
        boolean onTouch(View v, MotionEvent event);
    }

事件分发流程图

参考:
Android事件分发机制完全解析，带你从源码的角度彻底理解(上)
Android事件分发机制完全解析，带你从源码的角度彻底理解(下)
安卓事件分发学习之dispatchTouchEvent方法
深入理解onTouch与onInterceptTouchEvent透传原理
android事件拦截处理机制详解

Android事件分发机制面试题

https://www.androidos.net.cn/android/9.0.0_r8/xref/frameworks/base/core/java/android/app/Activity.java
Https://www.androidos.net.cn/android/9.0.0_r8/xref/frameworks/base/core/java/com/android/internal/policy/PhoneWindow.java
https://www.androidos.net.cn/android/9.0.0_r8/xref/frameworks/base/core/java/com/android/internal/policy/DecorView.java
https://www.androidos.net.cn/android/9.0.0_r8/xref/frameworks/base/core/java/android/view/ViewGroup.java

作者：yzpyzp