android中的事件处理，以及解决滑动冲突问题都离不开事件分发机制，android中的事件流，即MotionEvent都会经历一个从分发，拦截到处理的一个过程。即dispatchTouchEvent(),onInterceptEvent()到onTouchEvent()的一个过程，在dispatchTouchEvent()负责了事件的分发过程，在dispatchTouchEvent()中会调用onInterceptEvent()与onTouchEvent()，如果onInterceptEvent()返回true，那么会调用到当前view的onTouchEvent()方法，如果不拦截，事件就会下发到子view的dispatchTouchEvent()中进行同样的操作。本文将带领大家从源码角度来分析android是如何进行事件分发的。

android中的事件分发流程最先从activity的dispatchTouchEvent()开始:

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

这里调用了getWindow().superDispatchTouchEvent(ev)，这里可以看出activity将MotionEvent传寄给了Window。而Window是一个抽象类，superDispatchTouchEvent()也是一个抽象方法，这里用到的是window的子类phoneWindow。

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

从这里可以看出，event事件被传到了DecorView,也就是我们的顶层view.我们继续跟踪：

public boolean superDispatchTouchEvent(MotionEvent event) {
    return super.dispatchTouchEvent(event);
}

这里调用到了父类的dispatchTouchEvent()方法，而DecorView是继承自FrameLayout，FrameLayout继承了ViewGroup,所以这里会调用到ViewGroup的dispatchTouchEvent()方法。

所以整个事件流从activity开始，传递到window，最后再到我们的view(viewGroup也是继承自view)中，而view才是我们整个事件处理的核心阶段。

我们来看一下viewGroup的dispatchTouchEvent()中的实现：

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();
        }

这是dispatchTouchEvent()开始时截取的一段代码，我们来看一下，首先，当我们手指按下view时，会调用到resetTouchState()方法，在resetTouchState()中：

private void resetTouchState() {
    clearTouchTargets();
    resetCancelNextUpFlag(this);
    mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
    mNestedScrollAxes = SCROLL_AXIS_NONE;
}

我们继续跟踪clearTouchTargets()方法：

private void clearTouchTargets() {
    TouchTarget target = mFirstTouchTarget;
    if (target != null) {
        do {
            TouchTarget next = target.next;
            target.recycle();
            target = next;
        } while (target != null);
        mFirstTouchTarget = null;
    }
}

在clearTouchTargets()方法中，我们最终将mFirstTouchTarget赋值为null，我们继续回到dispatchTouchEvent()中，接着执行了下段代码：

// 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;
        }

当view被按下或mFirstTouchTarget != null 的时候，从前面可以知道，当每次view被按下时，也就是重新开始一次事件流的处理时，mFirstTouchTarget都会被设置成null，一会我们看mFirstTouchTarget是什么时候被赋值的。
从disallowIntercept属性我们大概能猜到是用来判断是否需要坐拦截处理，而我们知道可以通过调用父view的requestDisallowInterceptTouchEvent(true)可以让我们的父view不能对事件进行拦截，我们先来看看requestDisallowInterceptTouchEvent()方法中的实现：

@Override
public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {

    if (disallowIntercept == ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0)) {
        // We're already in this state, assume our ancestors are too
        return;
    }

    if (disallowIntercept) {
        mGroupFlags |= FLAG_DISALLOW_INTERCEPT;
    } else {
        mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
    }

    // Pass it up to our parent
    if (mParent != null) {
        mParent.requestDisallowInterceptTouchEvent(disallowIntercept);
    }
}

这里也是通过设置标志位做判断处理，所以这里是通过改变mGroupFlags标志，然后在dispatchTouchEvent()刚发中变更disallowIntercept的值判断是否拦截，当为true时，即需要拦截，这个时候便会跳过onInterceptTouchEvent()拦截判断，并标记为不拦截，即intercepted = false，我们继续看viewGroup的onInterceptTouchEvent()处理：

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;
}

即默认情况下，只有在ACTION_DOWN时，viewGroup才会表现为拦截。

我们继续往下看：

final int childrenCount = mChildrenCount;
if (newTouchTarget == null && childrenCount != 0) {
      final float x = ev.getX(actionIndex);
      final float y = ev.getY(actionIndex);
      // Find a child that can receive the event.
      // Scan children from front to back.
      final ArrayList<View> preorderedList = buildTouchDispatchChildList();
      final boolean customOrder = preorderedList == null
                            && isChildrenDrawingOrderEnabled();
      final View[] children = mChildren;
      for (int i = childrenCount - 1; i >= 0; i--) {
            final int childIndex = getAndVerifyPreorderedIndex(
                                childrenCount, i, customOrder);
                        final View child = getAndVerifyPreorderedView(
                                preorderedList, children, childIndex);

                        // If there is a view that has accessibility focus we want it
                        // to get the event first and if not handled we will perform a
                        // normal dispatch. We may do a double iteration but this is
                        // safer given the timeframe.
                        if (childWithAccessibilityFocus != null) {
                            if (childWithAccessibilityFocus != child) {
                                continue;
                            }
                            childWithAccessibilityFocus = null;
                            i = childrenCount - 1;
                        }

                        if (!canViewReceivePointerEvents(child)
                                || !isTransformedTouchPointInView(x, y, child, null)) {
                            ev.setTargetAccessibilityFocus(false);
                            continue;
                        }

                        newTouchTarget = getTouchTarget(child);
                        if (newTouchTarget != null) {
                            // Child is already receiving touch within its bounds.
                            // Give it the new pointer in addition to the ones it is handling.
                            newTouchTarget.pointerIdBits |= idBitsToAssign;
                            break;
                        }

                        resetCancelNextUpFlag(child);
                        if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
                            // Child wants to receive touch within its bounds.
                            mLastTouchDownTime = ev.getDownTime();
                            if (preorderedList != null) {
                                // childIndex points into presorted list, find original index
                                for (int j = 0; j < childrenCount; j++) {
                                    if (children[childIndex] == mChildren[j]) {
                                        mLastTouchDownIndex = j;
                                        break;
                                    }
                                }
                            } else {
                                mLastTouchDownIndex = childIndex;
                            }
                            mLastTouchDownX = ev.getX();
                            mLastTouchDownY = ev.getY();
                            newTouchTarget = addTouchTarget(child, idBitsToAssign);
                            alreadyDispatchedToNewTouchTarget = true;
                            break;
                        }

                        // The accessibility focus didn't handle the event, so clear
                        // the flag and do a normal dispatch to all children.
                        ev.setTargetAccessibilityFocus(false);
                    }
                    if (preorderedList != null) preorderedList.clear();
                }

这段代码首先会通过一个循环去遍历所有的子view，最终会调用到dispatchTransformedTouchEvent()方法，我们继续看dispatchTransformedTouchEvent()的实现：

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;
}

这段代码就比较明显了，如果child不为null，始终会调用到child.dispatchTouchEvent();否则调用super.dispatchTouchEvent();

如果child不为null时，事件就会向下传递，如果子view处理了事件，即dispatchTransformedTouchEvent()即返回true。继续向下执行到addTouchTarget()方法，我们继续看addTouchTarget()方法的执行结果：

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

这个时候我们发现mFirstTouchTarget又出现了，这时候会给mFirstTouchTarget重新赋值，即mFirstTouchTarget不为null。也就是说，如果事件被当前view或子view消费了，那么在接下来的ACTION_MOVE或ACTION_UP事件中，mFirstTouchTarget就不为null。但如果我们继承了该viewGroup，并在onInterceptTouchEvent()的ACTION_MOVE中拦截了事件，那么后续事件将不会下发，将由该viewGroup直接处理，从下面代码我们可以得到：

// 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;
            }

当存在子view并且事件被子view消费时，即在ACTION_DOWN阶段mFirstTouchTarget会被赋值，即在接下来的ACTION_MOVE事件中，由于intercepted为true，所以将ACTION_CANCEL 事件传递过去，从dispatchTransformedTouchEvent()中可以看到：

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;
    }

并将mFirstTouchTarget 最终赋值为 next，而此时mFirstTouchTarget位于TouchTarget链表尾部，所以mFirstTouchTarget会赋值为null，那么接下来的事件将不会进入到onInterceptTouchEvent()中。也就会直接交由该view处理。

如果我们没有进行事件的拦截，而是交由子view去处理，由于ViewGroup的onInterceptTouchEvent()默认并不会拦截除了ACTION_DOWN以外的事件，所以后续事件将继续交由子view去处理，如果存在子view且事件位于子view内部区域的话。

所以无论是否进行拦截，事件流都会交由view的dispatchTouchEvent()中进行处理，我们接下来跟踪一下view中的dispatchTouchEvent()处理过程：

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;
        if (li != null && li.mOnTouchListener != null
                && (mViewFlags & ENABLED_MASK) == ENABLED
                && li.mOnTouchListener.onTouch(this, event)) {
            result = true;
        }

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

当被按下时，即ACTION_DOWN时，view会停止内部的滚动，如果view没有被覆盖或遮挡时，首先会进行mListenerInfo是否为空的判断，我们看下mListenerInfo是在哪里初始化的：

ListenerInfo getListenerInfo() {
    if (mListenerInfo != null) {
        return mListenerInfo;
    }
    mListenerInfo = new ListenerInfo();
    return mListenerInfo;
}

这里可以看出，mListenerInfo一般不会是null，知道在我们使用它时调用过这段代码，而当view被加入window中的时候，会调用下面这段代码，从注释中也可以看出来：

/**
 * Add a listener for attach state changes.
 *
 * This listener will be called whenever this view is attached or detached
 * from a window. Remove the listener using
 * {@link #removeOnAttachStateChangeListener(OnAttachStateChangeListener)}.
 *
 * @param listener Listener to attach
 * @see #removeOnAttachStateChangeListener(OnAttachStateChangeListener)
 */
public void addOnAttachStateChangeListener(OnAttachStateChangeListener listener) {
    ListenerInfo li = getListenerInfo();
    if (li.mOnAttachStateChangeListeners == null) {
        li.mOnAttachStateChangeListeners
                = new CopyOnWriteArrayList<OnAttachStateChangeListener>();
    }
    li.mOnAttachStateChangeListeners.add(listener);
}

到这里我们就知道，mListenerInfo一开始就是被初始化好了的，所以li不可能为null，li.mOnTouchListener != null即当设置了TouchListener时不为null，并且view是enabled状态，一般情况view都是enable的。这个时候会调用到onTouch()事件，当onTouch()返回true时，这个时候result会赋值true。而当result为true时，onTouchEvent()将不会被调用。

从这里可以看出，onTouch()会优先onTouchEvent()调用；
当view设置touch监听并返回true时，那么它的onTouchEvent()将被屏蔽。否则会调用onTouchEvent()处理。

那么让我们继续来看看onTouchEvent()中的事件处理：

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

首先，当view状态是DISABLED时，只要view是CLICKABLE或LONG_CLICKABLE或CONTEXT_CLICKABLE，都会返回true,而button默认是CLICKABLE的，textview默认不是CLICKABLE的，而view一般默认都不是LONG_CLICKABLE的。

我们继续向下看：

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

如果有代理事件，仍然会返回true.

if (((viewFlags & CLICKABLE) == CLICKABLE ||
            (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||
            (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {
        switch (action) {
            case MotionEvent.ACTION_UP:
                boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
                if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
                    // take focus if we don't have it already and we should in
                    // touch mode.
                    boolean focusTaken = false;
                    if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
                        focusTaken = requestFocus();
                    }

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

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

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

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

                    if (prepressed) {
                        postDelayed(mUnsetPressedState,
                                ViewConfiguration.getPressedStateDuration());
                    } else if (!post(mUnsetPressedState)) {
                        // If the post failed, unpress right now
                        mUnsetPressedState.run();
                    }

                    removeTapCallback();
                }
                mIgnoreNextUpEvent = false;
                break;

            case MotionEvent.ACTION_DOWN:
                mHasPerformedLongPress = false;

                if (performButtonActionOnTouchDown(event)) {
                    break;
                }

                // Walk up the hierarchy to determine if we're inside a scrolling container.
                boolean isInScrollingContainer = isInScrollingContainer();

                // For views inside a scrolling container, delay the pressed feedback for
                // a short period in case this is a scroll.
                if (isInScrollingContainer) {
                    mPrivateFlags |= PFLAG_PREPRESSED;
                    if (mPendingCheckForTap == null) {
                        mPendingCheckForTap = new CheckForTap();
                    }
                    mPendingCheckForTap.x = event.getX();
                    mPendingCheckForTap.y = event.getY();
                    postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
                } else {
                    // Not inside a scrolling container, so show the feedback right away
                    setPressed(true, x, y);
                    checkForLongClick(0, x, y);
                }
                break;

            case MotionEvent.ACTION_CANCEL:
                setPressed(false);
                removeTapCallback();
                removeLongPressCallback();
                mInContextButtonPress = false;
                mHasPerformedLongPress = false;
                mIgnoreNextUpEvent = false;
                break;

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

                // Be lenient about moving outside of buttons
                if (!pointInView(x, y, mTouchSlop)) {
                    // Outside button
                    removeTapCallback();
                    if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
                        // Remove any future long press/tap checks
                        removeLongPressCallback();

                        setPressed(false);
                    }
                }
                break;
        }

        return true;
    }

当view是CLICKABLE或LONG_CLICKABLE或CONTEXT_CLICKABLE状态时，当手指抬起时，如果设置了click监听，最终会调用到performClick()，触发click()事件。这点从performClick()方法中可以看出：

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

    sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
    return result;
}

从这里我们也可以得出，click事件会在onTouchEvent()中被调用，如果view设置了onTouch()监听并返回true，那么click事件也会被屏蔽掉，不过我们可以在onTouch()中通过调用view的performClick()继续执行click()事件，这个就看我们的业务中的需求了。

从这里我们可以看出，如果事件没有被当前view或子view处理，即返回false，那么事件就会交由外层view继续处理，直到被消费。

如果事件一直没有被处理，会最终传递到Activity的onTouchEvent()中。

到这里我们总结一下：
事件是从Activity->Window->View(ViewGroup)的一个传递流程；

如果事件没有被中途拦截，那么它会一直传到最内层的view控件；

如果事件被某一层拦截，那么事件将不会向下传递，交由该view处理。如果该view消费了事件，那么接下来的事件也会交由该view处理；如果该view没有消费该事件，那么事件会交由外层view处理，...并最终调用到activity的onTouchEvent()中，除非某一层消费了该事件；

一个事件只能交由一个view处理；

DispatchTouchEvent()总是会被调用，而且最先被调用，onInterceptTouchEvent()和onTouchEvent()在DispatchTouchEvent()内部调用；

子view不能干扰ViewGroup对ACTION_DOWN事件的处理；

子view可以通过requestDisallowInterceptTouchEvent(true)控制父view不对事件进行拦截，跳过onInterceptTouchEvent()方法的执行。