Android下的Touch事件分发

触摸事件的分发过程有三个重要的方法

Touch 事件相关方法	方法功能	ViewGroup	Activity
public boolean dispatchTouchEvent(MotionEvent ev)	事件分发	Yes	Yes
public boolean onInterceptTouchEvent(MotionEvent ev)	事件拦截	Yes	No
public boolean onTouchEvent(MotionEvent ev)	事件响应	Yes	Yes

public boolean dispatchTouchEvent(MotionEvent ev)

用来分发事件的，如果当前事件能传递到该View，该方法一定调用。

Touch 事件发生时 Activity 的 dispatchTouchEvent(MotionEvent ev) 方法会以隧道方式（从根元素依次往下传递直到最内层子元素或在中间某一元素中由于某一条件停止传递）将事件传递给最外层 View 的 dispatchTouchEvent(MotionEvent ev) 方法，并由该 View 的 dispatchTouchEvent(MotionEvent ev) 方法对事件进行分发。dispatchTouchEvent 的事件分发逻辑如下：

如果 return true，事件会分发给当前 View 并由 dispatchTouchEvent 方法进行消费，同时事件会停止向下传递；
如果 return false，事件分发分为两种情况：
1. 如果当前 View 获取的事件直接来自 Activity，则会将事件返回给 Activity 的 onTouchEvent 进行消费；
2. 如果当前 View 获取的事件来自外层父控件，则会将事件返回给父 View 的 onTouchEvent 进行消费。
如果返回系统默认的 super.dispatchTouchEvent(ev)，事件会自动的分发给当前 View 的 onInterceptTouchEvent 方法。

public boolean onInterceptHoverEvent(MotionEvent event)

用来拦截事件的，如果返回值为true，表示拦截。否则不拦截。
在外层 View 的 dispatchTouchEvent(MotionEvent ev) 方法返回系统默认的 super.dispatchTouchEvent(ev) 情况下，事件会自动的分发给当前 View 的 onInterceptTouchEvent 方法。onInterceptTouchEvent 的事件拦截逻辑如下：

如果 onInterceptTouchEvent 返回 true，则表示将事件进行拦截，并将拦截到的事件交由当前 View 的 onTouchEvent 进行处理；
如果 onInterceptTouchEvent 返回 false，则表示将事件放行，当前 View 上的事件会被传递到子 View 上，再由子 View 的 dispatchTouchEvent 来开始这个事件的分发；
如果 onInterceptTouchEvent 返回 super.onInterceptTouchEvent(ev)，事件默认会被拦截，并将拦截到的事件交由当前 View 的 onTouchEvent 进行处理。

public boolean onTouchEvent(MotionEvent event)

处理当前事件的。如果返回值为true表示消耗该事件。否则无法再接收同一个序列的事件。
如果dispatchTouchEvent 方法 return false，事件会以冒泡方式返回给上层的 onTouchEvent 进行消费

在 dispatchTouchEvent 返回 super.dispatchTouchEvent(ev) 并且 onInterceptTouchEvent 返回 true 或返回 super.onInterceptTouchEvent(ev) 的情况下 onTouchEvent 会被调用。onTouchEvent 的事件响应逻辑如下：

如果事件传递到当前 View 的 onTouchEvent 方法，而该方法返回了 false，那么这个事件会从当前 View 向上传递，并且都是由上层 View 的 onTouchEvent 来接收，如果传递到上面的 onTouchEvent 也返回 false，这个事件就会“消失”，而且接收不到下一次事件。
如果返回了 true 则会接收并消费该事件。
如果返回 super.onTouchEvent(ev) 默认处理事件的逻辑和返回 false 时相同。

同一个序列的事件是；DOWN事件－－》多个MOVE事件－－》UP事件。

事件传递的大概过程

Activity－－》Window－－》View。
某个View的onTouchEvent（）方法被调用。如果返回true，传递回Window，Window再传递回Activity，事件处理结束。否则返回false，再同样的传递回Activity。

ViewGroup重载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.            // 标志FLAG_DISALLOW_INTERCEPT可以通过requestDisallowInterceptTouchEvent方法设置。            // resetTouchState对DOWN事件重置状态，因此该标志不影响ViewGroup处理DOWN事件的操作。            cancelAndClearTouchTargets(ev);            resetTouchState();        }        // Check for interception.        final boolean intercepted;        if (actionMasked == MotionEvent.ACTION_DOWN                || mFirstTouchTarget != null) {            // 如果子View调用了requestDisallowInterceptTouchEvent（）方法后，ViewGroup将无法拦截除DOWN事件以外的其他事件。                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;        // 如果ViewGroup不拦截，ViewGroup遍历所有的子View，判断子View是否满足当前的事件。        // 满足的条件有两个：子View是否播放动画和事件的坐标是否在子View的区域。        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);                        // 调用了子View的dispatchTouchEvent（）方法。                        // 如果返回了true，就会通过addTouchTarget()方法对mFirstTouchTarget赋值并停止遍历子View。                        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();                            // mFirstTouchTarget是一个单链表的数据结构。                            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.        // 如果遍历全部的子View都没有成功处理的，mFirstTouchTarget成员变量为null        if (mFirstTouchTarget == null) {            // No touch targets so treat this as an ordinary view.            // 第三个参数为null，就会调用super.dispatchTouchEvent（）方法            // super是View，所以调用了View的dispatchTouchEvent（）方法。            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;}private void resetTouchState() {    clearTouchTargets();    resetCancelNextUpFlag(this);    mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;    mNestedScrollAxes = SCROLL_AXIS_NONE;}/** * 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) {    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;}