专栏名称: 唤之

一步步探索学习Android Touch事件分发传递机制（三）

唤之 · 掘金 · 程序员 · 2017-12-08 08:32

正文

前言

该系列文章分三篇：

一步步探索学习Android Touch事件分发传递机制（一）通过写demo打Log，以ACTION_DOWN事件为例，完整了解整个Android Touch事件分发传递机制。

一步步探索学习Android Touch事件分发传递机制（二）探索了ACTION_MOVE和ACTION_UP事件的分发传递规律。

一步步探索学习Android Touch事件分发传递机制（三）即本篇，将通过Android源码分析，从本质上认识Android Touch事件分发传递机制。

1. dispatchTouchEvent()方法源码分析

1.1 Activity的dispatchTouchEvent()方法

源码：

   /**
     * 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) {
	        //当判断是ACTION_DOWN事件，回调onUserInteraction()
            onUserInteraction();
        }
        if (getWindow().superDispatchTouchEvent(ev)) {
	        //getWindow（）方法拿到的是PhoneWindow的实例
            return true;
        }
        //如果没有找到消费该事件的子View，最终会交给Activity的onTouchEvent()处理
        return onTouchEvent(ev);
    }

分析：
- 该方法首先判断传递进来的是不是一个ACTION_DOWN事件，if 是，就触发一个叫做 onUserInteraction()的回调方法。
- onUserInteraction()方法在Activity.java中是个空实现。开发者可以在需要的时候重写它，比如用于判断是不是用户开始做与屏幕交互的事情了。
- 然后，判断调用了getWindow().superDispatchTouchEvent(ev)。getWindow()拿到的是PhoneWindow的实例。
- 这里需要简单说明一下Android窗口结构：
- 继续看源码，看看PhoneWindow.java中的superDispatchTouchEvent(ev)方法。
```
 @Override
    public boolean superDispatchTouchEvent(MotionEvent event) {
        return mDecor.superDispatchTouchEvent(event);
    }
```
- 可以看到，这个方法内部又去调用了DecorView的superDispatchTouchEvent(ev)方法。
- 一路这么调用，DecorView其实是FrameLayout的子类，它没重写这个方法，所以最后调用到了ViewGroup的dispatchTouchEvent()方法。

1.2 ViewGroup的dispatchTouchEvent()方法

源码：

    @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<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();
                    }

                    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：

这个方法的代码逻辑比较多，有两百多行。我们去繁就简，理出主要的逻辑就行，毕竟我们只关心Touch事件的流向和处理逻辑。

首先来看一下，ViewGroup的dispatchTouchEvent()方法首先是要判断该事件自己要不要拦截下来自己处理。

     // 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;
            }

这里面我们看到一个很关键的boolean变量***disallowIntercept***，这个变量是控制是不是不允许父控件去拦截该事件的，取值是看mGroupFlags的取值。

这里面涉及到一个方法：***requestDisallowInterceptTouchEvent()***方法。这个方法确定mGroupFlags的取值，控制请求父布局不拦截该事件，而是交给自己去做处理。这个方法在处理滑动冲突等场景时经常用到。但在这里为了整个源码分析的逻辑简洁清晰，不再具体分析该方法的代码。

还有大家可以注意到一个判断条件： ***if (actionMasked == MotionEvent.ACTION_DOWN|| mFirstTouchTarget != null)***，也就是说ViewGroup去判断这个事件该不该去拦截，首先是这个事件得是ACTION_DOWN事件或者该事件的mFirstTouchTarget（目标子View）是不为空的才会考虑要不要拦截。
这说明mFirstTouchTarget为空的情况下，ACTION_MOVE和ACTION_UP事件是不会经过这个拦截判断的，而是直接intercepted = true表示事件被直接拦截掉。这一点刚好印证了我在一步步探索学习Android Touch事件分发传递机制（二）中提到的ACTION_MOVE和ACTION_UP事件的分发规律。
那么mFirstTouchTarget（目标子View）这个变量到底是什么呢？什么时候会为空呢？

可以看到ViewGroup的dispatchTouchEvent()方法的后续代码，是一个for循环：

...
 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.
		             
		                        }
		                        ...

这个for循环一个一个的遍历子View，寻找看事件的发生坐标在哪个View的范围中，如果找到了，就设置mFirstTouchTarget为child,并且把alreadyDispatchedToNewTouchTarget设置为true。
那么事件最终交给自己处理还是目标子View（mFirstTouchTarget）处理？
很简单，经过上面的分析可以知道，如果遍历完之后mFirstTouchTarget不为null，就传给mFirstTouchTarget（目标子View）处理；如果为null，就自己消费掉。
那其实不管是给目标子View处理还是自己处理，都会跑到View的dispatchTouchEvent()方法。看源码可以知道，当mFirstTouchTarget为null的时候，ViewGoup会调用super.dispatchTouchEvent(event),毕竟ViewGroup本质上是View的子类，所以其实ViewGroup调用的还是View的dispatchTouchEvent()方法。那么我们下面分析View的dispatchTouchEvent()方法。

1.3 View的dispatchTouchEvent()方法

源码：

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

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

分析：
- 代码量相对于ViewGroup的dispatchTouchEvnet()方法来说少很多。
- 主要是首先判断这个View本身有没有设置OnTouchListener监听，如果有，就直接跑去调用该接口下的onTouch()方法。该方法如果return true,这事件就是被消费掉了。return false,事件还是会传回给onTouchEvent()方法。
注意：值得注意的是，ViewGroup本身并没有重写View的onTouchEvnet()方法，所以这里如果回传，也是调用的父类View.java的onTouchEvent()方法。

2. onTouchEvent()方法源码分析

2.1 Activity的onTouchEvent()方法

源码：

 /**
     * Called when a touch screen event was not handled by any of the views
     * under it.  This is most useful to process touch events that happen
     * outside of your window bounds, where there is no view to receive it.
     *
     * @param event The touch screen event being processed.
     *
     * @return Return true if you have consumed the event, false if you haven't.
     * The default implementation always returns false.
     */
    public boolean onTouchEvent(MotionEvent event) {
        if (mWindow.shouldCloseOnTouch(this, event)) {
            finish();
            return true;
        }

        return false;
    }

分析：
- 这个方法代码很少，默认Activity的onTouchEvent()方法是返回false的，也就是说默认不处理触摸事件。
- 只有在PhoneWindow的shouldCloseOnTouch（）方法返回true才会处理触摸事件，直接finish整个Activity.

2.2 View的onTouchEvent()方法

前面说了，ViewGroup没有重写View的onTouchEvent()方法，所以继承VIewGroup时，调用的还是View的onTouchEvent()。

源码：

/**
     * Implement this method to handle touch screen motion events.
     * <p>
     * If this method is used to detect click actions, it is recommended that
     * the actions be performed by implementing and calling
     * {@link #performClick()}. This will ensure consistent system behavior,
     * including:
     * <ul>
     * <li>obeying click sound preferences
     * <li>dispatching OnClickListener calls
     * <li>handling {@link AccessibilityNodeInfo#ACTION_CLICK ACTION_CLICK} when
     * accessibility features are enabled
     * </ul>
     *
     * @param event The motion event.
     * @return True if the event was handled, false otherwise.
     */
    public boolean onTouchEvent(MotionEvent event) {
        final float x = event.getX();
        final float y = event.getY();
        final int viewFlags = mViewFlags;
        final int action = event.getAction();

        final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE
                || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
                || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;

        if ((viewFlags & ENABLED_MASK) == DISABLED) {
            if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
                setPressed(false);
            }
            mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
            // A disabled view that is clickable still consumes the touch
            // events, it just doesn't respond to them.
            return clickable;
        }
        if (mTouchDelegate != null) {
            if (mTouchDelegate.onTouchEvent(event)) {
                return true;
            }
        }

        if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
            switch (action) {
                case MotionEvent.ACTION_UP:
                    mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
                    if ((viewFlags & TOOLTIP) == TOOLTIP) {
                        handleTooltipUp();
                    }
                    if (!clickable) {
                        removeTapCallback();
                        removeLongPressCallback();
                        mInContextButtonPress = false;
                        mHasPerformedLongPress = false;
                        mIgnoreNextUpEvent = false;
                        break;
                    }
                    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:
                    if (event.getSource() == InputDevice.SOURCE_TOUCHSCREEN) {
                        mPrivateFlags3 |= PFLAG3_FINGER_DOWN;
                    }
                    mHasPerformedLongPress = false;

                    if (!clickable) {
                        checkForLongClick(0, x, y);
                        break;
                    }

                    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:
                    if (clickable) {
                        setPressed(false);
                    }
                    removeTapCallback();
                    removeLongPressCallback();
                    mInContextButtonPress = false;
                    mHasPerformedLongPress = false;
                    mIgnoreNextUpEvent = false;
                    mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
                    break;

                case MotionEvent.ACTION_MOVE:
                    if (clickable) {
                        drawableHotspotChanged(x, y);
                    }

                    // Be lenient about moving outside of buttons
                    if (!pointInView(x, y, mTouchSlop)) {
                        // Outside button
                        // Remove any future long press/tap checks
                        removeTapCallback();
                        removeLongPressCallback();
                        if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
                            setPressed(false);
                        }
                        mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
                    }
                    break;
            }

            return true;
        }

        return false;
    }

分析：
- 代码同样很长，那么我们只去理清主要的逻辑。
- 当事件传递到该方法处，首先会判断这个View是不是enabled状态，是不是clickable状态。
- 然后会根据Touch事件的类型做出不同的响应。比如View接收到Down事件和up事件等时候的表现效果。
注意：注意到一段比较重要的代码 if (!post(mPerformClick)) { performClick(); } 这段代码在判断是up事件之后调用了performClick()方法，这个方法回去回调onClickListener接口里面的onClick()方法。 这结合前面dispatchTouchEvent()方法中ACTION_DOWN事件会去调用onTouch，可见onTouch比onClick优先。

3. onInterceptTouchEvent()方法源码分析

只有ViewGroup有onInterceptTouchEvent()方法

源码：

  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;
    }

分析：
- 这个方法默认是返回false，表示不拦截触摸事件的
- 只有在 ev.isFromSource(InputDevice.SOURCE_MOUSE) && ev.getAction() == MotionEvent.ACTION_DOWN && ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY) && isOnScrollbarThumb(ev.getX(), ev.getY()) 这么多条件同时成立时才会拦截。
有需要的话，比如处理滑动冲突的时候，可以重写该方法，retrun true，拦截触摸事件。

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