Android 触摸事件分发和处理机制解析（三）View篇

在我们自定义view或者嵌套view时，经常需要处理滑动事件，点击事件等各种交互事件。在处理过程中，我们可能会遇到事件不响应，滑动和点击事件冲突等问题，这时候，如果我们理解Android触摸事件的分发和处理，处理起来就会得心应手。

本系列分析前两篇，建议先看看：
Android 触摸事件分发和处理机制解析（一）Activity篇
Android 触摸事件分发和处理机制解析（二）ViewGroup篇

这一篇，我们就来看下View的触摸事件分发和处理。具体是 dispatchTouchEvent()，onTouchEvent()，和OnTouchListener的onTouch方法 。

强烈建议打开源码对照进行查看，便于熟悉代码。本文用的是Android 8.0 SDK的源码

先看View的dispatchTouchEvent()方法：

    /**
     * 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);
        }
        
		上面是辅助功能相关，不用管。
        boolean result = false;    //最终返回值

        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onTouchEvent(event, 0);
        }
		上面是调试代码，也不用管。
		
        final int actionMasked = event.getActionMasked();
        if (actionMasked == MotionEvent.ACTION_DOWN) {
            // Defensive cleanup for new gesture
            stopNestedScroll();
        }
        如果传入的动作是按下动作，则重置状态，准备处理一组新触摸事件的动作。
        
-----------------------重点是这段代码-------------------------------------------------
        if (onFilterTouchEventForSecurity(event)) {    过滤掉不需要处理的动作
            if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
                result = true;
            }
            如果是鼠标拖动引起的滑动事件，上面会处理
            下面这些，额外分析
            //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;
            }
        }
-----------------------重点是这段代码-------------------------------------------------
        if (!result && mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
        } 这是调试代码，不用管

        // 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();
        }
        最后把状态修正一下

        return result;
    }

接下来，我们重点分析中间那一段：

            //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;
            }

可以看到，这几行代码中出现了我们所关注的其它两个触摸事件处理方法：一个是OnTouchListener.onTouch()，一个是onTouchEvent()。
而且我们可以先得出一个结论：

结论一：
onTouch()是先于onTouchEvent()被调用的。
如果onTouch()返回false的话，onTouchEvent()才会被调用。

而如果onTouch返回true的话，onTouchEvent()是否就不会被调用了呢？这还得看其它几个判断条件：

ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null
                    && (mViewFlags & ENABLED_MASK) == ENABLED
                    && li.mOnTouchListener.onTouch(this, event)) {

在源码里找下这个mListenerInfo以及mOnTouchListener是在什么时候被赋值的，找到了唯一的一处：

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

那么这个getListenerInfo() 又会在哪儿被调用呢？我们会感兴趣的是这个：

    /**
     * Register a callback to be invoked when a touch event is sent to this view.
     * @param l the touch listener to attach to this view
     */
    public void setOnTouchListener(OnTouchListener l) {
        getListenerInfo().mOnTouchListener = l;
    }

源码里getListenerInfo()有多处调用，但是mOnTouchListener赋值的地方也只有这一处。
也就是说，在我们每次写代码时，如果调用了setOnTouchListener（）方法，并传入了一个OnTouchListener，则View类的mListenerInfo肯定就不会为null了，而且它的mOnTouchListener变量也会被赋值为我们传入的OnTouchListener。

看到这，我们再来看下刚才需要分析的代码：

ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null
                    && (mViewFlags & ENABLED_MASK) == ENABLED
                    && li.mOnTouchListener.onTouch(this, event)) {

也就是说，如果我们给View设置了onTouchListener的话，li != null && li.mOnTouchListener != null为true。
那么(mViewFlags & ENABLED_MASK) == ENABLED表示判断这个View是不是enable的状态。如果我们调用了setEnable(false)，则这儿为false。默认都是true。

因此把这段代码再串起来总结下，就是

结论二：如果我们没有给View设置OnTouchListener，也就不会有onTouch()。View的onTouchEvent()会直接被调用，其返回值就作为dispatchTouchEvent()的返回值。

结论三：如果我们给ViewView设置了OnTouchListener，则onTouch()会先执行。又分两种情况：
如果onTouch()返回true，则onTouchEvent()就不会执行了，dispatchTouchEvent()也返回true。
如果onTouch()返回false，则onTouchEvent()会执行，其返回值就作为dispatchTouchEvent()返回值。

那View的onTouchEvent()又是怎么处理的呢？

    /**
     * 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;
            }
        }

        如果进了下面这个流程，则最后都返回true。否则返回false。
        
        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);
                        }

--------------------------------重点部分--------------------------------------------------------
                        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();
                                }
                            }
                        }
--------------------------------重点部分--------------------------------------------------------
                        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;
    }

代码比较长，我们只关注重点部分。上面标识出来的那部分代码，总结一下也就一行：

performClick();

看下这个方法干了什么：

    /**
     * Call this view's OnClickListener, if it is defined.  Performs all normal
     * actions associated with clicking: reporting accessibility event, playing
     * a sound, etc.
     *
     * @return True there was an assigned OnClickListener that was called, false
     *         otherwise is returned.
     */
    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);

        notifyEnterOrExitForAutoFillIfNeeded(true);

        return result;
    }

看到这个方法里的判断，似曾相识对吧，mOnClickListener被赋值的逻辑和onTouchListener那段是一样的：如果我们给View设置了OnClickListener的话，mOnClickListener就是我们设置的这个对象。
区别在于，setOnClickListener()方法里，view会被自动地设置为clickable。

    /**
     * Register a callback to be invoked when this view is clicked. If this view is not
     * clickable, it becomes clickable.
     *
     * @param l The callback that will run
     *
     * @see #setClickable(boolean)
     */
    public void setOnClickListener(@Nullable OnClickListener l) {
        if (!isClickable()) {
            setClickable(true);
        }
        getListenerInfo().mOnClickListener = l;
    }

因此这一段串起来，结论就是：

结论四：onClick()方法是在onTouchEvent()中被调用的。如果我们给View设置了OnClickListener，则当触摸事件进行到抬起动作（ACTION_UP）时，onClick()方法会执行。

最后再列一下本篇的结论：

结论一：View的onTouch()是先于onTouchEvent()被调用的。两者都在View的dispatchTouchEvent()中被调用。

结论二：如果我们没有给View设置OnTouchListener（也就不会有onTouch()），View的onTouchEvent()会直接被调用，其返回值就作为dispatchTouchEvent()的返回值。

结论三：如果我们给ViewView设置了OnTouchListener，则onTouch()会先执行。又分两种情况：
如果onTouch()返回true，则onTouchEvent()就不会执行了，dispatchTouchEvent()也返回true。
如果onTouch()返回false，则onTouchEvent()会执行，其返回值就作为dispatchTouchEvent()返回值。

结论四：onClick()方法是在onTouchEvent()中被调用的。如果我们给View设置了OnClickListener，则当触摸事件进行到抬起动作（ACTION_UP）时，onClick()方法会执行。