View的事件分发机制最新源码剖析

先抛出几个问题

1：触发View事件dispatchTouchEvent与onTouchEvent那个函数先执行？
2：onTouch消费事件的具体含义是什么？
3：onTouch，onClick回调方法的先后执行顺序？

先建立这样一个布局：

Button分别监听onTouchListener & onClickListener

        button1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                sb.append("onclick is called!" + "\n");
            }
        });

        button1.setOnTouchListener(new View.OnTouchListener() {
            @Override
            public boolean onTouch(View v, MotionEvent event) {
                sb.append("ontouch is called! event = " + event.getAction() + "\n");
                return isCheck;
            }
        });

isCheck由Checkbox控制默认为false;
默认情况下点击button，打印日志如下：

02-19 10:20:48.154 19082-19082/com.example.hongentao.touchdemo I/ViewActivity: ontouch is called! event = 0
02-19 10:20:48.154 19082-19082/com.example.hongentao.touchdemo I/ViewActivity: ontouch is called! event = 2
02-19 10:20:48.154 19082-19082/com.example.hongentao.touchdemo I/ViewActivity: ontouch is called! event = 2
02-19 10:20:48.154 19082-19082/com.example.hongentao.touchdemo I/ViewActivity: ontouch is called! event = 1
02-19 10:20:48.154 19082-19082/com.example.hongentao.touchdemo I/ViewActivity: onclick is called

选择CheckBox，消费掉OnTouch事件，打印日志如下：

02-19 10:23:43.694 19082-19082/com.example.hongentao.touchdemo I/ViewActivity: ontouch is called! event = 0
02-19 10:23:43.694 19082-19082/com.example.hongentao.touchdemo I/ViewActivity: ontouch is called! event = 2
02-19 10:23:43.694 19082-19082/com.example.hongentao.touchdemo I/ViewActivity: ontouch is called! event = 2
02-19 10:23:43.694 19082-19082/com.example.hongentao.touchdemo I/ViewActivity: ontouch is called! event = 1

查看打印日志，发现消费掉Ontouch事件后（也就是将返回值设为true），onClick事件没有得到执行。

那到底是什么原因导致的呢？

打开源码我们来分析一下：

Button是自定义的View,代码如下：

public class MyButton extends Button {
    private String TAG = MyButton.class.getSimpleName();

    public MyButton(Context context) {
        super(context);
    }

    public MyButton(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public MyButton(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent event) {
        Log.i(TAG, "disPatchTouchEvent is called!");
        return super.dispatchTouchEvent(event);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.i(TAG, "onTouchEvent is called!");
        return super.onTouchEvent(event);
    }

}

这里打印日志发现永远都是dispatchTouchEvent事件先执行，只要触发View的任何事件都会首先触发该方法。

源码为api-23, 老版本源码略有不同

我们找找dispatchTouchEvent到底在哪里？

分别在Button，TextView中都没有找到dispatchTouchEvent事件，最终在View中找到了该方法。

进入源码瞅一下：

/**
     * 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)) {
            //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;
    }

分析关键代码：

if (onFilterTouchEventForSecurity(event)) {
            //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;
            }
        }

onFilterTouchEventForSecurity(event)是判断改点击事件是否应该被传递，然后进入下一级判断条件，分别为四级判断条件
1：li != null
2： li.mOntouchListener != null
3：(mViewFlags & ENABLED_MASK) == ENABLED
4：li.mOnTouchListener.onTouch(this, event)

首先分析第一个判断条件：
ListenerInfo li = mListenerInfo;
mListenerInfo在那里被赋值的呢？
跟踪源码：

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

    public void setOnClickListener(@Nullable OnClickListener l) {
        if (!isClickable()) {
            setClickable(true);
        }
        getListenerInfo().mOnClickListener = l;
    }

在Button setOnclickListener监听事件的时候，调用getListenerInfo方法，getListenerInfo就是一个单例事件，从而li被赋值。

分析第二个判断条件：
li.mOntouchListener != null
这个又是什么鬼？
来来来，继续看源码

 /**
     * 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;
    }

通过查看源码，mTouchListener就是在setOntouchListener的时候被赋值的，一目了然。

再来看第三个判断条件：
(mViewFlags & ENABLED_MASK) == ENABLED
这个比较简单，就是判断当前的View是否是可点击的。

第四个判断条件为：
li.mOnTouchListener.onTouch(this, event)
这个更直接，直接获取onTouch的返回值：

借用这四种判断条件显而易见可以分析出，当Button消费掉ontouch事件之后，直接renturn true,onclick不会被执行。 而如果ontouch没有被消费，通过源码onclick肯定是在onTouchEvent(event)事件中被执行。

查看下源码：

 /**
     * Implement this method to handle touch screen motion events.
     * 

     * 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:
     * 

     * 
obeying click sound preferences
     * 
dispatching OnClickListener calls
     * 
handling {@link AccessibilityNodeInfo#ACTION_CLICK ACTION_CLICK} when
     * accessibility features are enabled
     * 
     *
     * @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();

        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);
        }

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

        return false;
    }

源码好长直接挑重点：
当Event为 ACTION_UP之后，经过一系列的判断会进入该段代码：

   if (!post(mPerformClick)) {performClick();}                            

进入performClick源码中看下：

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;
    }

果然看见了onclick事件被执行，现在答案也一一被解答了。

现在分别解答下开篇提出的三个问题：

1：触发View事件dispatchTouchEvent与onTouchEvent那个函数先执行？
答：只要是触发View的任何事件，都会首先触发dispatchTouchEvent事件。
2：onTouch消费事件的具体含义是什么？
答：ouTouch消费事件后，后续注册的事件回调都不会被执行，例如onClick。
3：onTouch，onClick回调方法的先后执行顺序？
答：分两种情况：1：onTouch消费掉事件, onTouch事件会被执行，onClick事件不会被执行。 2：onTouch没有消费掉事件，onTouch先执行，onCLick事件后执行。