红橙Darren视频笔记 View事件分发源码分析基于API29

首先Android的事件分发是基于责任链设计模式的如果不理解责任链设计可以参考：
https://blog.csdn.net/u011109881/article/details/59631314
提示：本文结合自己的代码实践更有助于理解

一几个典型案例

public class MyView extends View {private static final String TAG = "MyView";public MyView(Context context) {this(context, null);}public MyView(Context context, @Nullable AttributeSet attrs) {this(context, attrs, 0);}public MyView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {super(context, attrs, defStyleAttr);this.setOnClickListener(new OnClickListener() {@Overridepublic void onClick(View v) {Log.d(TAG, "onClick: ");}});this.setOnTouchListener(new OnTouchListener() {@Overridepublic boolean onTouch(View v, MotionEvent event) {Log.d(TAG, "onTouch: " + event.getAction());return false;}});}@Overridepublic boolean onTouchEvent(MotionEvent event) {Log.d(TAG, "onTouchEvent: " + event.getAction());return super.onTouchEvent(event);}
}

此时打印log如下

12-19 15:09:35.292  8259  8259 D MyView  : onTouch: 0
12-19 15:09:35.292  8259  8259 D MyView  : onTouchEvent: 0
12-19 15:09:35.341  8259  8259 D MyView  : onTouch: 2
12-19 15:09:35.341  8259  8259 D MyView  : onTouchEvent: 2
12-19 15:09:35.359  8259  8259 D MyView  : onTouch: 2
12-19 15:09:35.359  8259  8259 D MyView  : onTouchEvent: 2
12-19 15:09:35.374  8259  8259 D MyView  : onTouch: 2
12-19 15:09:35.374  8259  8259 D MyView  : onTouchEvent: 2
12-19 15:09:35.391  8259  8259 D MyView  : onTouch: 2
12-19 15:09:35.391  8259  8259 D MyView  : onTouchEvent: 2
12-19 15:09:35.413  8259  8259 D MyView  : onTouch: 2
12-19 15:09:35.413  8259  8259 D MyView  : onTouchEvent: 2
12-19 15:09:35.459  8259  8259 D MyView  : onTouch: 2
12-19 15:09:35.459  8259  8259 D MyView  : onTouchEvent: 2
12-19 15:09:35.459  8259  8259 D MyView  : onTouch: 1
12-19 15:09:35.459  8259  8259 D MyView  : onTouchEvent: 1
12-19 15:09:35.459  8259  8259 D MyView  : onClick:

按照责任链的思路似乎onTouch的出在责任链发起端 onTouchEvent接着最后是 onClick事件

二分析原因 View的事件分发源码分析

二.一 OnTouchListener中的onTouch返回值可以决定onTouchEvent是否可以走到

    //事件分发的起点public boolean dispatchTouchEvent(MotionEvent event) {...//是否消费 false意味着不消费 事件继续向责任链后端传递// true意味着消费事件（吃掉事件） 事件无法向责任链后端传递boolean result = false;...if (actionMasked == MotionEvent.ACTION_DOWN) {//手势为下落的时候 停掉嵌套view的滚动// Defensive cleanup for new gesturestopNestedScroll();}if (onFilterTouchEventForSecurity(event)) {if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {result = true;}//noinspection SimplifiableIfStatementListenerInfo li = mListenerInfo;if (li != null && li.mOnTouchListener != null&& (mViewFlags & ENABLED_MASK) == ENABLED&& li.mOnTouchListener.onTouch(this, event)) {//要执行onTouch 需要满足条件//1 li != null 2 li.mOnTouchListener != null 3 (mViewFlags & ENABLED_MASK) == ENABLEDresult = 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;}

根据我们之前的分析来到责任链的最前端 line22的onTouch事件
1 li != null
2 li.mOnTouchListener != null
3 (mViewFlags & ENABLED_MASK) == ENABLED
要执行这个代码需要三个条件
为此我们要先看看li的类型ListenerInfo究竟是个啥

    static class ListenerInfo {/*** Listener used to dispatch focus change events.* This field should be made private, so it is hidden from the SDK.* {@hide}*/@UnsupportedAppUsageprotected OnFocusChangeListener mOnFocusChangeListener;/*** Listeners for layout change events.*/private ArrayList<OnLayoutChangeListener> mOnLayoutChangeListeners;protected OnScrollChangeListener mOnScrollChangeListener;/*** Listeners for attach events.*/private CopyOnWriteArrayList<OnAttachStateChangeListener> mOnAttachStateChangeListeners;/*** Listener used to dispatch click events.* This field should be made private, so it is hidden from the SDK.* {@hide}*/@UnsupportedAppUsagepublic OnClickListener mOnClickListener;/*** Listener used to dispatch long click events.* This field should be made private, so it is hidden from the SDK.* {@hide}*/@UnsupportedAppUsageprotected OnLongClickListener mOnLongClickListener;/*** Listener used to dispatch context click events. This field should be made private, so it* is hidden from the SDK.* {@hide}*/protected OnContextClickListener mOnContextClickListener;/*** Listener used to build the context menu.* This field should be made private, so it is hidden from the SDK.* {@hide}*/@UnsupportedAppUsageprotected OnCreateContextMenuListener mOnCreateContextMenuListener;@UnsupportedAppUsageprivate OnKeyListener mOnKeyListener;@UnsupportedAppUsageprivate OnTouchListener mOnTouchListener;@UnsupportedAppUsageprivate OnHoverListener mOnHoverListener;@UnsupportedAppUsageprivate OnGenericMotionListener mOnGenericMotionListener;@UnsupportedAppUsageprivate OnDragListener mOnDragListener;private OnSystemUiVisibilityChangeListener mOnSystemUiVisibilityChangeListener;OnApplyWindowInsetsListener mOnApplyWindowInsetsListener;OnCapturedPointerListener mOnCapturedPointerListener;private ArrayList<OnUnhandledKeyEventListener> mUnhandledKeyListeners;private WindowInsetsAnimationListener mWindowInsetsAnimationListener;/*** This lives here since it's only valid for interactive views.*/private List<Rect> mSystemGestureExclusionRects;/*** Used to track {@link #mSystemGestureExclusionRects}*/public RenderNode.PositionUpdateListener mPositionUpdateListener;}

我们看到ListenerInfo里面有我们熟悉的OnClickListener以及mOnTouchListener 那么我们在代码里调用setOnClickListener setOnTouchListener源码做了什么呢

    public void setOnClickListener(@Nullable OnClickListener l) {if (!isClickable()) {setClickable(true);}getListenerInfo().mOnClickListener = l;}public void setOnTouchListener(OnTouchListener l) {getListenerInfo().mOnTouchListener = l;}@UnsupportedAppUsageListenerInfo getListenerInfo() {if (mListenerInfo != null) {return mListenerInfo;}mListenerInfo = new ListenerInfo();return mListenerInfo;}

哦吼，发现了ListenerInfo是一系列Listener的集合，在我们调用setOnTouchListener的时候有两个条件已经满足
1 li != null 2 li.mOnTouchListener != null 那么剩下的条件3 (mViewFlags & ENABLED_MASK) == ENABLED如何判断呢
(mViewFlags & ENABLED_MASK) == ENABLED是判断当前点击的控件是否是enable的，默认都是enable的（这点我是听视频里面说的 有个实验可以验证这一点就是在代码里面调用View的setEnabled(false)方法发现即使注册了OnTouchListener 里面的onTouch也不会执行了）
说到这里前面三个条件都成立了那么li.mOnTouchListener.onTouch(this, event)开始执行，这就调用到我们代码里面的onTouch代码块了，而他的返回值决定了result(是否消费/吃掉事件)，也就是事件是否继续执行，如果我们在onTouch返回了false（不吃掉事件）result维持默认值false，那么事件继续传递 line28 if (!result && onTouchEvent(event))得以继续执行，否则result=true onTouch无法继续执行（可以实验在OnTouchListener 里面的onTouch返回true看看）

二.二onTouchEvent中的返回值可以决定onClick是否可以走到

同样是基于之前责任链设计思路 onClick必定在onTouchEvent之后调用，那么是在onTouchEvent内部呢还是在onTouchEvent调用之后呢？我们查看onTouchEvent的内部代码可以发现答案是前者

    public boolean onTouchEvent(MotionEvent event) {...if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {switch (action) {case MotionEvent.ACTION_UP:...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)) {performClickInternal();}}}...break;case MotionEvent.ACTION_DOWN:...break;case MotionEvent.ACTION_CANCEL:...break;case MotionEvent.ACTION_MOVE:...break;}return true;}return false;}private final class PerformClick implements Runnable {@Overridepublic void run() {recordGestureClassification(TOUCH_GESTURE_CLASSIFIED__CLASSIFICATION__SINGLE_TAP);performClickInternal();}}private boolean performClickInternal() {// Must notify autofill manager before performing the click actions to avoid scenarios where// the app has a click listener that changes the state of views the autofill service might// be interested on.notifyAutofillManagerOnClick();return performClick();}public boolean performClick() {// We still need to call this method to handle the cases where performClick() was called// externally, instead of through performClickInternal()notifyAutofillManagerOnClick();final boolean result;final ListenerInfo li = mListenerInfo;if (li != null && li.mOnClickListener != null) {playSoundEffect(SoundEffectConstants.CLICK);li.mOnClickListener.onClick(this);//最终目的地result = true;} else {result = false;}sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);notifyEnterOrExitForAutoFillIfNeeded(true);return result;}

可以看到在MotionEvent.ACTION_UP会调用PerformClick方法其中会performClickInternal，最后调用的performClick中会调用到onClick事件
因此总结下来 View的事件分发的顺序是这样的
dispatchTouchEvent->OnTouchListener onTouch DOWN事件->onTouchEvent DOWN事件
->OnTouchListener onTouchMOVE事件->onTouchEvent MOVE事件
->OnTouchListener onTouchUP事件->onTouchEventUP事件
->OnClickListener onClick
其中任意一个环节事件被吃掉（返回true）则后续的链子都断掉（收不到后续事件），不过这仅限于后面的层级，（不是直接把链条中间断开，后面的事件都接收不到的意思）
换一种说法，我们可以可以将OnTouchListener onTouchEvent OnClickListener 想象成3层不透光的纸，上面有个开关，return false可以打开开关让光投到下一层，在上面的一系列事件中，比如我们 OnTouchListener onTouch MOVE的时候关闭了开关，那么后面两层的事件都无法接受到后续事件，但是注意，OnTouchListener onTouchUP事件还是可以接收到的，开关只影响后面的层级。说到这个开关其实只有OnTouchListener onTouchEvent 有而已，活用这个开关，可以让我们在事件中只接受特定的事件。
https://blog.csdn.net/fengluoye2012/article/details/83782042
这里有个参考链接里面的图片很适合理解事件分发