当我们触发点击事件时，会由Activity处理，再交给ViewGroup处理，ViewGroup又会交给子View处理，那么应该由哪一个子View处理呢？这就需要事件分发的机制了，下面我们就来分析一下View的事件分发机制。
Activity的构成
当我们写Activity时会调用setContentView（）方法来加载布局，setContentView（）实现如下：
public void setContentView(View view) {
getWindow().setContentView(view);
initWindowDecorActionBar();
}
接下来我们看一下getWindow（）：
mWindow又是什么呢？最终我们在attach（）方法发现了mWindow：
final void attach(Context context, ActivityThread aThread,
Instrumentation instr, IBinder token, int ident,
Application application, Intent intent, ActivityInfo info,
CharSequence title, Activity parent, String id,
NonConfigurationInstances lastNonConfigurationInstances,
Configuration config, String referrer, IVoiceInteractor voiceInteractor,
Window window, ActivityConfigCallback activityConfigCallback, IBinder assistToken,
IBinder shareableActivityToken) {
attachBaseContext(context);
mFragments.attachHost(null /*parent*/);
//构造PhoneWindow
mWindow = new PhoneWindow(this, window, activityConfigCallback);
而getWindow（）又指的是PhoneWindow，所以来看看PhoneWindow的setContentView（）方法:
@Override
public void setContentView(View view, ViewGroup.LayoutParams params) {
// Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
// decor, when theme attributes and the like are crystalized. Do not check the feature
// before this happens.
if (mContentParent == null) {
installDecor(); //1
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
view.setLayoutParams(params);
final Scene newScene = new Scene(mContentParent, view);
transitionTo(newScene);
} else {
mContentParent.addView(view, params);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
mContentParentExplicitlySet = true;
}
我们看看注释1处installDecor（）方法里面做了什么：
private void installDecor() {
mForceDecorInstall = false;
if (mDecor == null) {
mDecor = generateDecor(-1); //1
mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
mDecor.setIsRootNamespace(true);
if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
}
} else {
mDecor.setWindow(this);
}
if (mContentParent == null) {
mContentParent = generateLayout(mDecor); //2
...
}
...
}
}
然后我们查看注释1处generateDecor方法里做了什么：
protected DecorView generateDecor(int featureId) {
// System process doesn't have application context and in that case we need to directly use
// the context we have. Otherwise we want the application context, so we don't cling to the
// activity.
Context context;
if (mUseDecorContext) {
Context applicationContext = getContext().getApplicationContext();
if (applicationContext == null) {
context = getContext();
} else {
context = new DecorContext(applicationContext, this);
if (mTheme != -1) {
context.setTheme(mTheme);
}
}
} else {
context = getContext();
}
return new DecorView(context, featureId, this, getAttributes());
}
这里创建了一个DecorView，这个DecorView就是Activity中的根View。接着查看DecorView的源码,我们发现DecorView是PhoneWindow类的内部类，并且继承了FrameLayout。
我们再回到installDecor（）方法中，查看generateLayout（mDecor）做了什么:
protected ViewGroup generateLayout(DecorView decor) {
...
//根据不同的情况加载不同的布局给layoutResource
int layoutResource;
int features = getLocalFeatures();
// System.out.println("Features: 0x" + Integer.toHexString(features));
if ((features & ((1 << FEATURE_LEFT_ICON) | (1 << FEATURE_RIGHT_ICON))) != 0) {
if (mIsFloating) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
R.attr.dialogTitleIconsDecorLayout, res, true);
layoutResource = res.resourceId;
} else {
layoutResource = R.layout.screen_title_icons;
}
// XXX Remove this once action bar supports these features.
removeFeature(FEATURE_ACTION_BAR);
// System.out.println("Title Icons!");
} else if ((features & ((1 << FEATURE_PROGRESS) | (1 << FEATURE_INDETERMINATE_PROGRESS))) != 0
&& (features & (1 << FEATURE_ACTION_BAR)) == 0) {
// Special case for a window with only a progress bar (and title).
// XXX Need to have a no-title version of embedded windows.
layoutResource = R.layout.screen_progress;
// System.out.println("Progress!");
} else if ((features & (1 << FEATURE_CUSTOM_TITLE)) != 0) {
// Special case for a window with a custom title.
// If the window is floating, we need a dialog layout
if (mIsFloating) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
R.attr.dialogCustomTitleDecorLayout, res, true);
layoutResource = res.resourceId;
} else {
layoutResource = R.layout.screen_custom_title;
}
// XXX Remove this once action bar supports these features.
removeFeature(FEATURE_ACTION_BAR);
} else if ((features & (1 << FEATURE_NO_TITLE)) == 0) {
// If no other features and not embedded, only need a title.
// If the window is floating, we need a dialog layout
if (mIsFloating) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
R.attr.dialogTitleDecorLayout, res, true);
layoutResource = res.resourceId;
} else if ((features & (1 << FEATURE_ACTION_BAR)) != 0) {
layoutResource = a.getResourceId(
R.styleable.Window_windowActionBarFullscreenDecorLayout,
R.layout.screen_action_bar);
} else {
layoutResource = R.layout.screen_title; //1
}
// System.out.println("Title!");
} else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {
layoutResource = R.layout.screen_simple_overlay_action_mode;
} else {
// Embedded, so no decoration is needed.
layoutResource = R.layout.screen_simple;
// System.out.println("Simple!");
}
...
return contentParent;
}
PhoneWindow的generateLayout（）方法比较长，这里只截取了一小部分关键的代码，其主要内容就是根据不同的情况加载不同的布局给layoutResource。现在查看1处的布局R.layout.screen_title：
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:fitsSystemWindows="true">
<!-- Popout bar for action modes -->
<ViewStub android:id="@+id/action_mode_bar_stub"
android:inflatedId="@+id/action_mode_bar"
android:layout="@layout/action_mode_bar"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:theme="?attr/actionBarTheme" />
<FrameLayout
android:layout_width="match_parent"
android:layout_height="?android:attr/windowTitleSize"
style="?android:attr/windowTitleBackgroundStyle">
<TextView android:id="@android:id/title"
style="?android:attr/windowTitleStyle"
android:background="@null"
android:fadingEdge="horizontal"
android:gravity="center_vertical"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</FrameLayout>
<FrameLayout android:id="@android:id/content"
android:layout_width="match_parent"
android:layout_height="0dip"
android:layout_weight="1"
android:foregroundGravity="fill_horizontal|top"
android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>
由此看出，一个 Activity 包含一个 Window 对象，这个对象是由 PhoneWindow 来实现的。PhoneWindow 将 DecorView 作为整个应用窗口的根 View，而这个 DecorView 又将屏幕划分为两个区域：一个是 TitleView，另一个是 ContentView，而我们平常做应用所写的布局正是展示在ContentView中的。
解析View的事件分发机制
当我们点击屏幕是，就产生了点击事件，这个事件被封装成一个类 MotionEvent。
而当这个 MotionEvent 产生后，那么系统就会将这个 MotionEvent 传递给 View 的层级，MotionEvent在View中的层级传递过程就是点击事件分发。
事件分发的三个重要方法：
dispatchTouchEvent（MotionEvent ev）:用来进行事件的分发onInterceptTouchEvent（MotionEvent ev）:用来进行事件的拦截onTouchEvent（MotionEvent ev）:用来处理点击事件
View的事件分发机制
首先分析 ViewGroup 的 dispatchTouchEvent（MotionEvent ev）方法：
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
...
// Handle an initial down.
//判断事件是否为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);
//如果是，说明这是一个新的事件序列，需要初始化，将mFirstTouchTarget置为null
resetTouchState();
}
// Check for interception
final boolean intercepted;
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) { //1
//不允许拦截标志
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; //2
//如果允许拦截
if (!disallowIntercept) {
intercepted = onInterceptTouchEvent(ev);
ev.setAction(action); // restore action in case it was changed
} else {
intercepted = false;
}
}
//如果是up、move事件
else {
// There are no touch targets and this action is not an initial down
// so this view group continues to intercept touches.
intercepted = true;
}
...
return handled;
}
-
mFirstTouchTarget: 当前ViewGroup是否拦截了事件，如果拦截了，mFirstTouchTarget=null。 -
FLAG_DISALLOW_INTERCEPT：禁止ViewGroup 拦截除了DOWN之外的事件 -
当ViewGroup要拦截事件的时候，那么后续的事件序列都将交给它处理，而不用再调用
onInterceptTouchEvent（）方法了。所以，onInterceptTouchEvent（）方法并不是每次事件都会调用的。
接下来查看onInterceptTouchEvent（）方法：
public boolean onInterceptTouchEvent(MotionEvent ev) {
if (ev.isFromSource(InputDevice.SOURCE_MOUSE) //1
&& ev.getAction() == MotionEvent.ACTION_DOWN
&& ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
&& isOnScrollbarThumb(ev.getX(), ev.getY())) {
return true;
}
return false;
}
如果满足1处条件，拦截，否则不拦截。接下来查看dispatchTouchEvent（）方法剩余部分：
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
...
final View[] children = mChildren;
//倒序遍历viewGroup子元素，即从上层view开始
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.
//判断子元素能否接受到点击事件
if (childWithAccessibilityFocus != null) {
if (childWithAccessibilityFocus != child) {
continue;
}
childWithAccessibilityFocus = null;
i = childrenCount - 1;
}
//判断触摸点是否在子view范围内或子view是否在播放动画
if (!child.canReceivePointerEvents()
|| !isTransformedTouchPointInView(x, y, child, null)) {
//如果均不符合则表示不符合条件
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)) { //3
// 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();
...
}
接下来查看3处 dispatchTransformedTouchEvent 方法：
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);
//如果没有子view
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
handled = child.dispatchTouchEvent(event);
}
event.setAction(oldAction);
return handled;
}
}
如果没有子view，则调用父类的 dispatchTouchEvent 方法，ViewGroup是继承View，接下来查看View的 dispatchTouchEvent 方法：
public boolean dispatchTouchEvent(MotionEvent event) {
boolean result = false;
if (onFilterTouchEventForSecurity(event)) {
if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
result = true;
}
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
//事件被消费
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
if (!result && onTouchEvent(event)) {
result = true;
}
}
}
如果 OnTouchListener 不为null并且 onTouch 方法返回true，则表示事件被消费，就不会执行onTouchEvent（event）；否则执行。接下来查看onTouchEvent（）方法：
public boolean onTouchEvent(MotionEvent event) {
final int action = event.getAction();
final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE
//可被点击
|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
//可被长按点击
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE; //1
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)) {
performClickInternal(); //2
}
}
}
...
}
return true
}
return false;
}
从1处看出只要View的CLICKABLE和LONG_CLICKABLE有一个为true，那么onTouchEvent（）就会返回true消耗这个事件。接着在ACTION_UP事件中会调用performClickInternal()方法：
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();
}
接下来查看 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) { //1
playSoundEffect(SoundEffectConstants.CLICK);
li.mOnClickListener.onClick(this);
result = true;
} else {
result = false;
}
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
notifyEnterOrExitForAutoFillIfNeeded(true);
return result;
}
由1处看出，如果View设置了点击事件 OnClickListener，那么它的 onClick 方法就会被执行。
点击事件分发的传递规则
由前面事件分发机制的分析可知点击事件分发的这3个重要方法的关系，用伪代码来简单表示：
public boolean dispatchTouchEvent(MotionEvent ev) {
boolean result=false;
if(onInterceptTouchEvent(ev)){
result=onTouchEvent(ev);
}else{
result=child.dispatchTouchEvent(ev);
}
return result;
}
事件由上而下的传递原则：
- 当点击事件后会由 Activity 来处理，传递给PhoneWindow，再传递给DecorView，最后传递给顶层的 ViewGroup。
- 对于根ViewGroup，点击事件首先传递给它的
dispatchTouchEvent（），如果该ViewGrouponInterceptTouchEvent（）方法返回true，则表示它要拦截这个事件，就将事件交给它的onTouchEvent（）方法处理；否则表示它不拦截该事件，就将事件交给它的子元素的dispatchTouchEvent（）来处理。 - 如此反复下去，如果传递给底层View，就会调用其
dispatchTouchEvent（）方法，最终调用View的onTouchEvent（）方法。
总结：事件由下而上传递返回值的规则如下：为true，则处理了，不继续向上传递；为false，则不处理，继续向上传递。
