本文详细解析了Android事件分发机制，从ViewGroup事件分发开始，深入分析了事件传递过程，包括如何在ViewGroup和View之间传递事件，以及如何通过事件分发来处理触摸、点击等操作。
继上次转发的一篇文章Android 事件分发机制,这两天决定自己跟踪源码一探究竟.
此次我们直接从ViewGroup事件分发开始.
首先,先要了解什么是ViewGroup
ViewGroup和View有什么区别呢？实际上,ViewGroup就是一组View的集合,它包括子View或者子ViewGroup.是Android中所有布局的父类或者间接父类,例如RelativeLayout,LinearLayout等布局都是继承自ViewGroups.说到底,ViewGroup也是一个View,只不过它比起其他View,多了包含子View和定义布局参数的方法.
了解了ViewGroup后,我们开始来分析事件分发流程.
这里用一个Demo来开始.这个Demo的布局由一个自定义的MyLayout(继承自RelativeLayout)组成,里面包含一个Button.然后分别实现了Mylayout的OnTouch事件,和Button的OnClick点击事件以及OnTouch事件.
布局文件activity_main.xml:
<com.example.touchevent.MyLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/myLayout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".MainActivity" >
<Button
android:id="@+id/btn1"
android:text="Button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerInParent="true"/>
</com.example.touchevent.MyLayout>package com.example.touchevent;
import android.content.Context;
import android.util.AttributeSet;
import android.widget.RelativeLayout;
public class MyLayout extends RelativeLayout {
public MyLayout(Context context, AttributeSet attrs) {
super(context, attrs);
// TODO Auto-generated constructor stub
}
}
package com.example.touchevent;
import android.os.Bundle;
import android.app.Activity;
import android.util.Log;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.View.OnTouchListener;
import android.widget.Button;
public class MainActivity extends Activity {
private Button button;
private MyLayout layout;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
layout = (MyLayout)this.findViewById(R.id.myLayout);
layout.setOnTouchListener(new OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
// TODO Auto-generated method stub
Log.v("Event", "layout --> setOnTouchListener --> onTouch");
return false;
}
});
button = (Button)this.findViewById(R.id.btn1);
button.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
Log.v("Event", "button --> setOnClickListener --> onClick");
}
});
button.setOnTouchListener(new OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
// TODO Auto-generated method stub
Log.v("Event", "button --> setOnTouchListener --> onTouch");
return false;
}
});
}
}
当我们只点击Button的时候:
这里OnTouch执行了两次,是因为一次是ACTION_DOWN,一次是ACTION_UP(如果你手指点击的时候移动了下
,也会触发ACTION_MOVE).
当我们点击空白区域的时候:
从两次打印结果来看,我们发现,当我们点击Button的时候,MyLayout的OnTouch事件没有被触发,并且Button的OnTouch方法先于OnClick方法执行.我们可以理解为,
当点击Button的时候,OnClick最后把事件消费掉了.
从Android 事件分发机制里得知,ViewGroup中有一个onInterceptTouchEvent(MotionEvent ev)方法,
这个方法用来拦截TouchEvent.我们来看看源码:
public boolean onInterceptTouchEvent(MotionEvent ev) {
return false;
}
这里我们发现无论点击哪里,都只会触发MyLayout的OnTouch方法,Button的点击事件都被拦截了.
这里我们可以分析出Android的Touch的事件传递都是先从ViewGroup开始,然后再视情况传递到View.从Android 事件分发机制文章得知,
只要触摸事件发生,系统都会调用dispatchTouchEvent(MotionEvent ev) 这个方法进行事件的分发.至于到底是先调用控件所在
布局的dispatchTouchEvent方法还是控件View本身的dispatchTouchEvent方法?很明显从上边的这些打印信息可以得知.系统会先调用
控件所在布局的dispatchTouchEvent方法,然后在这个方法中找到被点击的控件,去调用该控件的dispatchTouchEvent方法.
这里可能有点绕~请大家自己体会,多读几遍.
根据上边的分析,现在我们来找MyLayout布局中的dispatchTouchEvent方法,发现RelativeLayout中并没有这个方法,没关系我们接着
从它的类寻找.在ViewGroup中我们终于找到了这个方法:
/**
* {@inheritDoc}
*/
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
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;
}
// 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 (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 View[] children = mChildren;
final boolean customOrder = isChildrenDrawingOrderEnabled();
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = customOrder ?
getChildDrawingOrder(childrenCount, i) : i;
final View child = children[childIndex];
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
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();
mLastTouchDownIndex = childIndex;
mLastTouchDownX = ev.getX();
mLastTouchDownY = ev.getY();
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;
}
}
}
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;
}这个代码咋看起来,尼玛有点略吓人.根据注释,找到49行的if语句,发现只有条件为真时，才能进入具体的内部处理.来看看这个条件
!canceled&&!intercepted 其中canceled是用来检查是否取消动作的,intercepted表示是否拦截.很显然这两个值都为false的时
候,程序才能进入到if语句内部来遍历子view,检查点击的是哪个子view.这里由于!caneled正常情况下均为true,我们就不分析了.
来看看intercepted是如何得到的,正常情况下当触摸按下的时候,也就是ACTION_DOWN时,intercepted是在第30行
onInterceptTouchEvent(ev)赋值的,也就是上边我们贴出的只返回false的那个方法,可想而知,如果这个方法返回true.会导致所
有的点击事件都被屏蔽拦截.进入到if语句内部第70行有个for循环,它遍历了所有子View找到可以接收事件的子view(也就是触摸坐
标等属性在这个子View范围中).注意第88行dispatchTransformedTouchEvent,这个函数就是将这个事件转化为子View的事件,
并调用chiled.dispatchTouchEvent(event).
dispatchTransformedTouchEvent函数代码:
/**
* Transforms a motion event into the coordinate space of a particular child view,
* filters out irrelevant pointer ids, and overrides its action if necessary.
* If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.
*/
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);
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
handled = child.dispatchTouchEvent(event);
}
event.setAction(oldAction);
return handled;
}
// Calculate the number of pointers to deliver.
final int oldPointerIdBits = event.getPointerIdBits();
final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;
// If for some reason we ended up in an inconsistent state where it looks like we
// might produce a motion event with no pointers in it, then drop the event.
if (newPointerIdBits == 0) {
return false;
}
// If the number of pointers is the same and we don't need to perform any fancy
// irreversible transformations, then we can reuse the motion event for this
// dispatch as long as we are careful to revert any changes we make.
// Otherwise we need to make a copy.
final MotionEvent transformedEvent;
if (newPointerIdBits == oldPointerIdBits) {
if (child == null || child.hasIdentityMatrix()) {
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
event.offsetLocation(offsetX, offsetY);
handled = child.dispatchTouchEvent(event);
event.offsetLocation(-offsetX, -offsetY);
}
return handled;
}
transformedEvent = MotionEvent.obtain(event);
} else {
transformedEvent = event.split(newPointerIdBits);
}
// Perform any necessary transformations and dispatch.
if (child == null) {
handled = super.dispatchTouchEvent(transformedEvent);
} else {
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
transformedEvent.offsetLocation(offsetX, offsetY);
if (! child.hasIdentityMatrix()) {
transformedEvent.transform(child.getInverseMatrix());
}
handled = child.dispatchTouchEvent(transformedEvent);
}
// Done.
transformedEvent.recycle();
return handled;
}是有返回值的.若控件是可以点击的,则dispatchTouchEvent返回值必定是true.如果我们点击的是MyLayout的空白区域,
ViewGroup的dispatchTouchEvent方法就不会进入到49行的内部语句中,而是进入到114行语句中,在里面同样调用了
dispatchTransformedTouchEvent,只不过参数由child变为canceled.使得dispatchTransformedTouchEvent代码中
的第13行条件为真.因此调用了super.dispatchTouchEvent(event).那这个super.dispatchTouchEven在何处调用呢?
当然是View中,因为ViewGroup的父类就是View.
接下来我们来看看View中的dispatchTouchEven是如何处理的.
/**
* 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 (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
if (onFilterTouchEventForSecurity(event)) {
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
return true;
}
if (onTouchEvent(event)) {
return true;
}
}
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
return false;
}
li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)一共有4个条件必须同时为真时才能进入if内部执行.
li一般为真,就不分析了.先来看看第二个条件li.mOnTouchListener.它在哪里赋值的呢？对！就是这里:
/**
* 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;
}(mViewFlags & ENABLED_MASK) == ENABLED意思为当前控件是否为enable的,按钮默认均为enable.
第四个条件,li.mOnTouchListener.onTouch(this, event),其实就是回调了我们注册touch事件时实现的
onTouch函数,我们在代码里返回了true使得条件为真.如果条件为假,代码就会继续执行21行的onTouchEvent(event)
函数.这里我们分析了半天没有看到onClick函数在哪里执行.那么我们来看看onTouchEvent的代码:
/**
* 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 int viewFlags = mViewFlags;
if ((viewFlags & ENABLED_MASK) == DISABLED) {
if (event.getAction() == 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));
}
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}
if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {
switch (event.getAction()) {
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);
}
if (!mHasPerformedLongPress) {
// 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();
}
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();
}
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
} else {
// Not inside a scrolling container, so show the feedback right away
setPressed(true);
checkForLongClick(0);
}
break;
case MotionEvent.ACTION_CANCEL:
setPressed(false);
removeTapCallback();
removeLongPressCallback();
break;
case MotionEvent.ACTION_MOVE:
final int x = (int) event.getX();
final int y = (int) event.getY();
// 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;
}performClick()函数.
/**
* 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() {
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
li.mOnClickListener.onClick(this);
return true;
}
return false;
}
/**
* 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(OnClickListener l) {
if (!isClickable()) {
setClickable(true);
}
getListenerInfo().mOnClickListener = l;
}这里我们基本把整个Android的事件分发机制从ViewGroup开始分析 一直分析到了View中.大致了解了它的流程.下面通过网上的一份流程图来总结下:
本文部分参考了guolin大神的http://blog.youkuaiyun.com/guolin_blog/article/details/9153747等人的博客~
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