前言
Android事件分发机制可以说是我们android工程师面试中必考题,弄懂它的原理是我们避不开的任务,随意长痛不如短痛,花点时间干掉他,我们开始吧
Android事件分发机制简介
Android事件分发机制的发生在View与View之间或者ViewGroup与View之间具有镶嵌的视图上,而且视图上必须为点击可用。当一个点击事件产生后,它的传递过程遵循如下顺序:Activity -> Window->View,即事件先传递给Activity,再到Window,再到顶级View,才开始我们的事件分发;
Android事件分发相关概念
Android事件分发机制主要由三个重要的方法共同完成的
1. dispatchTouchEvent:用于点击事件的分发
2. onInterceptTouchEvent:用于点击事件的拦截
3. onTouchEvent:用于处理点击事件
主要注意View中是没有onInterceptTouchEvent()方法的
Android事件分发机制的分发例子
这里两个ViewGroup嵌套View来演示,
一、MyView
继承View并覆写其三个构造方法,覆写dispatchTouchEvent和onTouchEvent
public class MyView extends View {
public MyView(Context context) {
super(context);
} public MyView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
} public MyView(Context context, AttributeSet attrs) {
super(context, attrs);
} @Override
public boolean dispatchTouchEvent(MotionEvent event) {
System.out.println("MyView dispatchTouchEvent");
return super.dispatchTouchEvent(event);
} @Override
public boolean onTouchEvent(MotionEvent event) {
System.out.println("MyView onTouchEvent");
return super.onTouchEvent(event);
}
}
二、MyViewGroup01和MyViewGroup02
MyViewGroup01和MyViewGroup02是一样的代码,这里以01为例,继承ViewGroup并覆写其三个构造方法,覆写dispatchTouchEvent和onTouchEvent和onInterceptTouchEvent方法
public class MyViewGroup01 extends LinearLayout {
public MyViewGroup01(Context context) {
super(context);
} public MyViewGroup01(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
} public MyViewGroup01(Context context, AttributeSet attrs) {
super(context, attrs);
} @Override
public boolean dispatchTouchEvent(MotionEvent ev) {
System.out.println("MyViewGroup01 dispatchTouchEvent");
return super.dispatchTouchEvent(ev);
} @Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
System.out.println("MyViewGroup01 onInterceptTouchEvent");
return super.onInterceptTouchEvent(ev);
} @Override
public boolean onTouchEvent(MotionEvent event) {
System.out.println("MyViewGroup01 onTouchEvent");
return super.onTouchEvent(event);
}
}
三、MyView和MyViewGroup布局文件
这里以ViewGroup和Group嵌套,由上面可以知道事件最后分配到布局的顶级View,这里的顶级View是MyViewGroup02,然后开始事件的传递
<?xml version="1.0" encoding="utf-8"?>
<com.handsome.boke2.TouchEvent.MyViewGroup02 xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="200dp"
android:layout_height="200dp"
android:background="#0f0"> <com.handsome.boke2.TouchEvent.MyViewGroup01
android:layout_width="100dp"
android:layout_height="100dp"
android:background="#f00"> <com.handsome.boke2.TouchEvent.MyView
android:layout_width="50dp"
android:layout_height="50dp"
android:background="#00f" />
</com.handsome.boke2.TouchEvent.MyViewGroup01>
</com.handsome.boke2.TouchEvent.MyViewGroup02>
分析事件传递
点击MyView(即蓝色部分):先接收事件的是父容器(MyViewGroup02)往下分发,而事件的分发过程中分为两步骤
分发过程
处理过程
其正常的分发事件结果为
//分发过程
MyViewGroup02 dispatchTouchEvent
MyViewGroup02 onInterceptTouchEvent
MyViewGroup01 dispatchTouchEvent
MyViewGroup01 onInterceptTouchEvent
MyView dispatchTouchEvent
//处理过程
MyView onTouchEvent
MyViewGroup01 onTouchEvent
MyViewGroup02 onTouchEvent
1、dispatchTouchEvent(分发事件)
如果在MyViewGroup01中的dispatchTouchEvent方法中返回true,表示需要在MyViewGroup01消费了整个事件,即不会再分发,也不会再处理。dispatchTouchEvent方法中返回true的打印信息
//分发过程
MyViewGroup02 : dispatchTouchEvent
MyViewGroup02: onIntetceptTouchEvent
MyViewGroup01: dispatchTouchEvent
如果在MyViewGroup01中disPatchTouchEvent返回false,表示在MyViewGroup01点击事件在本层不再进行分发,并交由上层控件OnTouchEvent方法进行处理。dispatchTouchEvent方法中返回false的打印信息
//分发过程
MyViewGroup02: dispatchTouchEvent
MyViewGroup02: onInterceptTouchEvent
MyViewGroup02: dispatchTouchEvent
//处理过程
MyViewGroup02: onTouchEnvent
2. OnInterceptTouchEvent ( 拦截事件)
如果在MyViewGroup01中的onInterceptTouchEvent中返回true,表示需要在MyViewGroup01拦截这个点击事件,不再继续下发,即MyView不再执行dispatchTouchEvent方法。但是只是分发结束而已,接着开始处理事件。下面是onInterceptTouchEvent方法中返回True的打印信息
//分发事件
MyViewGroup02 : dispatchTouchEvent
MyViewGroup02: onInterceptTouchEvent
MyViewGroup01: dispthTouchEvent
MyViewGroup01: onInterceptTouchEvent
//处理事件
MyViewGroup01: onTouchEvent
MyViewGroup02: onTouchEvent
如果在MyViewGroup01的onInterceptTouchEvent中返回false,表示MyViewGroup01不会拦截这个点击事件,继续往下分发。下面是OnInterceptTouchEvent方法中返回false的打印信息
//分发事件
MyViewGroup02 : disptchTouchEvent
MyViewGroup02: onInterceptTouchEvent
MyViewGroup01:dispatchTouchEvent
MyViewGroup01: onInterceptTouchEvent
//处理过程
MyView : dispatchTouchEvent
MyView: onTouchEvent
MyViewGroup01: onTouchEvent
MyViewGroup02: onTouchEvent
3. onTouchEvent (消费事件)
如果MyViewGroup01中的onTouchEvent方法中返回true, 表示MyViewGroup01可以将该事件直接消费掉了,即分发结束后,处理事件的时候,直接处理MyGroup01就可以结束了。下面是onTouchEvent方法中的打印信息
//分发过程
MyViewGroup02: dispatchTouchEvent
MyViewGroup02: onInterceptTouchEvent
MyViewGroup01: dispatchTouchEvent
MyViewGroup01: onInterceptTouchEvent
MyView: dispatchTouchEvent
MyView:onInterceptTouchEvent
//事件处理
MyView: onTouchEvent
MyViewGroup01:onTouchEvent
如果MyViewGroup01中的onTouchEvent方法中返回false,表示MyViewGroup01不可以将该事件直接消费掉,即事件继续往上处理。下面是OnTouchEvent方法中返回false的打印信息
//分发事件
MyViewGroup02: dispatchTouchEvent
MyViewGroup02: onIntercepterTouchEvent
MyViewGroup01: dispatchTouchEvent
MyViewGroup01: onInterceptTouchEvent
MyView ondisptchTouchEvent
//处理事件
MyView onTouchEvent
MyViewGroup01 : onTouchEvent
MyViewGroup02: onTouchEvent
总结起来
1. dispatchTouchEvent
return true; 表示该view内部消化掉了所有事件
return false: 表示事件在本层不再继续分发,并交由上层控件的onTouchEvent方法进行消费
return super.dispatchTouchEvent(ev) 默认事件将分发给本层事件拦截onInterceptTouchEvent
2. onInterceptTouchEvent
return true: 表示将事件进行拦截,并将拦截到的事件交由本层的 onTouchEvent进行处理
return false: 表示不进行事件拦截处理,事件得以分发到子View
return super.onInterceptTouchEvent(ev) 默认表示不拦截该事件,并将事件传递给下一层的view的dispatchTouchEvent;
3. onTouchEvent
return true: 表示onTouchEvent处理完事件后消费了此事件
return false : 表示不响应事件,那么该事件将不断的向上层view的onTouchEvent方法传递;
return super.onTouchEvent(ev) 表示不响应事件,与return false一样;
Android事件分发机制的分发流程
ViewGroup事件分发源码分析
我们这里以ViewGroup的dispatchTouchEvent()开始讲解,这里主要讲两件事情:
询问是否拦截事件
遍历子View并分发事件
一: dispatchTouchEvent源码询问拦截事件
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
......
// Check for interception.
final boolean intercepted;
//这里检查是否拦截事件
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;
} ......
}
ViewGroup在两种情况下都会判断是否要拦截当前事件事件类型为Action_down: 当我们触发的点击事件,也就是说Action_move和Action_up事件来时,则不触发拦截事件;
mFirstTouchTarget !=null: 当ViewGroup不拦截事件并将事件交给自View的时候该不等式成立;反过来,事件被ViewGroup拦截时,该不等式不成立;
二,dispatchTouchEvent源码遍历子View并分发事件
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
......
final View[] children = mChildren;
//遍历所有子View
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = getAndVerifyPreorderedIndex(
childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(
preorderedList, children, childIndex);
//判断子View是否能接收点击事件
if (childWithAccessibilityFocus != null) {
if (childWithAccessibilityFocus != child) {
continue;
}
childWithAccessibilityFocus = null;
i = childrenCount - 1;
}
//判断子元素在播放动画时落在子元素的区域内
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
//判断子元素点击事件是否落在子元素的区域内
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);
//事件传递到子View,下面追踪该方法
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
// 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);
} ......
}
ViewGroup直接使用for遍历所有子View,对子View的各种状态进行判断,最好调用dispatchTransformTouchEvent(ev,false,child,idBitsToAssign)将事件传递给子View,下面是dispatchTransformedTouchEvent()方法部分源码if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
handled = child.dispatchTouchEvent(event);
}
其最后就是分发给子View的dispatchTouch()方法,那么事件就分发到子View中去了
View事件分发源码分析
view对点击事件处理过程主要有:
view的dispatchTouchEvent() 判断分发事件
view的onTouchEvent:处理事件的具体做法
一、dispatchTouchEvent源码判断分发事件部分
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;
//这里开始判断
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
if (!result && onTouchEvent(event)) {
result = true;
}
}
......
return result;
}
从源码判断处看出,首先会判断有没有设置mOnTouchListener,如果mOnTouchListener不为空,那么onTouchEvent就不会被调用,这里可以得到一个结论,若在View中设置了OnTouchListener,那么它的优先级是高于onTouchEvent的,这样可以更好的让我们自己setOnTouchEventListener()处理点击事件
二、onTouchEvent源码处理事件的具体做法部分
public boolean onTouchEvent(MotionEvent event) {
......
//当View处于不可用状态下,也会消耗点击事件
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);
} ......
//对点击事件的具体处理
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) {
......
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();
}
}
}
......
}
} return true;
}
......
}
从对点击事件的具体处理可以看出,只要View的CLICKABLE和LONG_CLICKABLE有一个为true,那么它就会消耗这个事件,即OnTouchEvent方法返回true。在Action_UP事件中,会触发PerformClick()方法,如果View设置了OnClickListener,那么PerformClick方法内部会调用它的OnClick()方法,这里就不分析它的点击事件了
View事件方法执行顺序
onTouchListener > onTouchEvent > onLongClickListener > onClickListener