一直对View的事件分发机制不太明白,在项目开发中也遇到过,在网上也找到一些解决问题方法,但是其原理并不太了解,现在辞职了有时间,今天写写View的事件分发,结合android源码一起来学习下,如果讲的不对,往指出一起学习提高,言归正传。
新建一个android项目,里面只有一个activity,有一个button，我们给Button设置setOnClickListener(),setOnTouchListener(),通过log看看结果：
btnClick.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
Log.i("com.example.demo","button click ");
}
});
Button的Touch事件：
<span style="font-size:18px;">btnClick.setOnTouchListener(new OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
Log.i("com.example.demo","button touch "+event.getAction());
return false;
}
});</span>
log：
com.example.demo(30220): button touch 0 com.example.demo(30220): button touch 2 com.example.demo(30220): button touch 2 com.example.demo(30220): button touch 2 com.example.demo(30220): button touch 2 com.example.demo(30220): button touch 2 com.example.demo(30220): button touch 1 com.example.demo(30220): button click
可以看到onTouch优先于OnClick,而且OnTouch执行了好几次,因为OnTouch事件由DWON,MOVE,UP这三部分构成,所以onTouch执行了好几次,那么为什么执行的顺序是先onTouch后onClick呢?
观察onClick和onTouch会发现onTouch()方法有返回值,默认是返回false,如果我们改为返回true，会有什么不同,点击打印log看看：
com.example.demo(3280): button touch 0 com.example.demo(3280): button touch 2 com.example.demo(3280): button touch 2 com.example.demo(3280): button touch 2 com.example.demo(3280): button touch 2 com.example.demo(3280): button touch 1
通过log结果发现Onclick事件没有执行,我们可以理解onTouch返回true时,Button的事件被消费了,就相当于把view的事件拦截了就不会再继续向下传递,因此OnClick事件没有执行,
首先知道一点，只要你触摸到了界面上的任何一个控件，就一定会调用该控件的dispatchTouchEvent方法。这个方法优先于onTouch和onClick先执行,当我们去点击按钮的时候，就会去调用Button类里的dispatchTouchEvent方法,那我们去Button源码中找这个方法,Button源码很少,没有这个方法,Button源码如下：
虽然没有这个方法,但我们看出Button继承了TextView，那就到TextView中取找,但是在TextView中并没有找到dispatchTouchEvent()方法,那就只能找TextView的父类了,而TextView的父类就是View对象了,那在View源码中找dispatchTouchEvent()方法看看它执行逻辑：
我们首先翻译下这个方法的说明：
@param event The motion event to be dispatched,事件动作事件派遣
@return True
if the event was handled by the view, false otherwise.如果这个事件被处理了就返回true,否则会返回false,
现在看下dispatchTouchEvent()方法里的代码,看源码得有个方法,不是所有的代码都要看懂，
在dispatchTouchEvent()方法中重点是看
if (mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED &&
mOnTouchListener.onTouch(this, event)) {
return true;
}
if (onTouchEvent(event)) {
return true;
}
首先看第一个if语句,
mOnTouchListener变量在什么时候初始化呢?我们追踪下,发现它的初始化时在
<pre name="code" class="java"> public void setOnTouchListener(OnTouchListener l) {
mOnTouchListener = l;
}
因此这个是我们在setOntouchListener的时候,mOnTouchListener就可以赋值了,因此这个变量不会为null,
第二个条件(mViewFlags & ENABLED_MASK) == ENABLED是判断当前点击的控件是否是enable的，按钮默认都是enable的，因此这个条件恒定为true
if条件的第三个条件mOnTouchListener.onTouch(this, event)我们点击onTouch()方法里发现：
public interface OnTouchListener {
/**
* Called when a touch event is dispatched to a view. This allows listeners to
* get a chance to respond before the target view.
*
* @param v The view the touch event has been dispatched to.
* @param event The MotionEvent object containing full information about
* the event.
* @return True if the listener has consumed the event, false otherwise.
*/
boolean onTouch(View v, MotionEvent event);
}
发现它是一个接口中的方法,用于回调的,其实就是我们设置onTouchListener方法的返回值,
而我们返回的是true，因此这个if条件判断返回的是true,那么就不会执行下面的语句了
if (onTouchEvent(event)) {
return true;
}
我们看看onTouchEvent(event)方法的逻辑：源码如下：
public boolean onTouchEvent(MotionEvent event) {
final int viewFlags = mViewFlags;
if ((viewFlags & ENABLED_MASK) == DISABLED) {
if (event.getAction() == MotionEvent.ACTION_UP && (mPrivateFlags & PRESSED) != 0) {
mPrivateFlags &= ~PRESSED;
refreshDrawableState();
}
// 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 & PREPRESSED) != 0;
if ((mPrivateFlags & 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.
mPrivateFlags |= PRESSED;
refreshDrawableState();
}
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 |= PREPRESSED;
if (mPendingCheckForTap == null) {
mPendingCheckForTap = new CheckForTap();
}
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
} else {
// Not inside a scrolling container, so show the feedback right away
mPrivateFlags |= PRESSED;
refreshDrawableState();
checkForLongClick(0);
}
break;
case MotionEvent.ACTION_CANCEL:
mPrivateFlags &= ~PRESSED;
refreshDrawableState();
removeTapCallback();
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 & PRESSED) != 0) {
// Remove any future long press/tap checks
removeLongPressCallback();
// Need to switch from pressed to not pressed
mPrivateFlags &= ~PRESSED;
refreshDrawableState();
}
}
break;
}
return true;
}
return false;
}
我们发现这方法里面的源码太多了,但是onTouchEvent()方法其实我们只要看case MotionEvent.ACTION_UP里面的代码,因为我们手触摸到最后倒是要执行这里,在经过种种判断之后,代码会走到这里：
if (!post(mPerformClick)) {
performClick();
}
然后执行performClick()方法,performClick方法里面的代码：
public boolean performClick() {
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
if (mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
mOnClickListener.onClick(this);
return true;
}
return false;
}
首先看if条件,mOnClickListener这个变量就是我们点击的时候设置的,因此不会为null,然后我们看一个重要的方法,也是回调方法,
mOnClickListener.onClick(this);
这就是设置view的点击事件,通过源码我们现在应该明白了最初我们设置onClick和onTouch事件的传递顺序,
总结：
1：如果view对象setOnTouchListener方法返回true，那么view对象就不会执行click事件，如果setTouchListener设置为false，view才会执行click事件,
还有一个重要的知识,我们知道touch事件由DWON,MOVE,UP组成,如下：
btnClick.setOnTouchListener(new OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
if(event.getAction()==MotionEvent.ACTION_DOWN){
Log.i("com.example.demo","ACTION_DOWN");
return false;
}else if(event.getAction()==MotionEvent.ACTION_MOVE){
Log.i("com.example.demo","ACTION_MOVE");
}else{
Log.i("com.example.demo","ACTION_UP");
}
return false;
}
});
运行结果：
晚上继续