为了更好的研究View的事件转发，我们自定以一个MyButton继承Button，然后把跟事件传播有关的方法进行复写，然后添加上日志：
然后把我们自定义的按钮加到主布局文件中：
public class MyButton extends Button{
public MyButton(Context context) {
super(context);
}
public MyButton(Context context, AttributeSet attrs) {
super(context, attrs);
}
public MyButton(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
switch (ev.getAction()) {
case MotionEvent.ACTION_DOWN:
LogUtil.logI("MyButton","___ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
LogUtil.logI("MyButton","___ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
LogUtil.logI("MyButton","___ACTION_UP");
break;
}
return super.dispatchTouchEvent(ev);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
LogUtil.logI("MyButton","onTouchEventACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
LogUtil.logI("MyButton","onTouchEventACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
LogUtil.logI("MyButton","onTouchEvent ACTION_UP");
break;
}
return super.dispatchTouchEvent(event);
}
}
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"> <myview.MyLinerLayout
android:id="@+id/ll"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center">
<myview.MyButton
android:id="@+id/btn"
android:layout_width="100dp"
android:layout_height="100dp"
android:text="Button" />
</myview.MyLinerLayout>
</LinearLayout>
在Activity中设置Button的Touch事件：
public class MainActivity extends Activity
{
protected static final String TAG = "MyButton";
private Button mButton ;
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main); mButton = (Button) findViewById(R.id.id_btn);
mButton.setOnTouchListener(new OnTouchListener()
{
@Override
public boolean onTouch(View v, MotionEvent event)
{
int action = event.getAction(); switch (action)
{
case MotionEvent.ACTION_DOWN:
Log.e(TAG, "onTouch ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.e(TAG, "onTouch ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.e(TAG, "onTouch ACTION_UP");
break;
default:
break;
} return false;
}
});
} }
点击按钮之后查看打印结果：
E/MyButton(879): dispatchTouchEvent ACTION_DOWN
E/MyButton(879): onTouch ACTION_DOWN
E/MyButton(879): onTouchEvent ACTION_DOWN
E/MyButton(879): dispatchTouchEvent ACTION_MOVE
E/MyButton(879): onTouch ACTION_MOVE
E/MyButton(879): onTouchEvent ACTION_MOVE
E/MyButton(879): dispatchTouchEvent ACTION_UP
E/MyButton(879): onTouch ACTION_UP
E/MyButton(879): onTouchEvent ACTION_UP
可以看到，不管是DOWN，MOVE，UP都会按照下面的顺序执行
1、dispatchTouchEvent
2、 setOnTouchListener的onTouch
3、onTouchEvent
先来看触摸事件的入口函数：
public boolean dispatchTouchEvent(MotionEvent event) {
if (!onFilterTouchEventForSecurity(event)) {
return false;
}
if (mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED &&
mOnTouchListener.onTouch(this, event)) {
return true;
}
return onTouchEvent(event);
}
可以看出如果mOnTouchListener不为空，并且view是ENABLED的，并且mOnTouchListener.onTouch(this, event)的返回值==true，那么
dispatchTouchEvent的返回值就是true，onTouchEvent方法就得不到执行。
public void setOnTouchListener(OnTouchListener l) {
mOnTouchListener = l;
}
如果view设置了setOnTouchListener的监听，那么mOnTouchListener！=null就成立，一般view是ENABLED的，所以onTouch函数的返回值就决定了onTouchEvent函数能否得到执行。
来看下onTouchEvent函数的源码：
public boolean onTouchEvent(MotionEvent event) {
final int viewFlags = mViewFlags;
//如果当前View是Disabled状态且是可点击则会消费掉事件（return true);可以忽略，不是我们的重点
if ((viewFlags & ENABLED_MASK) == DISABLED) {
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 (!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) {
mPrivateFlags |= PRESSED;
refreshDrawableState();
postDelayed(mUnsetPressedState,
ViewConfiguration.getPressedStateDuration());
} else if (!post(mUnsetPressedState)) {
// If the post failed, unpress right now
mUnsetPressedState.run();
}
removeTapCallback();
}
break;
case MotionEvent.ACTION_DOWN:
if (mPendingCheckForTap == null) {
mPendingCheckForTap = new CheckForTap();
}
mPrivateFlags |= PREPRESSED;
mHasPerformedLongPress = false;
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
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
int slop = mTouchSlop;
if ((x < 0 - slop) || (x >= getWidth() + slop) ||
(y < 0 - slop) || (y >= getHeight() + slop)) {
// 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;
}
首先分析down事件：
可以看到down的时候会将mPrivateFlags设置一个PREPRESSED的表示，设置mHasPerformedLongPress=false；表示长按事件还未触发
这是一个延迟115ms的异步任务，也就是115ms之后就会执行CheckForTap类中的run()方法。
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
private final class CheckForTap implements Runnable {
public void run() {
mPrivateFlags &= ~PREPRESSED;
mPrivateFlags |= PRESSED;
refreshDrawableState();
if ((mViewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) {
postCheckForLongClick(ViewConfiguration.getTapTimeout());
}
}
}
将mPrivateFlags设置为PRESSED
里面还包含了一个postCheckForLongClick()的方法，来看下postCheckForLongClick()方法：
private void postCheckForLongClick(int delayOffset) {
mHasPerformedLongPress = false;
if (mPendingCheckForLongPress == null) {
mPendingCheckForLongPress = new CheckForLongPress();
}
mPendingCheckForLongPress.rememberWindowAttachCount();
//执行一个延迟500-115ms异步任务
postDelayed(mPendingCheckForLongPress,
ViewConfiguration.getLongPressTimeout() - delayOffset);
}
class CheckForLongPress implements Runnable {
private int mOriginalWindowAttachCount;
public void run() {
if (isPressed() && (mParent != null)
&& mOriginalWindowAttachCount == mWindowAttachCount) {
if (performLongClick()) {
mHasPerformedLongPress = true;
}
}
}
如果performLongClick()返回true那么mHasPerformedLongPress=true。
public boolean performLongClick() {
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_LONG_CLICKED);
boolean handled = false;
if (mOnLongClickListener != null) {
// 回调用户实现的长按操作监听函数（OnLongClickListener）
handled = mOnLongClickListener.onLongClick(View.this);
}
if (!handled) {
// 如果OnLongClickListener的onLongClick返回false
// 则需要继续处理该长按事件，这里是显示上下文菜单
handled = showContextMenu();
}
if (handled) {
// 长按操作事件被处理了，此时应该给用户触觉上的反馈
performHapticFeedback(HapticFeedbackConstants.LONG_PRESS);
}
return handled;
}
可以看到用户的长按事件是在这个方法里面实现的回调,并且返回一个boolean值。
总结下就是：
当用户按下，首先会设置标识为PREPRESSED，如果115后，没有抬起，会将View的标识设置为PRESSED且去掉PREPRESSED标识，然后发出一个检测长按的延迟任务，延时为：ViewConfiguration.getLongPressTimeout() - delayOffset（500ms -115ms），这个115ms刚好是检测到PREPRESSED时间；也就是用户从DOWN触发开始算起，如果500ms内没有抬起则认为触发了长按事件。
在来看下up事件：
如果包含PRESSED或者PREPRESSED则进入执行体，也就是无论是115ms内或者之后抬起都会进入执行体，如果mHasPerformedLongPress==false，进入IF,否则直接跳出。
如果mPerformClick为null，初始化一个实例，然后立即通过handler添加到消息队列尾部，如果添加失败则直接执行 performClick();添加成功，在mPerformClick的run方法中就是执行performClick()方法。
public boolean performClick() {
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
if (mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
mOnClickListener.onClick(this);
return true;
}
return false;
}
可以看到这个方法里面会执行用户的onClick()的回调，并且返回true，表示消费了这个事件。
总结下up事件：如果用户设置了长按回调并且返回了true，那么performClick()方法就不会执行，那么我们的onClick()方法也就不会执行。
总结下整个的onTouchEvent方法，只要view具有onClick或者onLongClick那么都会返回true，否则返回false。这也就是Button可以不用设置点击或者长按的回调，而TextView需要设置回调之后
才会进入里面的if()体的原因：
public void setOnClickListener(@Nullable OnClickListener l) {
if (!isClickable()) {
setClickable(true);
}
getListenerInfo().mOnClickListener = l;
}
调用这个方法之后，如果不可点击就会设置成可以点击。
整个View的事件转发流程是：
View.dispatchEvent->View.setOnTouchListener->View.onTouchEvent
在dispatchTouchEvent中会进行OnTouchListener的判断，如果OnTouchListener不为null且返回true，则表示事件被消费，onTouchEvent不会被执行；否则执行onTouchEvent。
