对于一个Android开发者来说View事件分发机制是不得不了解的.不了解事件分发机制的话很多自定义控件都无法写出来.今天我们一起来看看View的事件分发机制是如何实现的.
首先来看个栗子:
自定义一个MyButton继承Button并重写dispatchTouchEvent和onTouchEvent方法
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
switch (ev.getAction()){
case MotionEvent.ACTION_DOWN:
Log.e(TAG, "dispatchTouchEvent ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.e(TAG, "dispatchTouchEvent ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.e(TAG, "dispatchTouchEvent ACTION_UP");
break;
}
return super.dispatchTouchEvent(ev);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
switch (event.getAction()){
case MotionEvent.ACTION_DOWN:
Log.e(TAG, "onTouchEvent ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.e(TAG, "onTouchEvent ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.e(TAG, "onTouchEvent ACTION_UP");
break;
}
return super.onTouchEvent(event);
}xml里面调用
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/mFrameLayout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true">
<event.test.hxy.com.testenent.MyButton
android:id="@+id/my_bt"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:layout_constraintStart_toStartOf="parent"
android:layout_gravity="center"
android:text="按钮"/>
</FrameLayout>
MyButton mButton= (MyButton) findViewById(R.id.my_bt);
mButton.setOnTouchListener(new View.OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
switch (event.getAction()) {
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;
}
return false;
}
});
mButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Log.e(TAG, "onClick");
}
});
mButton.setOnLongClickListener(new View.OnLongClickListener() {
@Override
public boolean onLongClick(View v) {
Log.e(TAG, "onLongClick");
return false;
}
});当我们只点击一下的时候日志打出
08-14 11:47:38.173 15712-15712/fun.hxy.com.testalg E/MyButton: dispatchTouchEvent ACTION_DOWN
08-14 11:47:38.173 15712-15712/fun.hxy.com.testalg E/MainActivity: onTouch ACTION_DOWN
08-14 11:47:38.174 15712-15712/fun.hxy.com.testalg E/MyButton: onTouchEvent ACTION_DOWN
08-14 11:47:38.199 15712-15712/fun.hxy.com.testalg E/MyButton: dispatchTouchEvent ACTION_UP
08-14 11:47:38.199 15712-15712/fun.hxy.com.testalg E/MainActivity: onTouch ACTION_UP
08-14 11:47:38.199 15712-15712/fun.hxy.com.testalg E/MyButton: onTouchEvent ACTION_UP
08-14 11:47:38.200 15712-15712/fun.hxy.com.testalg E/MainActivity: onClick我们发现时间都是从dispatchTouchEvent开始的然后onTouch->onTouchEvent->onClick
为什么会这样呢?我们来看一下View的源码:
public boolean dispatchTouchEvent(MotionEvent event) {
1 ....
2 if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
3 if (!result && onTouchEvent(event)) {
result = true;
}
.....
return result;
}在View的dispatchTouchEvent里面我们可以发现View先判断li.mOnTouchListener.onTouch(this, event)先执行了onTouch()事件在到第三行的onTouchEvent(),并且如果onTouch()的返回值返回了true的话就不会在往下执行onTouchEvent()方法了;
我们在看看onTouchEvent()的源码:
public boolean onTouchEvent(MotionEvent event) {
......
switch (action) {
case MotionEvent.ACTION_UP:
if (!focusTaken) {
if (!post(mPerformClick)) {
performClick();
}
}
break;
}
return true;
}
return false;
}我们发现onClick的方法是在onTouch()方法的up里面执行的.这就是为什么View点击的时候会先调用onTouch() -> onTouchEvent() -> onClick(),面试的时候经常会问到
我们在来看一下长按事件:
现在我们长按button按钮
08-14 14:16:16.725 11084-11084/fun.hxy.com.testalg E/MyButton: dispatchTouchEvent ACTION_DOWN
08-14 14:16:16.726 11084-11084/fun.hxy.com.testalg E/MainActivity: onTouch ACTION_DOWN
08-14 14:16:16.726 11084-11084/fun.hxy.com.testalg E/MyButton: onTouchEvent ACTION_DOWN
08-14 14:16:16.875 11084-11084/fun.hxy.com.testalg E/MyButton: dispatchTouchEvent ACTION_MOVE
08-14 14:16:16.875 11084-11084/fun.hxy.com.testalg E/MainActivity: onTouch ACTION_MOVE
08-14 14:16:16.875 11084-11084/fun.hxy.com.testalg E/MyButton: onTouchEvent ACTION_MOVE
08-14 14:16:16.890 11084-11084/fun.hxy.com.testalg E/MyButton: dispatchTouchEvent ACTION_MOVE
08-14 14:16:16.890 11084-11084/fun.hxy.com.testalg E/MainActivity: onTouch ACTION_MOVE
08-14 14:16:16.890 11084-11084/fun.hxy.com.testalg E/MyButton: onTouchEvent ACTION_MOVE
08-14 14:16:16.906 11084-11084/fun.hxy.com.testalg E/MyButton: dispatchTouchEvent ACTION_MOVE
......
08-14 14:16:17.142 11084-11084/fun.hxy.com.testalg E/MainActivity: onLongClick
08-14 14:16:17.189 11084-11084/fun.hxy.com.testalg E/MyButton: dispatchTouchEvent ACTION_MOVE
08-14 14:16:17.190 11084-11084/fun.hxy.com.testalg E/MainActivity: onTouch ACTION_MOVE
08-14 14:16:17.190 11084-11084/fun.hxy.com.testalg E/MyButton: onTouchEvent ACTION_MOVE
.....
08-14 14:16:17.948 11084-11084/fun.hxy.com.testalg E/MyButton: dispatchTouchEvent ACTION_UP
08-14 14:16:17.949 11084-11084/fun.hxy.com.testalg E/MainActivity: onTouch ACTION_UP
08-14 14:16:17.949 11084-11084/fun.hxy.com.testalg E/MyButton: onTouchEvent ACTION_UP
08-14 14:16:17.954 11084-11084/fun.hxy.com.testalg E/MainActivity: onClick
我们发现长按的时候会执行ACTION_DOWN因为手指按住可能会有一点抖,会执行不等次的ACTION_MOVE,然后先执行onLongClick后面放手在执行onClick.
再次翻看onTouchEvent()的源码:
public boolean onTouchEvent(MotionEvent event) {
switch (action) {
case MotionEvent.ACTION_UP:
......
if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
if (!post(mPerformClick)) {
performClick();
}
}
......
break;
case MotionEvent.ACTION_DOWN:
......
checkForLongClick(0, x, y);
......
break;
}
return false;
}我们发现在ACTION_DOWN的时候,系统就会执行checkForLongClick方法
private void checkForLongClick(int delayOffset, float x, float y) {
if (mPendingCheckForLongPress == null) {
mPendingCheckForLongPress = new CheckForLongPress();
}
postDelayed(mPendingCheckForLongPress,
ViewConfiguration.getLongPressTimeout() - delayOffset);
}这个checkForLongClick里面执行了一个延迟操作延迟五百毫秒时候就会执行CheckForLongPress这个延迟事件
@Override
public void run() {
if (isPressed() && (mParent != null)
&& mOriginalWindowAttachCount == mWindowAttachCount) {
if (performLongClick(mX, mY)) {
mHasPerformedLongPress = true;
}
}
}这个方法里执行了
public boolean performLongClick(float x, float y) {
mLongClickX = x;
mLongClickY = y;
final boolean handled = performLongClick();
mLongClickX = Float.NaN;
mLongClickY = Float.NaN;
return handled;
}我们可以看到performLongClick这个方法的返回值为boolean类型,所以当onLongClickListener方法返回值改为true的时候,mHasPerformedLongPress 被置为true,从上面又可以看到只有在mHasPerformedLongPress为false的时候performClick()方法才会执行.
好了,到这里View的事件机制差不多就看完一遍了,View的事件机制还是比较简单的,下一篇我将跟大家一起学习一下ViewGroup的事件机制.希望大家都能有所收获.