開發一段時間的Android，或多或少對Android的事件有一些瞭解，對諸如dispatchTouchEvent、onTouchEvent方法有些瞭解。但真正在面試中被問起，整個機制，或者具體的分析ViewGroup+ViewGroup+View的具體回調順序，就懵了。百度出的第一位博客講解的很到位：
當一個Touch事件(觸摸事件爲例)到達根節點，即Acitivty的ViewGroup時，它會依次下發，下發的過程是調用子View(ViewGroup)的dispatchTouchEvent方法實現的。簡單來說，就是ViewGroup遍歷它包含着的子View，調用每個View的dispatchTouchEvent方法，而當子View爲ViewGroup時，又會通過調用ViwGroup的dispatchTouchEvent方法繼續調用其內部的View的dispatchTouchEvent方法。上述例子中的消息下發順序是這樣的：①-②-⑤-⑥-⑦-③-④。dispatchTouchEvent方法只負責事件的分發，它擁有boolean類型的返回值，當返回爲true時，順序下發會中斷。在上述例子中如果⑤的dispatchTouchEvent返回結果爲true，那麼⑥-⑦-③-④將都接收不到本次Touch事件。
ViewGroup+ViewGroup+View事件傳遞的具體分析
題目：比如最上面有一個ViewGroup，下面有一個ViewGroup，最下面有一個Button，那麼點擊Button，請問事件是如何傳遞的，這期間會回調哪些方法，順序如何？
看了上面的講解，其實心中已有一定的瞭解。但不能止於此。於是我使用代碼驗證並查看Android的源代碼來看看到底是怎麼回事，來找到最後的答案。
新建Activity，layout的結構爲LinearLayout - RelativeLayout - Button，剛好是題目所示的結構。爲了驗證回調函數，我將這三個View都繼承了，同時也在Activity中覆蓋dispatchTouchEvent和onTouchEvent：
layout佈局如下：
<?xml version="1.0" encoding="utf-8"?>
<com.idengpan.life100.viewtouch.TouchLinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@color/white"
android:orientation="vertical" >
<com.idengpan.life100.viewtouch.TouchRelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent" >
<com.idengpan.life100.viewtouch.TouchButton
android:id="@+id/btn_test"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="測試" />
</com.idengpan.life100.viewtouch.TouchRelativeLayout>
</com.idengpan.life100.viewtouch.TouchLinearLayout>三個自定義的View/ViewGroup如下：
public class TouchLinearLayout extends LinearLayout {
public TouchLinearLayout(Context context, AttributeSet attrs) {
super(context, attrs);
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
System.out.println("TouchLinearLayout onInterceptTouchEvent");
boolean result = super.onInterceptTouchEvent(ev);
System.out.println("TouchLinearLayout onInterceptTouchEvent return " + result);
return result;
}
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
System.out.println("TouchLinearLayout dispatchTouchEvent");
boolean result = super.dispatchTouchEvent(ev);
System.out.println("TouchLinearLayout dispatchTouchEvent return " + result);
return result;
}
@Override
public boolean onTouchEvent(MotionEvent event) {
System.out.println("TouchLinearLayout onTouchEvent");
boolean result = super.onTouchEvent(event);
System.out.println("TouchLinearLayout onTouchEvent return " + result);
return result;
}
}
public class TouchRelativeLayout extends RelativeLayout{
public TouchRelativeLayout(Context context, AttributeSet attrs) {
super(context, attrs);
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
System.out.println("TouchRelativeLayout onInterceptTouchEvent");
boolean result = super.onInterceptTouchEvent(ev);
System.out.println("TouchRelativeLayout onInterceptTouchEvent return " + result);
return result;
}
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
System.out.println("TouchRelativeLayout dispatchTouchEvent");
boolean result = super.dispatchTouchEvent(ev);
System.out.println("TouchRelativeLayout dispatchTouchEvent return " + result);
return result;
}
@Override
public boolean onTouchEvent(MotionEvent ev) {
System.out.println("TouchRelativeLayout onTouchEvent");
boolean result = super.onTouchEvent(ev);
System.out.println("TouchRelativeLayout onTouchEvent return " + result);
return result;
}
}
public class TouchButton extends Button {
public TouchButton(Context context, AttributeSet attrs) {
super(context, attrs);
}
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
System.out.println("TouchButton dispatchTouchEvent");
boolean result = super.dispatchTouchEvent(ev);
System.out.println("TouchButton dispatchTouchEvent return " + result);
return result;
}
@Override
public boolean onTouchEvent(MotionEvent ev) {
System.out.println("TouchButton onTouchEvent");
boolean result = super.onTouchEvent(ev);
System.out.println("TouchButton onTouchEvent return " + result);
return result;
}
}
//Activity中覆蓋：
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
System.out.println("activity dispatchTouchEvent");
boolean result = super.dispatchTouchEvent(ev);
System.out.println("activity dispatchTouchEvent return " + result);
return result;
}
@Override
public boolean onTouchEvent(MotionEvent ev) {
System.out.println("activity onTouchEvent");
boolean result = super.onTouchEvent(ev);
System.out.println("activity onTouchEvent return " + result);
return result;
}點擊Button，可以看到回調的執行順序是：
I/System.out(7034): activity dispatchTouchEvent
I/System.out(7034): TouchLinearLayout dispatchTouchEvent
I/System.out(7034): TouchLinearLayout onInterceptTouchEvent
I/System.out(7034): TouchLinearLayout onInterceptTouchEvent return false
I/System.out(7034): TouchRelativeLayout dispatchTouchEvent
I/System.out(7034): TouchRelativeLayout onInterceptTouchEvent
I/System.out(7034): TouchRelativeLayout onInterceptTouchEvent return false
I/System.out(7034): TouchButton dispatchTouchEvent
I/System.out(7034): TouchButton onTouchEvent
I/System.out(7034): TouchButton onTouchEvent return true
I/System.out(7034): TouchButton dispatchTouchEvent return true
I/System.out(7034): TouchRelativeLayout dispatchTouchEvent return true
I/System.out(7034): TouchLinearLayout dispatchTouchEvent return true
I/System.out(7034): activity dispatchTouchEvent return true
看到日誌輸出，更是一目瞭然了。去查看了下ViewGroup的源碼，它的onInterceptTouchEvent方法未做任何處理，總是返回false的。
public boolean onInterceptTouchEvent(MotionEvent ev) {
return false;
}再看看ViewGroup中的dispatchTouchEvent方法：
public boolean dispatchTouchEvent(MotionEvent ev) {
....
// 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;
}
...
//dispatchTransformedTouchEvent方法中
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;
}
...
}
}那View裏的dispatchTouchEvent方法執行的很明顯，直接實現onTouch監聽或執行onTouchEvent方法：
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;
}而ViewGroup中沒有覆蓋onTouchEvent方法，都是執行View中的onTouchEvent方法。
在View的onTouchEvent方法中，會在ACTION_UP事件中調用performCLick方法，從而響應Button.onClick事件:
public boolean onTouchEvent(MotionEvent event) {
...
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.
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();
}
}
}
....
}所以這個題目的答案就是，點擊事件從Activity傳遞到ViewGroup，最上層的ViewGroup調用dispatchTouchEvent進行事件分發，接着會調用下面一層ViewGroup的dispatchTouchEvent方法，然後就是Button的dispatchTouchEvent方法，而Button的dispatchTouchEvent方法會調用Button的onTouchEvent方法，在這個方法裏，點擊事件被消費，返回了true，接着Button的dispatchTouchEvent方法也返回true，然後中間ViewGroup的dispatchTouchEvent方法也返回true，然後是最上層的dispatchTouchEvent方法也返回true，最後是Activity的dispatchTouchEvent方法也返回true。事件一步一步向下傳遞，然後一步一步向上冒泡反饋。