Android中View的分發攔截機制是一塊重要的內容，網上也有很多大神進行過相關的分析。
在這篇文章裏我將以自己的理解儘量全面地分析整個流程，有些分析結果是很多文章沒有提及的。
整個分析過程將通過demo與源碼進行，做到有理有據。
###demo結構
這個demo的地址在這兒
首先要知道，ViewGroup的相關方法有dispatchTouchEvent、onInterceptTouchEvent、onTouchEvent、requestDisallowInterceptTouchEvent，
View的相關方法有dispatchTouchEvent、onTouchEvent，一般來說，這些方法返回true表示已經消耗了觸摸事件，不會再向下分發事件，返回false則相反。
demo中需要重寫相關方法並添加日誌輸出，便於我們觀察並理清正確的過程。
自定義ViewGroupA 繼承FrameLayout如下：
/**
* Author: Sbingo
* Date: 2017/6/28
*/
public class ViewGroupA extends FrameLayout{
Logger myLogger = Logger.getLogger("Sbingo ViewGroupA");
public ViewGroupA(Context context) {
super(context);
}
public ViewGroupA(Context context, AttributeSet attrs) {
super(context, attrs);
}
public ViewGroupA(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
myLogger.log(Level.INFO, "dispatchTouchEvent");
return super.dispatchTouchEvent(ev);
}
@Override
public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {
myLogger.log(Level.INFO, "requestDisallowInterceptTouchEvent");
super.requestDisallowInterceptTouchEvent(disallowIntercept);
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
myLogger.log(Level.INFO, "onInterceptTouchEvent");
return super.onInterceptTouchEvent(ev);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
myLogger.log(Level.INFO, "onTouchEvent");
return super.onTouchEvent(event);
}
}
類似地，自定義ViewGroupB 繼承FrameLayout，添加相關打印日誌。
接着自定義MyView繼承TextView如下:
/**
* Author: Sbingo
* Date: 2017/6/28
*/
public class MyView extends android.support.v7.widget.AppCompatTextView {
Logger myLogger = Logger.getLogger("Sbingo MyView");
public MyView(Context context) {
super(context);
}
public MyView(Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
}
public MyView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
myLogger.log(Level.INFO, "dispatchTouchEvent");
return super.dispatchTouchEvent(ev);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
myLogger.log(Level.INFO, "onTouchEvent：" + event.getAction());
return true;
}
}
給它們定義如下的佈局層次：
<com.sbingo.viewsample.ViewGroupA
android:id="@+id/vga"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@android:color/black">
<com.sbingo.viewsample.ViewGroupB
android:id="@+id/vgb"
android:layout_width="300dp"
android:layout_height="400dp"
android:layout_gravity="center"
android:background="@color/colorPrimary"
tools:layout_editor_absoluteX="0dp"
tools:layout_editor_absoluteY="0dp">
<com.sbingo.viewsample.MyView
android:id="@+id/my_view"
android:layout_width="150dp"
android:layout_height="200dp"
android:layout_gravity="center"
android:background="@color/colorAccent" />
</com.sbingo.viewsample.ViewGroupB>
</com.sbingo.viewsample.ViewGroupA>
佈局很簡單，MyView在ViewGroupB正中間，ViewGroupB在ViewGroupA正中間。
一切準備就緒，開始玩起來！
###事實來說話，log作用大
點擊MyView
06-28 11:33:32.947 24137-24137/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
06-28 11:33:32.948 24137-24137/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
06-28 11:33:32.948 24137-24137/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
06-28 11:33:32.948 24137-24137/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
06-28 11:33:32.948 24137-24137/com.sbingo.viewsample I/Sbingo MyView: dispatchTouchEvent
06-28 11:33:32.948 24137-24137/com.sbingo.viewsample I/Sbingo MyView: onTouchEvent
06-28 11:33:32.949 24137-24137/com.sbingo.viewsample I/Sbingo ViewGroupB: onTouchEvent
06-28 11:33:32.949 24137-24137/com.sbingo.viewsample I/Sbingo ViewGroupA: onTouchEvent
觸摸事件到達MyView後沒有被消耗，又向上傳遞
點擊ViewGroupB
06-28 11:33:48.721 24137-24137/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
06-28 11:33:48.721 24137-24137/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
06-28 11:33:48.721 24137-24137/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
06-28 11:33:48.721 24137-24137/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
06-28 11:33:48.721 24137-24137/com.sbingo.viewsample I/Sbingo ViewGroupB: onTouchEvent
06-28 11:33:48.721 24137-24137/com.sbingo.viewsample I/Sbingo ViewGroupA: onTouchEvent
如果點擊的是ViewGroupB，ViewGroupB就會攔截事件，但因爲也沒有消耗事件，事件又向上傳遞
點擊ViewGroupA
06-28 11:35:04.516 24137-24137/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
06-28 11:35:04.517 24137-24137/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
06-28 11:35:04.517 24137-24137/com.sbingo.viewsample I/Sbingo ViewGroupA: onTouchEvent
如果點擊的是ViewGroupA，ViewGroupA就直接攔截了觸摸事件。
這3種輸出是最基本的分發流程，相信大部分開發是沒有疑問的。
點擊MyView，MyView消耗事件
06-28 11:49:05.194 13115-13115/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
06-28 11:49:05.194 13115-13115/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
06-28 11:49:05.194 13115-13115/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
06-28 11:49:05.195 13115-13115/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
06-28 11:49:05.195 13115-13115/com.sbingo.viewsample I/Sbingo MyView: dispatchTouchEvent
06-28 11:49:05.195 13115-13115/com.sbingo.viewsample I/Sbingo MyView: onTouchEvent：0
06-28 11:49:05.327 13115-13115/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
06-28 11:49:05.327 13115-13115/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
06-28 11:49:05.328 13115-13115/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
06-28 11:49:05.328 13115-13115/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
06-28 11:49:05.328 13115-13115/com.sbingo.viewsample I/Sbingo MyView: dispatchTouchEvent
06-28 11:49:05.328 13115-13115/com.sbingo.viewsample I/Sbingo MyView: onTouchEvent：1
MyView消耗觸摸事件後，事件不再向上傳遞，之後該事件的所有動作都傳到MyView這裏，其中可能包含多次的move（示例中沒有這種情況）。
點擊MyView，ViewGroupB攔截
06-28 11:48:15.129 12066-12066/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
06-28 11:48:15.129 12066-12066/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
06-28 11:48:15.129 12066-12066/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
06-28 11:48:15.129 12066-12066/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
06-28 11:48:15.130 12066-12066/com.sbingo.viewsample I/Sbingo ViewGroupB: onTouchEvent
06-28 11:48:15.130 12066-12066/com.sbingo.viewsample I/Sbingo ViewGroupA: onTouchEvent
這和直接點擊ViewGroupB的輸出一樣。
現在爲ViewGroupB和MyView設置OnTouchListener和OnClickListener，並添加打印日誌。
點擊MyView，MyView在onTouchEvent消耗事件
06-28 16:28:24.140 6251-6251/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
06-28 16:28:24.140 6251-6251/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
06-28 16:28:24.140 6251-6251/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
06-28 16:28:24.141 6251-6251/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
06-28 16:28:24.141 6251-6251/com.sbingo.viewsample I/Sbingo MyView: dispatchTouchEvent
06-28 16:28:24.141 6251-6251/com.sbingo.viewsample I/Sbingo MyView: onTouch
06-28 16:28:24.141 6251-6251/com.sbingo.viewsample I/Sbingo MyView: onTouchEvent：0
06-28 16:28:24.238 6251-6251/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
06-28 16:28:24.238 6251-6251/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
06-28 16:28:24.238 6251-6251/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
06-28 16:28:24.238 6251-6251/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
06-28 16:28:24.238 6251-6251/com.sbingo.viewsample I/Sbingo MyView: dispatchTouchEvent
06-28 16:28:24.238 6251-6251/com.sbingo.viewsample I/Sbingo MyView: onTouch
06-28 16:28:24.239 6251-6251/com.sbingo.viewsample I/Sbingo MyView: onTouchEvent：1
點擊MyView，MyView不消耗事件
06-28 16:39:52.048 17495-17495/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
06-28 16:39:52.049 17495-17495/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
06-28 16:39:52.049 17495-17495/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
06-28 16:39:52.049 17495-17495/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
06-28 16:39:52.049 17495-17495/com.sbingo.viewsample I/Sbingo MyView: dispatchTouchEvent
06-28 16:39:52.049 17495-17495/com.sbingo.viewsample I/Sbingo MyView: onTouch
06-28 16:39:52.049 17495-17495/com.sbingo.viewsample I/Sbingo MyView: onTouchEvent：0
06-28 16:39:52.136 17495-17495/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
06-28 16:39:52.137 17495-17495/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
06-28 16:39:52.137 17495-17495/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
06-28 16:39:52.137 17495-17495/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
06-28 16:39:52.137 17495-17495/com.sbingo.viewsample I/Sbingo MyView: dispatchTouchEvent
06-28 16:39:52.137 17495-17495/com.sbingo.viewsample I/Sbingo MyView: onTouch
06-28 16:39:52.137 17495-17495/com.sbingo.viewsample I/Sbingo MyView: onTouchEvent：1
06-28 16:39:52.162 17495-17495/com.sbingo.viewsample I/Sbingo MyView: onClick
可以發現
1.onTouch方法先於onTouchEvent方法執行
2.當MyView不消耗觸摸事件時，onClick方法纔得到執行，否則不會進入onClick方法。
3.不管MyView是否消耗事件，事件都沒有向上傳遞
點擊MyView，MyView的onTouch方法消耗事件
07-07 11:08:42.158 21627-21627/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
07-07 11:08:42.158 21627-21627/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
07-07 11:08:42.158 21627-21627/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
07-07 11:08:42.158 21627-21627/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
07-07 11:08:42.159 21627-21627/com.sbingo.viewsample I/Sbingo MyView: dispatchTouchEvent
07-07 11:08:42.159 21627-21627/com.sbingo.viewsample I/Sbingo MyView: onTouch
07-07 11:08:42.255 21627-21627/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
07-07 11:08:42.255 21627-21627/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
07-07 11:08:42.255 21627-21627/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
07-07 11:08:42.255 21627-21627/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
07-07 11:08:42.256 21627-21627/com.sbingo.viewsample I/Sbingo MyView: dispatchTouchEvent
07-07 11:08:42.256 21627-21627/com.sbingo.viewsample I/Sbingo MyView: onTouch
剛纔已經發現onTouch方法先於onTouchEvent方法執行，現在onTouch方法消耗事件後，onTouchEvent方法便不會執行，之後該事件的所有動作都傳到MyView的onTouch方法。
點擊MyView，ViewGroupB攔截
06-28 16:44:51.593 20418-20418/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
06-28 16:44:51.594 20418-20418/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
06-28 16:44:51.594 20418-20418/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
06-28 16:44:51.594 20418-20418/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
06-28 16:44:51.594 20418-20418/com.sbingo.viewsample I/Sbingo ViewGroupB: onTouch
06-28 16:44:51.594 20418-20418/com.sbingo.viewsample I/Sbingo ViewGroupB: onTouchEvent
06-28 16:44:51.691 20418-20418/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
06-28 16:44:51.691 20418-20418/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
06-28 16:44:51.692 20418-20418/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
06-28 16:44:51.692 20418-20418/com.sbingo.viewsample I/Sbingo ViewGroupB: onTouch
06-28 16:44:51.692 20418-20418/com.sbingo.viewsample I/Sbingo ViewGroupB: onTouchEvent
06-28 16:44:51.698 20418-20418/com.sbingo.viewsample I/Sbingo ViewGroupB: onClick
當ViewGroupB攔截事件後，之後對於該事件的所有動作ViewGroupB的dispatchTouchEvent方法將返回true,
ViewGroupB的onInterceptTouchEvent方法將不再執行。
去除MyView的OnClickListener,點擊MyView，MyView不消耗事件
07-07 11:29:06.078 7138-7138/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
07-07 11:29:06.078 7138-7138/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
07-07 11:29:06.078 7138-7138/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
07-07 11:29:06.078 7138-7138/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
07-07 11:29:06.079 7138-7138/com.sbingo.viewsample I/Sbingo MyView: dispatchTouchEvent
07-07 11:29:06.079 7138-7138/com.sbingo.viewsample I/Sbingo MyView: onTouch
07-07 11:29:06.079 7138-7138/com.sbingo.viewsample I/Sbingo MyView: onTouchEvent：0
07-07 11:29:06.079 7138-7138/com.sbingo.viewsample I/Sbingo ViewGroupB: onTouch
07-07 11:29:06.079 7138-7138/com.sbingo.viewsample I/Sbingo ViewGroupB: onTouchEvent
07-07 11:29:06.125 7138-7138/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
07-07 11:29:06.126 7138-7138/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
07-07 11:29:06.126 7138-7138/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
07-07 11:29:06.126 7138-7138/com.sbingo.viewsample I/Sbingo ViewGroupB: onTouch
07-07 11:29:06.126 7138-7138/com.sbingo.viewsample I/Sbingo ViewGroupB: onTouchEvent
07-07 11:29:06.144 7138-7138/com.sbingo.viewsample I/Sbingo ViewGroupB: onClick
接着去除ViewGroupB的OnClickListener,點擊MyView，MyView不消耗事件
07-07 11:42:36.186 17561-17561/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
07-07 11:42:36.186 17561-17561/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
07-07 11:42:36.187 17561-17561/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
07-07 11:42:36.187 17561-17561/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
07-07 11:42:36.187 17561-17561/com.sbingo.viewsample I/Sbingo MyView: dispatchTouchEvent
07-07 11:42:36.187 17561-17561/com.sbingo.viewsample I/Sbingo MyView: onTouch
07-07 11:42:36.187 17561-17561/com.sbingo.viewsample I/Sbingo MyView: onTouchEvent：0
07-07 11:42:36.187 17561-17561/com.sbingo.viewsample I/Sbingo ViewGroupB: onTouch
07-07 11:42:36.187 17561-17561/com.sbingo.viewsample I/Sbingo ViewGroupB: onTouchEvent
07-07 11:42:36.187 17561-17561/com.sbingo.viewsample I/Sbingo ViewGroupA: onTouchEvent
可以發現
- 當沒有OnClickListener時，觸摸事件就會向上傳遞，否則即使不消耗事件也不會；
- 向上傳遞時其實也是先到onTouch再到onTouchEvent方法。
到這裏，我們已經基本瞭解了分發流程，對於如何攔截也都知道了。
那麼如果子View不想被攔截，如何實現反攔截呢？
ViewGroup有一個requestDisallowInterceptTouchEvent`方法，是專門用來反攔截的，傳入true表示子View希望反攔截，false表示由父佈局決定是否攔截。
我們來試一試這個方法：
app啓動時調用ViewGroupB的requestDisallowInterceptTouchEvent(true)方法，點擊MyView，ViewGroupB攔截
07-07 13:22:20.082 12098-12098/com.sbingo.viewsample I/Sbingo ViewGroupB: requestDisallowInterceptTouchEvent
07-07 13:22:20.082 12098-12098/com.sbingo.viewsample I/Sbingo ViewGroupA: requestDisallowInterceptTouchEvent
07-07 13:22:41.056 12098-12098/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
07-07 13:22:41.056 12098-12098/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
07-07 13:22:41.057 12098-12098/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
07-07 13:22:41.057 12098-12098/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
07-07 13:22:41.057 12098-12098/com.sbingo.viewsample I/Sbingo ViewGroupB: onTouch
07-07 13:22:41.057 12098-12098/com.sbingo.viewsample I/Sbingo ViewGroupB: onTouchEvent
07-07 13:22:41.057 12098-12098/com.sbingo.viewsample I/Sbingo ViewGroupA: onTouchEvent
當調用ViewGroupB的requestDisallowInterceptTouchEvent(true)方法時，
會自動向上調用ViewGroupA的requestDisallowInterceptTouchEvent方法，
之後在ViewGroupA的onInterceptTouchEvent方法中返回了true進行攔截，攔截成功了。
可見此時requestDisallowInterceptTouchEvent(true)並沒有起作用。
這次因爲
1.每次點擊時，都會重置。所以我們在啓動時的設置沒有作用。
2.ACTION_DOWN不能被攔截，否則後續事件都不會向下傳遞。
ViewGroupB只在ACTION_DOWN時不攔截，其餘情況攔截,點擊MyView
07-07 13:36:44.715 25397-25397/com.sbingo.viewsample I/Sbingo ViewGroupB: requestDisallowInterceptTouchEvent
07-07 13:36:44.715 25397-25397/com.sbingo.viewsample I/Sbingo ViewGroupA: requestDisallowInterceptTouchEvent
07-07 13:36:49.350 25397-25397/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
07-07 13:36:49.350 25397-25397/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
07-07 13:36:49.350 25397-25397/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
07-07 13:36:49.351 25397-25397/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
07-07 13:36:49.352 25397-25397/com.sbingo.viewsample I/Sbingo MyView: dispatchTouchEvent
07-07 13:36:49.352 25397-25397/com.sbingo.viewsample I/Sbingo MyView: onTouch
07-07 13:36:49.352 25397-25397/com.sbingo.viewsample I/Sbingo MyView: onTouchEvent：0
07-07 13:36:49.435 25397-25397/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
07-07 13:36:49.435 25397-25397/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
07-07 13:36:49.435 25397-25397/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
07-07 13:36:49.436 25397-25397/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
07-07 13:36:49.436 25397-25397/com.sbingo.viewsample I/Sbingo MyView: dispatchTouchEvent
07-07 13:36:49.436 25397-25397/com.sbingo.viewsample I/Sbingo MyView: onTouch
07-07 13:36:49.436 25397-25397/com.sbingo.viewsample I/Sbingo MyView: onTouchEvent：3
MyView只收到了ACTION_DOWN和ACTION_CANCEL兩個動作。
接下來演示如何正確地實現反攔截：
ViewGroupB只在ACTION_DOWN時不攔截，其餘情況攔截,
重寫MyView的dispatchTouchEvent方法如下：
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
myLogger.log(Level.INFO, "dispatchTouchEvent");
switch (ev.getAction()) {
case MotionEvent.ACTION_DOWN:
getParent().requestDisallowInterceptTouchEvent(true);
break;
default:
}
return super.dispatchTouchEvent(ev);
}
點擊MyView，MyView消耗事件
07-07 14:13:50.178 26911-26911/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
07-07 14:13:50.179 26911-26911/com.sbingo.viewsample I/Sbingo ViewGroupA: onInterceptTouchEvent
07-07 14:13:50.179 26911-26911/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
07-07 14:13:50.179 26911-26911/com.sbingo.viewsample I/Sbingo ViewGroupB: onInterceptTouchEvent
07-07 14:13:50.179 26911-26911/com.sbingo.viewsample I/Sbingo MyView: dispatchTouchEvent
07-07 14:13:50.179 26911-26911/com.sbingo.viewsample I/Sbingo ViewGroupB: requestDisallowInterceptTouchEvent
07-07 14:13:50.179 26911-26911/com.sbingo.viewsample I/Sbingo ViewGroupA: requestDisallowInterceptTouchEvent
07-07 14:13:50.179 26911-26911/com.sbingo.viewsample I/Sbingo MyView: onTouch
07-07 14:13:50.179 26911-26911/com.sbingo.viewsample I/Sbingo MyView: onTouchEvent：0
07-07 14:13:50.251 26911-26911/com.sbingo.viewsample I/Sbingo ViewGroupA: dispatchTouchEvent
07-07 14:13:50.251 26911-26911/com.sbingo.viewsample I/Sbingo ViewGroupB: dispatchTouchEvent
07-07 14:13:50.251 26911-26911/com.sbingo.viewsample I/Sbingo MyView: dispatchTouchEvent
07-07 14:13:50.251 26911-26911/com.sbingo.viewsample I/Sbingo MyView: onTouch
07-07 14:13:50.251 26911-26911/com.sbingo.viewsample I/Sbingo MyView: onTouchEvent：1
這裏我們更換了調用requestDisallowInterceptTouchEvent方法的時機，
調用getParent().requestDisallowInterceptTouchEvent(true)後，
之後對於該事件的所有動作，ViewGroupA和ViewGroupB的onInterceptTouchEvent方法都沒有執行，
直接執行了子View的dispatchTouchEvent方法， 反攔截成功。
至此，所有可能的流程基本都已分析過。
至於爲什麼這樣就能反攔截成功，請接着往下看。
###源碼分析
對於以上的分析結論，基本是沒有疑問的。
主要困惑應該在於requestDisallowInterceptTouchEvent方法的使用上，我們着重講一下這個方法。先看一下它的源碼：
@Override
public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {
if (disallowIntercept == ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0)) {
// We're already in this state, assume our ancestors are too
return;
}
if (disallowIntercept) {
mGroupFlags |= FLAG_DISALLOW_INTERCEPT;
} else {
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
}
// Pass it up to our parent
if (mParent != null) {
mParent.requestDisallowInterceptTouchEvent(disallowIntercept);
}
}
這個方法對mGroupFlags進行位運算，並依次調用父佈局的requestDisallowInterceptTouchEvent方法，這和之前的log輸出相同。
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
......
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;
// Handle an initial down.
if (actionMasked == MotionEvent.ACTION_DOWN) {
// Throw away all previous state when starting a new touch gesture.
// The framework may have dropped the up or cancel event for the previous gesture
// due to an app switch, ANR, or some other state change.
cancelAndClearTouchTargets(ev);
resetTouchState();
}
// 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;
}
......
}
在dispatchTouchEvent方法中，如果觸摸動作是ACTION_DOWN，第15行就會調用resetTouchState方法:
/**
* Resets all touch state in preparation for a new cycle.
*/
private void resetTouchState() {
clearTouchTargets();
resetCancelNextUpFlag(this);
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
mNestedScrollAxes = SCROLL_AXIS_NONE;
}
這個重置方法的第7行又對mGroupFlags進行了位運算，再回到dispatchTouchEvent方法，看第22行的判斷，使得ACTION_DOWN情況下第24行的onInterceptTouchEvent總是得到執行。
也就是說，ACTION_DOWN時，會重置觸摸狀態，onInterceptTouchEvent方法必定會執行。
所以之前在app啓動時調用requestDisallowInterceptTouchEvent(true)方法，反攔截無效；在MyView收到ACTION_DOWN時調用requestDisallowInterceptTouchEvent(true)方法就有效，之後的動作就跳過onInterceptTouchEvent方法，直接分發給了MyView的dispatchTouchEvent方法。
第24、27、32行3處的intercepted賦值分別表示以下3種情況：
ACTION_DOWN或允許攔截、
不允許攔截（就是反攔截情況）
已經決定攔截且不是ACTION_DOWN動作。
說了這麼久的攔截，攔截方法是怎麼樣的呢？來看一下：
public boolean onInterceptTouchEvent(MotionEvent ev) {
if (ev.isFromSource(InputDevice.SOURCE_MOUSE)
&& ev.getAction() == MotionEvent.ACTION_DOWN
&& ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
&& isOnScrollbarThumb(ev.getX(), ev.getY())) {
return true;
}
return false;
}
可以看到，一般情況下是不會攔截的。
###總結
根據上面的所有分析，可以畫出如下的事件分發攔截流程圖：
這張圖涵蓋了大部分的情況，有助於我們理解和記憶。
除了這張圖以外，關於事件分發攔截，我還有以下總結：
- ViewGroup比View多了兩個攔截相關的方法。
- 有很多方法可以獲取觸摸事件，可以根據業務場景在合適的時機獲取並實現業務邏輯。
- 父佈局攔截可以一直攔截，也可以根據業務邊界動態改變。
- 子View反攔截也可以分爲靜態和動態實現，但要注意反攔截方法的調用時機。
- 核心要點就是熟悉整個流程，在合適的時機和View層次實現業務邏輯。