ViewGroup事件传递机制

前言

上一篇View的事件传递机制总结介绍了View的事件传递的过程，今天当然要看看ViewGroup的事件传递情况了。

示例代码

public class MyViewGroup extends LinearLayout {    public MyViewGroup(Context context, AttributeSet attrs) {        super(context, attrs);    }    @Override    public boolean dispatchTouchEvent(MotionEvent ev) {        System.out.println("======MyViewGroup dispatchTouchEvent");        return super.dispatchTouchEvent(ev);    }    @Override    public boolean onInterceptTouchEvent(MotionEvent ev) {        System.out.println("======MyViewGroup onInterceptTouchEvent");        return super.onInterceptTouchEvent(ev);    }}

public class MainActivity extends AppCompatActivity {    private Button button;    private ViewGroup viewGroup;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        button = (Button) findViewById(R.id.button);        viewGroup = (ViewGroup) findViewById(R.id.viewgroup);        button.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                System.out.println("====Button onClick");            }        });        button.setOnTouchListener(new View.OnTouchListener() {            @Override            public boolean onTouch(View v, MotionEvent event) {                System.out.println("====Button onTouch");                return false;            }        });        viewGroup.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                System.out.println("======MyViewGroup onClick");            }        });        viewGroup.setOnTouchListener(new View.OnTouchListener() {            @Override            public boolean onTouch(View v, MotionEvent event) {                System.out.println("======MyViewGroup onTouch");                return false;            }        });    }}

我们分别点击Button和空白区域，观察控制台输出

点击按钮输出如下：

03-29 02:21:47.918 1891-1891/? I/System.out: ======MyViewGroup dispatchTouchEvent03-29 02:21:47.918 1891-1891/? I/System.out: ======MyViewGroup onInterceptTouchEvent03-29 02:21:47.918 1891-1891/? I/System.out: ====Button onTouch03-29 02:21:48.038 1891-1891/? I/System.out: ======MyViewGroup dispatchTouchEvent03-29 02:21:48.038 1891-1891/? I/System.out: ======MyViewGroup onInterceptTouchEvent03-29 02:21:48.038 1891-1891/? I/System.out: ====Button onTouch03-29 02:21:48.038 1891-1891/? I/System.out: ====Button onClick

点击空白区域输出如下：

03-29 02:24:48.541 1891-1891/? I/System.out: ======MyViewGroup dispatchTouchEvent03-29 02:24:48.541 1891-1891/? I/System.out: ======MyViewGroup onInterceptTouchEvent03-29 02:24:48.541 1891-1891/? I/System.out: ======MyViewGroup onTouch03-29 02:24:48.631 1891-1891/? I/System.out: ======MyViewGroup dispatchTouchEvent03-29 02:24:48.631 1891-1891/? I/System.out: ======MyViewGroup onTouch03-29 02:24:48.631 1891-1891/? I/System.out: ======MyViewGroup onClick

从上面的打印可以看出，当ViewGoup内嵌套Button的时候，点击Button只会执行Button的onTouch和onClick事件，当点击Button以为的区域时，才会执行ViewGroup的触摸事件。

接下来我们看看源码

 public boolean dispatchTouchEvent(MotionEvent ev) {         final int action = ev.getAction();         final float xf = ev.getX();         final float yf = ev.getY();         final float scrolledXFloat = xf + mScrollX;         final float scrolledYFloat = yf + mScrollY;         final Rect frame = mTempRect;         boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;         if (action == MotionEvent.ACTION_DOWN) {             if (mMotionTarget != null) {                 // this is weird, we got a pen down, but we thought it was                 // already down!                 // XXX: We should probably send an ACTION_UP to the current                 // target.                 mMotionTarget = null;             }             // If we're disallowing intercept or if we're allowing and we didn't             // intercept             if (disallowIntercept || !onInterceptTouchEvent(ev)) {                 // reset this event's action (just to protect ourselves)                 ev.setAction(MotionEvent.ACTION_DOWN);                 // We know we want to dispatch the event down, find a child                 // who can handle it, start with the front-most child.                 final int scrolledXInt = (int) scrolledXFloat;                 final int scrolledYInt = (int) scrolledYFloat;                 final View[] children = mChildren;                 final int count = mChildrenCount;                 for (int i = count - 1; i >= 0; i--) {                     final View child = children[i];                     if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE                             || child.getAnimation() != null) {                         child.getHitRect(frame);                         if (frame.contains(scrolledXInt, scrolledYInt)) {                             // offset the event to the view's coordinate system                             final float xc = scrolledXFloat - child.mLeft;                             final float yc = scrolledYFloat - child.mTop;                            ev.setLocation(xc, yc);                             if (child.dispatchTouchEvent(ev))  {                                 // Event handled, we have a target now.                                mMotionTarget = child;                                 return true;                            }                             // The event didn't get handled, try the next view.                             // Don't reset the event's location, it's not                             // necessary here.                        }                     }                 }             }         }         boolean isUpOrCancel = (action == MotionEvent.ACTION_UP) ||                 (action == MotionEvent.ACTION_CANCEL);         if (isUpOrCancel) {             // Note, we've already copied the previous state to our local            // variable, so this takes effect on the next event             mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;         }         // The event wasn't an ACTION_DOWN, dispatch it to our target if         // we have one.         final View target = mMotionTarget;         if (target == null) {             // We don't have a target, this means we're handling the            // event as a regular view.             ev.setLocation(xf, yf);            return super.dispatchTouchEvent(ev);         }         // if have a target, see if we're allowed to and want to intercept its         // events         if (!disallowIntercept && onInterceptTouchEvent(ev)) {             final float xc = scrolledXFloat - (float) target.mLeft;             final float yc = scrolledYFloat - (float) target.mTop;             ev.setAction(MotionEvent.ACTION_CANCEL);             ev.setLocation(xc, yc);             if (!target.dispatchTouchEvent(ev)) {                 // target didn't handle ACTION_CANCEL. not much we can do                // but they should have.             }            // clear the target            mMotionTarget = null;             // Don't dispatch this event to our own view, because we already             // saw it when intercepting; we just want to give the following            // event to the normal onTouchEvent().            return true;         }         if (isUpOrCancel) {             mMotionTarget = null;         }         // finally offset the event to the target's coordinate system and         // dispatch the event.         final float xc = scrolledXFloat - (float) target.mLeft;         final float yc = scrolledYFloat - (float) target.mTop;         ev.setLocation(xc, yc);         return target.dispatchTouchEvent(ev);     }

1.获取当前手指在屏幕上触摸点击的坐标位置，用于判断当前手指触摸点击的是View区域还是ViewGroup区域。
2.获得disallowIntercept的值，disallowIntercept指的是是否禁用掉事件拦截功能，默认值是false，你可以调用requestDisallowInterceptTouchEvent方法修改它。
3.由于disallowIntercept默认值是false，所以条件是否满足完全取决于方法onInterceptTouchEvent返回值取反。而我们进入该方法会发现里面的实现仅仅是返回一个false。也就是if条件满足
4.通过一个for循环遍历当前ViewGroup下所以子View
5.获取遍历子View在屏幕上的坐标位置，然后代码第36行，判断当前屏幕手指触摸点击坐标是否包含遍历的子View在屏幕上的坐标位置范围？如果包含，者表示当前手指触摸点击的地方是该子View，也就是点击了Button。否则表示当前手指触摸并没有点击到ViewGroup中的子View，也就是点击到了空白区域。
6.调用子View的dispatchTouchEvent方法来处理View的触摸事件分发，这里一步就是我们上一篇博客分析的View的事件传递总结入口。在这篇博客中我们知道，当View是可点击的或者长安点击或者设置了setOnClickListener点击监听事件的，View#dispatchTouchEvent方法一律返回true，否则返回false。所以当条件满足，也就是子View设置了点击事件时ViewGroup#dispatchTouchEvent方法返回true，触摸对象mMotionTarget = child赋值成当前点击的子Viwe执行结束。因此这也验证了上面示例代码，当button设置了点击事件时只执行了Button的onClick事件，并没有执行任何关于ViewGroup的触摸点击事件。

总结：

• onInterceptTouchEvent方法是用于ViewGroup对子View的触摸事件拦截功能，默认返回false，不拦截子View的触摸事件，可以重写该方法，返回true来拦截子View的触摸事件传递。此时只会执行ViewGroup的触摸事件传递。

• 当子View是不可点击的且没有设置setOnClickListener点击监听事件时，会先执行子View的触摸事件，然后在执行ViewGroup的触摸事件。

  现在俩验证以上两个结论。

onInterceptTouchEvent返回true

 @Override    public boolean onInterceptTouchEvent(MotionEvent ev) {        System.out.println("======MyViewGroup onInterceptTouchEvent");        return true;    }

打印如下：

03-29 02:48:50.932 2496-2496/? I/System.out: ======MyViewGroup dispatchTouchEvent03-29 02:48:50.932 2496-2496/? I/System.out: ======MyViewGroup onInterceptTouchEvent03-29 02:48:50.932 2496-2496/? I/System.out: ======MyViewGroup onTouch03-29 02:48:51.032 2496-2496/? I/System.out: ======MyViewGroup dispatchTouchEvent03-29 02:48:51.032 2496-2496/? I/System.out: ======MyViewGroup onTouch03-29 02:48:51.042 2496-2496/? I/System.out: ======MyViewGroup onClick

由打印可以看出，当onInterceptTouchEvent返回true时，是不会执行子View的触摸点击事件的。也就是父View拦截了子View的触摸事件，不允许向下传递。

子View不可点击且没设置setOnClickListener

我们把Button修改成ImageView,并且不设置点击事件

public class MainActivity extends AppCompatActivity {    private ImageView imageView;    private ViewGroup viewGroup;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        imageView = (ImageView) findViewById(R.id.imageview);        viewGroup = (ViewGroup) findViewById(R.id.viewgroup);        imageView.setOnTouchListener(new View.OnTouchListener() {            @Override            public boolean onTouch(View v, MotionEvent event) {                System.out.println("====Button onTouch");                return false;            }        });        viewGroup.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                System.out.println("======MyViewGroup onClick");            }        });        viewGroup.setOnTouchListener(new View.OnTouchListener() {            @Override            public boolean onTouch(View v, MotionEvent event) {                System.out.println("======MyViewGroup onTouch");                return false;            }        });    }}

打印如下：

03-29 02:56:35.059 2735-2735/? I/System.out: ======MyViewGroup dispatchTouchEvent03-29 02:56:35.059 2735-2735/? I/System.out: ======MyViewGroup onInterceptTouchEvent03-29 02:56:35.059 2735-2735/? I/System.out: ====Button onTouch03-29 02:56:35.059 2735-2735/? I/System.out: ======MyViewGroup onTouch03-29 02:56:35.149 2735-2735/? I/System.out: ======MyViewGroup dispatchTouchEvent03-29 02:56:35.149 2735-2735/? I/System.out: ======MyViewGroup onTouch03-29 02:56:35.159 2735-2735/? I/System.out: ======MyViewGroup onClick

由打印可以看出，既执行了子View的触摸事件，也执行了ViewGroup的触摸事件，由于ImageView默认情况是不可点击的，因此：当子View不可点击或者么有设置setOnClickListener点击事件时，点击子View是先执行View的触摸事件，然后在执行ViewGroup的触摸事件的。

总结：
1.Android事件分发是先传递到ViewGroup，再由ViewGroup传递到View的。
2.在ViewGroup中可以通过onInterceptTouchEvent方法对事件传递进行拦截，onInterceptTouchEvent方法返回true代表不允许事件继续向子View传递，返回false代表不对事件进行拦截，默认返回false。
3.子View中如果将传递的事件消费掉，ViewGroup中将无法接收到任何事件。