事件分发机制

先从底端的View看起。

public class View {

    // 经过Activity与ViewGroup的分发到了View
    public boolean dispatchTouchEvent(MotionEvent event) {

        ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnTouchListener != null
                && (mViewFlags & ENABLED_MASK) == ENABLED
                && li.mOnTouchListener.onTouch(this, event)) {
            result = true;
        }

        if (!result && onTouchEvent(event)) {
            result = true;
        }
    }
    
    //如果设置了onTouchListener 执行回调函数onTouch
    public boolean onTouchEvent(MotionEvent event) {

        if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
            switch (action) {
                case MotionEvent.ACTION_UP:
                    performClick();
                ...
            }
            return true;
        }
    }

    // 执行OnClickListener回调消耗事件
    public boolean performClick() {

        final boolean result;
        final ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnClickListener != null) {
            li.mOnClickListener.onClick(this);
            result = true;
        } else {
            result = false;
        }

        return result;
    }

}

ViewGroup相较View多了拦截处理和给子View分发的操作。

public class ViewGroup{
    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 {

            intercepted = true;
        }

        if (!canceled && !intercepted) {
            for (int i = childrenCount - 1; i >= 0; i--) {
                final int childIndex = getAndVerifyPreorderedIndex(
                        childrenCount, i, customOrder);
                final View child = getAndVerifyPreorderedView(
                        preorderedList, children, childIndex);

                //子View处理了事件
                if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
                    // Child wants to receive touch within its bounds.
                    mLastTouchDownTime = ev.getDownTime();
                    if (preorderedList != null) {
                        // childIndex points into presorted list, find original index
                        for (int j = 0; j < childrenCount; j++) {
                            if (children[childIndex] == mChildren[j]) {
                                mLastTouchDownIndex = j;
                                break;
                            }
                        }
                    } else {
                        mLastTouchDownIndex = childIndex;
                    }
                    mLastTouchDownX = ev.getX();
                    mLastTouchDownY = ev.getY();
                    newTouchTarget = addTouchTarget(child, idBitsToAssign);
                    alreadyDispatchedToNewTouchTarget = true;
                    break;
                }
                
                // 如果子View都没有处理，或者View.onTouch 返回false

                if (mFirstTouchTarget == null) {
                    // No touch targets so treat this as an ordinary view.
                    handled = dispatchTransformedTouchEvent(ev, canceled, null,
                            TouchTarget.ALL_POINTER_IDS);
                }
            }
        }
    }

    //让子View去尝试执行
    private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
                                                  View child, int desiredPointerIdBits) {
        final boolean handled;

        // 如果子View都没有处理，或者View.onTouch 返回false ，将会掉父类View的dispatchTouchEvent，将自己当成View去处理onTouch onTouchEvent和performClick
        if (child == null) {
            handled = super.dispatchTouchEvent(transformedEvent);
        } else {
            final float offsetX = mScrollX - child.mLeft;
            final float offsetY = mScrollY - child.mTop;
            transformedEvent.offsetLocation(offsetX, offsetY);
            if (!child.hasIdentityMatrix()) {
                transformedEvent.transform(child.getInverseMatrix());
            }

            handled = child.dispatchTouchEvent(transformedEvent);
        }
        
        return handled;

    }
        
}

大致可理解为伪代码如下

public boolean dispatchTouchEvent(MotionEvent ev){
    
    boolean consume;
    
    if(onInterceptTouchEvent(ev)){
        consume = onTouchEvent(ev);
    }else {
        consume = child.dispatchTouchEvent(ev);
    }
    
    return consume;
}

再向上传递到Activity实际是顶部View DecorView做的处理，与ViewGroup无二意。
反过来从Activity->ViewGroup->View 就是事件分发的流程。