Android事件分发机制示例分析

public boolean onTouchEvent(MotionEvent event) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                break;
            case MotionEvent.ACTION_MOVE:
                break;
            case MotionEvent.ACTION_UP:
                break;
            case MotionEvent.ACTION_CANCEL:
                break;
        }
        return true;
    }

 public boolean dispatchTouchEvent(MotionEvent ev) {
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            onUserInteraction();
        }
        if (getWindow().superDispatchTouchEvent(ev)) {
            return true;
        }
        return onTouchEvent(ev);
    }

 @Override
    public boolean superDispatchTouchEvent(MotionEvent event) {
        return mDecor.superDispatchTouchEvent(event);
    }

public boolean superDispatchTouchEvent(MotionEvent event) {
            return super.dispatchTouchEvent(event);
  }

public class ViewGroup {
    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        boolean handled = false;
        final int action = ev.getAction();
        final int actionMasked = action & MotionEvent.ACTION_MASK;
        // Check for interception.
        //按下后，第一次调用时actionMasked=down，mFirstTouchTarget=null
        //一直按着 actionMasked=move, 如果mFirstTouchTarget！=null说明有子view处理了touch事件
        final boolean intercepted;
        if (actionMasked == MotionEvent.ACTION_DOWN
                || mFirstTouchTarget != null) {
            final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
            if (!disallowIntercept) {
                //调用子类复写的方法 如果为true表示拦截，false表示不拦截
                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.
            //如果没有子view处理touch事件，并且不是第一次按下的down事件，group会对touch事件进行拦截
            intercepted = true;
        }
        TouchTarget newTouchTarget = null;
        //第一次按下时，判断view是否处理事件的标志
        boolean alreadyDispatchedToNewTouchTarget = false;
        //如果不拦截，则进入事件分发
        if (!canceled && !intercepted) {
            //注意，只有ACTION_DOWN时才会遍历子View
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
                    || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
                final int childrenCount = mChildrenCount;
                if (newTouchTarget == null && childrenCount != 0) {
                    final ArrayList<View> preorderedList = buildTouchDispatchChildList();
                    final View[] children = mChildren;
                    //开始遍历子view
                    for (int i = childrenCount - 1; i >= 0; i--) {
                        final View child = getAndVerifyPreorderedView(
                                preorderedList, children, childIndex);
                        //isTransformedTouchPointInView 通过比对坐标，来判断事件是否落在了child上，
                        // true是，false不是
                        if (!canViewReceivePointerEvents(child)
                                || !isTransformedTouchPointInView(x, y, child, null)) {
                            ev.setTargetAccessibilityFocus(false);
                            // 如果事件没有落在child上，则跳过此次循环，执行下一次循环
                            continue;
                        }
                        //通过遍历TouchTarget链表查找TouchTarget，如果找到了，则break跳出循环
                        newTouchTarget = getTouchTarget(child);
                        if (newTouchTarget != null) {
                            // Child is already receiving touch within its bounds.
                            // Give it the new pointer in addition to the ones it is handling.
                            newTouchTarget.pointerIdBits |= idBitsToAssign;
                            break;
                        }
                        //第一次进来newTouchTarget==null，进入下面的逻辑
                        //如果dispatchTransformedTouchEvent 返回true，表示子view对事件进行了处理，则跳出此循环，
                        // 不会再向子view进行分发
                        if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
                            // Child wants to receive touch within its bounds.
                            //将child缓存到TouchTarget的链表中
                            newTouchTarget = addTouchTarget(child, idBitsToAssign);
                            //注意这个标志位，true，下面的逻辑会用到
                            alreadyDispatchedToNewTouchTarget = true;
                            break;
                        }
 
                    }
                    if (preorderedList != null) preorderedList.clear();
                }
            }
        }
        // Dispatch to touch targets.
        //ACTION_DOWN和ACTION_MOVE 的时都会执行下面逻辑
        //mFirstTouchTarget==null 说明没有子View处理touch事件，dispatchTransformedTouchEvent的第三个参数为null。
        if (mFirstTouchTarget == null) {
            // No touch targets so treat this as an ordinary view.
            handled = dispatchTransformedTouchEvent(ev, canceled, null,
                    TouchTarget.ALL_POINTER_IDS);
        } else {
            // Dispatch to touch targets, excluding the new touch target if we already
            // dispatched to it.  Cancel touch targets if necessary.
            //对 TouchTarget 链表进行遍历，TouchTarget里缓存了处理事件的view，
            //一次完整事件，move会调用多次，所以在ACTION_DOWN时找到的对应的view，缓存起来，
            // 而不是每次都遍历ViewGroup的子View，这样太损坏性能
            TouchTarget target = mFirstTouchTarget;
            //开始对链表进行遍历，为啥是一个链表呢，这是为了进行多点触摸，会有多个view响应触摸事件。
            while (target != null) {
                final TouchTarget next = target.next;
                //alreadyDispatchedToNewTouchTarget ==true ，并且缓存的target和newTouchTarget是同一个
                //直接将handled置为true，防止ACTION_DOWN时dispatchTransformedTouchEvent方法重复调用
                if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
                    handled = true;
                } else {
                    if (dispatchTransformedTouchEvent(ev, cancelChild,
                            target.child, target.pointerIdBits)) {
                        handled = true;
                    }
                }
                target = next;
            }
        }
        //将handled 返回dispatchTouchEvent的调用者。
        return handled;
    }
    
    private TouchTarget getTouchTarget(@NonNull View child) {
        for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {
            if (target.child == child) {
                return target;
            }
        }
        return null;
    }
    
    private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
        final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
        target.next = mFirstTouchTarget;
        mFirstTouchTarget = target;
        return target;
    }
    private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
                                                  View child, int desiredPointerIdBits) {
        final boolean handled;
        // 只看最关键的两行代码
        if (child == null) {
            //如果child为null，没有子view处理touch事件，则调用父类View的dispatchTouchEvent。
            handled = super.dispatchTouchEvent(transformedEvent);
        } else {
            //如果child不为空，则调用child view 的dispatchTouchEvent
            //这行代码很关键，如果child是ViewGroup则继续在ViewGoup的进行事件分发，这会是一个递归调用
            //如果child 是一个View，则调用View的dispatchTouchEvent，进行事件的分发。
            handled = child.dispatchTouchEvent(transformedEvent);
        }
        //以上两行代码最终会调用到View的dispatchTouchEvent方法。
        return handled;
    }
}

public class View {
    public boolean dispatchTouchEvent(MotionEvent ev) {
        boolean result = false;
        ListenerInfo li = mListenerInfo;
        //如果view设置了mOnTouchListener，先调用mOnTouchListener.onTouch，
        //如果返回true，则表示消费了此事件，不在向下传递，为啥这样说？那是因为，通过他的返回值，进行了逻辑判断
        if (li != null && li.mOnTouchListener != null
                && (mViewFlags & ENABLED_MASK) == ENABLED
                && li.mOnTouchListener.onTouch(this, event)) {
            result = true;
        }
        //如果result为false，则调用View.onTouchEvent方法
        if (!result && onTouchEvent(event)) {
            result = true;
        }
        return result;
    }
    public boolean onTouchEvent(MotionEvent event) {
        switch (action) {
            case MotionEvent.ACTION_UP:
                if (mPerformClick == null) {
                    mPerformClick = new PerformClick();
                }
                if (!post(mPerformClick)) {
                    //调用performClick mOnClickListener.onClick()方法
                    performClickInternal();
                }
                break;
            case MotionEvent.ACTION_DOWN:
                break;
            case MotionEvent.ACTION_CANCEL:
                break;
            case MotionEvent.ACTION_MOVE:
                break;
        }
        return false;
    }
    public boolean performClick() {
        final boolean result;
        final ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnClickListener != null) {
            playSoundEffect(SoundEffectConstants.CLICK);
            li.mOnClickListener.onClick(this);
            result = true;
        } else {
            result = false;
        }
        return result;
    }
}