源码3：ExecuteEvents

上面在分析输入模块的时候都提到了会有对应的事件发送。具体的实现就在ExecuteEvents 类里面

下面我们来分析一下这个事件系统

在最上面有一个委托定义

  public delegate void EventFunction<T1>(T1 handler, BaseEventData eventData);

带有两个参数一个是handler 是个泛型一个是eventData通常这个会包含位置信息偏移量点击物体等等

基于这个委托在当前类定义了很多委托的实例（下面放了部分原始代码）

private static readonly EventFunction<IPointerEnterHandler> s_PointerEnterHandler = Execute;

private static void Execute(IPointerEnterHandler handler, BaseEventData eventData)
{
	handler.OnPointerEnter(ValidateEventData<PointerEventData>(eventData));
}

private static readonly EventFunction<IPointerExitHandler> s_PointerExitHandler = Execute;

private static void Execute(IPointerExitHandler handler, BaseEventData eventData)
{
	handler.OnPointerExit(ValidateEventData<PointerEventData>(eventData));
}

private static readonly EventFunction<IPointerDownHandler> s_PointerDownHandler = Execute;

private static void Execute(IPointerDownHandler handler, BaseEventData eventData)
{
	handler.OnPointerDown(ValidateEventData<PointerEventData>(eventData));
}

private static readonly EventFunction<IPointerUpHandler> s_PointerUpHandler = Execute;

private static void Execute(IPointerUpHandler handler, BaseEventData eventData)
{
	handler.OnPointerUp(ValidateEventData<PointerEventData>(eventData));
}

从这里看出来上面有很多具体的handler类型，这些handler 也都是一个interface 并且是继承IEventSystemHandler 。同时定义了自己的具体方法。

所以从这里就不难看出来 UI上的具体行为事件就是通过调用这些handler接口实现的。（上面代码看不明白的就得去熟悉下c# 委托相关的类容）

Execute

具体调用上面的委托实例就是通过下面代码实现了

    private static readonly ObjectPool<List<IEventSystemHandler>> s_HandlerListPool = new ObjectPool<List<IEventSystemHandler>>(null, l => l.Clear());

    public static bool Execute<T>(GameObject target, BaseEventData eventData, EventFunction<T> functor) where T : IEventSystemHandler
    {
        var internalHandlers = s_HandlerListPool.Get();
        GetEventList<T>(target, internalHandlers);
        //  if (s_InternalHandlers.Count > 0)
        //      Debug.Log("Executinng " + typeof (T) + " on " + target);

        var internalHandlersCount = internalHandlers.Count;
        for (var i = 0; i < internalHandlersCount; i++)
        {
            T arg;
            try
            {
                arg = (T)internalHandlers[i];
            }
            catch (Exception e)
            {
                var temp = internalHandlers[i];
                Debug.LogException(new Exception(string.Format("Type {0} expected {1} received.", typeof(T).Name, temp.GetType().Name), e));
                continue;
            }

            try
            {
                functor(arg, eventData);
            }
            catch (Exception e)
            {
                Debug.LogException(e);
            }
        }

        var handlerCount = internalHandlers.Count;
        s_HandlerListPool.Release(internalHandlers);
        return handlerCount > 0;
    }

上面代码思路是：

1.定义一个对象池每次执行前先从对象池中获取List ，上面讲过所有的操作事件都是IEventSystemHandler接口的子类

2.获取当前对象上所有的IEventSystemHandler 返回到上面的List集合中

        /// <summary>
        /// Get the specified object's event event.
        /// </summary>
        private static void GetEventList<T>(GameObject go, IList<IEventSystemHandler> results) where T : IEventSystemHandler
        {
            // Debug.LogWarning("GetEventList<" + typeof(T).Name + ">");
            if (results == null)
                throw new ArgumentException("Results array is null", "results");

            if (go == null || !go.activeInHierarchy)
                return;

            var components = ListPool<Component>.Get();
            go.GetComponents(components);

            var componentsCount = components.Count;
            for (var i = 0; i < componentsCount; i++)
            {
                if (!ShouldSendToComponent<T>(components[i]))
                    continue;

                // Debug.Log(string.Format("{2} found! On {0}.{1}", go, s_GetComponentsScratch[i].GetType(), typeof(T)));
                results.Add(components[i] as IEventSystemHandler);
            }
            ListPool<Component>.Release(components);

3.遍历获取到的IEventSystemHandler ，将handler作为第一个参数，eventData作为第二个参数执行EventFunction方法，也就是上面讲的委托实例。它会调用handler中对应接口的方法

例如在TouchInputModule中我们调用松开事件

 ExecuteEvents.Execute(pointerEvent.pointerPress, pointerEvent, ExecuteEvents.pointerUpHandler);

ExecuteHierarchy

与Execute方法类似这里面还定义了一个ExecuteHierarchy执行事件方法

        /// <summary>
        /// Execute the specified event on the first game object underneath the current touch.
        /// </summary>
        private static readonly List<Transform> s_InternalTransformList = new List<Transform>(30);

        public static GameObject ExecuteHierarchy<T>(GameObject root, BaseEventData eventData, EventFunction<T> callbackFunction) where T : IEventSystemHandler
        {
            GetEventChain(root, s_InternalTransformList);

            var internalTransformListCount = s_InternalTransformList.Count;
            for (var i = 0; i < internalTransformListCount; i++)
            {
                var transform = s_InternalTransformList[i];
                if (Execute(transform.gameObject, eventData, callbackFunction))
                    return transform.gameObject;
            }
            return null;
        }

    private static void GetEventChain(GameObject root, IList<Transform> eventChain)
    {
        eventChain.Clear();
        if (root == null)
            return;

        var t = root.transform;
        while (t != null)
        {
            eventChain.Add(t);
            t = t.parent;
        }
    }

实际上他最终的执行还是通过Execute 方法，不过不同点就在与他执行前先沿着Hierarchy获取了包括当前物体以及往上的所有父对象。然后遍历查询这些对项直到有handler为止。

不过可以想象这样做的话下层层级对象的事件肯定会阻挡上层层级对象的事件。

其他方法

    //判断对象能否接受对应事件
    /// <summary>
    /// Whether the specified game object will be able to handle the specified event.
    /// </summary>
    public static bool CanHandleEvent<T>(GameObject go) where T : IEventSystemHandler
    {
        var internalHandlers = s_HandlerListPool.Get();
        GetEventList<T>(go, internalHandlers);
        var handlerCount = internalHandlers.Count;
        s_HandlerListPool.Release(internalHandlers);
        return handlerCount != 0;
    }

//获取实际能接受对应事件的对象
    /// <summary>
    /// Bubble the specified event on the game object, figuring out which object will actually receive the event.
    /// </summary>
    public static GameObject GetEventHandler<T>(GameObject root) where T : IEventSystemHandler
    {
        if (root == null)
            return null;

        Transform t = root.transform;
        while (t != null)
        {
            if (CanHandleEvent<T>(t.gameObject))
                return t.gameObject;
            t = t.parent;
        }