UGUI源码阅读（一）事件系统

1. 用于事件执行的静态类 ExecuteEvents：

触发时执行gameObject上的对应事件处理；

namespace UnityEngine.EventSystems
{
    public static class ExecuteEvents
    {
        //声明委托类型，各事件处理回调
        public delegate void EventFunction<T1>(T1 handler, BaseEventData eventData);

        //数据有效判断
        public static T ValidateEventData<T>(BaseEventData data) where T : class
        {
            if ((data as T) == null)
                throw new ArgumentException(String.Format("Invalid type: {0} passed to event expecting {1}", data.GetType(), typeof(T)));
            return data as T;
        }

        //指针移动回调
        private static readonly EventFunction<IPointerMoveHandler> s_PointerMoveHandler = Execute;

        private static void Execute(IPointerMoveHandler handler, BaseEventData eventData)
        {
            handler.OnPointerMove(ValidateEventData<PointerEventData>(eventData));
        }
        
        //...其余各事件处理回调
        
        //从子到父获取一系列Transform，冒泡效果
        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;
            }
        }

        /**
         * 只在一个GameObject上执行事件Handler，不冒泡
         * T为某类事件Handler
         */
        public static bool Execute<T>(GameObject target, BaseEventData eventData, EventFunction<T> functor) where T : IEventSystemHandler
        {
            var internalHandlers = ListPool<IEventSystemHandler>.Get();

            //获取target上T类型的所有事件Handler，且组件是激活的
            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
                {
                    //执行事件处理，如 arg.OnPointerMove(ValidateEventData<PointerEventData>(eventData));
                    functor(arg, eventData);
                }
                catch (Exception e)
                {
                    Debug.LogException(e);
                }
            }

            var handlerCount = internalHandlers.Count;
            ListPool<IEventSystemHandler>.Release(internalHandlers);
            return handlerCount > 0;
        }

        /// <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);

        /**
         * 冒泡执行事件处理，直到有个gameObject上执行了事件Handler
         */
        public static GameObject ExecuteHierarchy<T>(GameObject root, BaseEventData eventData, EventFunction<T> callbackFunction) where T : IEventSystemHandler
        {
            //从子到父获取一系列Transform
            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;    //某个gameObject上执行了事件Handler，就结束
            }
            return null;
        }

        //是否为类型T的事件Handler,且组件是激活的
        private static bool ShouldSendToComponent<T>(Component component) where T : IEventSystemHandler
        {
            var valid = component is T;
            if (!valid)
                return false;

            var behaviour = component as Behaviour;
            if (behaviour != null)
                return behaviour.isActiveAndEnabled;
            return true;
        }

        /// <summary>
        /// Get the specified object's event event.
        /// 获取指定gameObject上的类型T的事件handler
        /// </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);
            // Debug.LogWarning("end GetEventList<" + typeof(T).Name + ">");
        }

        /// <summary>
        /// Whether the specified game object will be able to handle the specified event.
        /// 指定的gameObject是否含有类型T的事件handler
        /// </summary>
        public static bool CanHandleEvent<T>(GameObject go) where T : IEventSystemHandler
        {
            var internalHandlers = ListPool<IEventSystemHandler>.Get();
            GetEventList<T>(go, internalHandlers);
            var handlerCount = internalHandlers.Count;
            ListPool<IEventSystemHandler>.Release(internalHandlers);
            return handlerCount != 0;
        }

        /// <summary>
        /// Bubble the specified event on the game object, figuring out which object will actually receive the event.
        /// 冒泡找到第一个gameObject，其包含类型T的事件handler
        /// </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;
            }
            return null;
        }
    }
}

2. EventSystem 组件：

主要用于驱动输入模块更新和处理输入；

    public class EventSystem : UIBehaviour
    {
        private List<BaseInputModule>