error CS0234: The type or namespace name 'EventSystems' does not exist in the namespace 'UnityEngine'. Are you missing an assembly reference?
Unity3D 引用UnityEngine.EventSystems时 提示不存在EventSystems命名空间可以尝试把
Unity5安装路径的\Editor\Data\UnityExtensions\Unity\GUISystem\UnityEngine.UI.dll 复制到工程目录