-- Instantiates a class

local function _instantiate(class, ...)

    -- 抽象类不能实例化

    if rawget(class, "__abstract") then

        error("asbtract class cannot be instantiated.")

    end

    -- 单例模式，如果实例已经生成，则直接返回

    if rawget(class, "__singleton") then

        -- _G[class]值为本class的实例

        if _G[class] then

            return _G[class]

        end

    end

    local inst = setmetatable({__class=class}, {__index = class})

    if inst.__init__ then

        inst:__init__(...)

    end

    --单例模式，如果实例未生成，则将实例记录到类中

    if rawget(class, "__singleton") then

        if not _G[class] then

            _G[class] = inst

            -- 对类对象增加实例获取接口

            class.getInstance = function ( self )

                return _G[class]

            end

            -- 对类对象增加实例销毁

            class.destroyInstance = function ( self )

                return _G[class]

            end

        end

    end

    return inst

end

-- LUA类构造函数

local function class(base)

    local metatable = {

        __call = _instantiate,

        __index = base

    }

    -- __parent 属性缓存父类，便于子类索引父类方法

    local _class = {__parent = base}

    -- 在class对象中记录 metatable ，以便重载 metatable.__index

    _class.__metatable = metatable

    return setmetatable(_class, metatable)

end

--- Test whether the given object is an instance of the given class.

-- @param object Object instance

-- @param class Class object to test against

-- @return Boolean indicating whether the object is an instance

-- @see class

-- @see clone

function instanceof(object, class)

    local meta = getmetatable(object)

    while meta and meta.__index do

        if meta.__index == class then

            return true

        end

        meta = getmetatable(meta.__index)

    end

    return false

end

Dmenu = class() -- 菜单类

Dmenu.__singleton = true -- 开启单例模式

local menu = Dmenu()
if Dmenu:getInstance() == menu then
     print("true")
end

Dmenu:destroyInstance()
menu:destroyInstance()