class - 如何在Lua中创建类，子类和属性？

我在Lua上学习课时很难。无用的谷歌搜索使我想到了有关元表的想法，并暗示了第三方库对于模拟/编写类是必需的。

这是一个示例(只是因为我注意到在提供示例代码时我得到了更好的答案):

public class ElectronicDevice
{
    protected bool _isOn;
    public bool IsOn { get { return _isOn; } set { _isOn = value; } }
    public  void Reboot(){_isOn = false; ResetHardware();_isOn = true; }
}

public class Router : ElectronicDevice
{
}

public class Modem :ElectronicDevice
{
    public void WarDialNeighborhood(string areaCode)
    {
        ElectronicDevice cisco = new Router();
        cisco.Reboot();
        Reboot();
        if (_isOn)
            StartDialing(areaCode);
    }
}

这是我第一次尝试使用哈维尔(Javier)建议的技术来翻译以上内容。

我接受了RBerteig的建议。但是，对派生类的调用仍然会产生:"attempt to call method 'methodName' (a nil value)"

--Everything is a table
ElectronicDevice = {};

--Magic happens
mt = {__index=ElectronicDevice};

--This must be a constructor
function ElectronicDeviceFactory ()
    -- Seems that the metatable holds the fields
    return setmetatable ({isOn=true}, mt)
end

-- Simulate properties with get/set functions
function ElectronicDevice:getIsOn()  return self.isOn end
function ElectronicDevice:setIsOn(value)  self.isOn = value end
function ElectronicDevice:Reboot() self.isOn = false;
    self:ResetHardware(); self.isOn = true; end
function ElectronicDevice:ResetHardware()  print('resetting hardware...') end

Router = {};
mt_for_router = {__index=Router}

--Router inherits from ElectronicDevice
Router = setmetatable({},{__index=ElectronicDevice});

--Constructor for subclass, not sure if metatable is supposed to be different
function RouterFactory ()
    return setmetatable ({},mt_for_router)
end

Modem ={};
mt_for_modem = {__index=Modem}

--Modem inherits from ElectronicDevice
Modem = setmetatable({},{__index=ElectronicDevice});

--Constructor for subclass, not sure if metatable is supposed to be different
function ModemFactory ()
    return setmetatable ({},mt_for_modem)
end

function Modem:WarDialNeighborhood(areaCode)
        cisco = RouterFactory();
        --polymorphism
        cisco.Reboot(); --Call reboot on a router
        self.Reboot(); --Call reboot on a modem
        if (self.isOn) then self:StartDialing(areaCode) end;
end

function Modem:StartDialing(areaCode)
    print('now dialing all numbers in ' .. areaCode);
end

testDevice = ElectronicDeviceFactory();
print("The device is on? " .. (testDevice:getIsOn() and "yes" or "no") );
testDevice:Reboot(); --Ok

testRouter = RouterFactory();
testRouter:ResetHardware(); -- nil value

testModem = ModemFactory();
testModem:StartDialing('123'); -- nil value

最佳答案

这是示例代码的文字转录示例，其中包含一个有用的Class库，该库可以移至另一个文件。

这绝不是Class的规范实现；您可以随意定义对象模型。

Class = {}

function Class:new(super)
    local class, metatable, properties = {}, {}, {}
    class.metatable = metatable
    class.properties = properties

    function metatable:__index(key)
        local prop = properties[key]
        if prop then
            return prop.get(self)
        elseif class[key] ~= nil then
            return class[key]
        elseif super then
            return super.metatable.__index(self, key)
        else
            return nil
        end
    end

    function metatable:__newindex(key, value)
        local prop = properties[key]
        if prop then
            return prop.set(self, value)
        elseif super then
            return super.metatable.__newindex(self, key, value)
        else
            rawset(self, key, value)
        end
    end

    function class:new(...)
        local obj = setmetatable({}, self.metatable)
        if obj.__new then
            obj:__new(...)
        end
        return obj
    end

    return class
end

ElectronicDevice = Class:new()

function ElectronicDevice:__new()
    self.isOn = false
end

ElectronicDevice.properties.isOn = {}
function ElectronicDevice.properties.isOn:get()
    return self._isOn
end
function ElectronicDevice.properties.isOn:set(value)
    self._isOn = value
end

function ElectronicDevice:Reboot()
    self._isOn = false
    self:ResetHardware()
    self._isOn = true
end

Router = Class:new(ElectronicDevice)

Modem = Class:new(ElectronicDevice)

function Modem:WarDialNeighborhood(areaCode)
    local cisco = Router:new()
    cisco:Reboot()
    self:Reboot()
    if self._isOn then
        self:StartDialing(areaCode)
    end
end

如果您坚持使用获取/设置属性的方法，则不需要__index和__newindex函数，而只需要一个__index表即可。在这种情况下，模拟继承的最简单方法是这样的:

BaseClass = {}
BaseClass.index = {}
BaseClass.metatable = {__index = BaseClass.index}

DerivedClass = {}
DerivedClass.index = setmetatable({}, {__index = BaseClass.index})
DerivedClass.metatable = {__index = DerivedClass.index}

换句话说，派生类的__index表“继承”了基类的__index表。之所以行之有效，是因为Lua在委托(delegate)给__index表时会有效地在其上重复查找，因此会调用__index表的元方法。

另外，请谨慎使用obj.Method(...)和obj:Method(...)。 obj:Method(...)是obj.Method(obj, ...)的语法糖，将这两个调用混合使用会产生异常错误。