lua Getter&Setter简单实现

lua是一门简单的语言，不带类和属性封装，但可以使用lua强大的元表模拟实现：

class.lua

local type = type
local rawset = rawset
local setmetatable = setmetatable

local traceCount = 0
local tracebacks = setmetatable({}, {__mode = "k"})

local function class(classname, super)
    local cls = {}

    cls.classname = classname
    cls.class = cls
    cls.Get = {}
    cls.Set = {}

    local Get = cls.Get
    local Set = cls.Set

    if super then
        -- copy super method
        for key, value in pairs(super) do
            if type(value) == "function" and key ~= "ctor" then
                cls[key] = value
            end
        end

        -- copy super getter
        for key, value in pairs(super.Get) do
            Get[key] = value
        end

        -- copy super setter
        for key, value in pairs(super.Set) do
            Set[key] = value
        end

        cls.super = super
    end

    function cls.__index(self, key)
        local func = cls[key]
        if func then
           return func
        end

        local getter = Get[key]
        if getter then
            return getter(self)
        end

        return nil
    end

    function cls.__newindex(self, key, value)
        local setter = Set[key]
        if setter then
            setter(self, value or false)
            return
        end

        if Get[key] then
            assert(false, "readonly property")
        end

        rawset(self, key, value)
    end

    function cls.new(...)
        local self = setmetatable({}, cls)
        local function create(cls, ...)
            if cls.super then
                create(cls.super, ...)
            end
            if cls.ctor then
                cls.ctor(self, ...)
            end
        end
        create(cls, ...)

        -- debug
        traceCount = traceCount + 1
        tracebacks[self] = traceCount

        return self
    end

    -- compat
    cls.dtor = nil
    function cls.delete(self)
        if tracebacks[self] < 0 then return end
        local destroy
        destroy = function(cls)
            if cls.dtor then
                cls.dtor(self)
            end
            if cls.super then
                destroy(cls.super)
            end
        end
        destroy(cls)
        tracebacks[self] = -tracebacks[self]
    end

    return cls
end

return class

示例：

local class = require "class"

local Point = class("Point")

function Point:ctor(x, y)
    self.x = x
    self.y = y
end

function Point.Get:x() return self._x end
function Point.Set:x(value) self._x = value end

function Point.Get:y() return self._y end
function Point.Set:y(value) self._y = value end

local p = Point.new(10, 20)
print(p.x, p.y)
p.x = 5
p.y = 5
print(p.x, p.y)

local class = require "class"

local Sprite = class("Sprite")

function Sprite:ctor()
    self.cobj = cc.Sprite:create()
end

function Sprite.Get:x() return self.cobj:getPositionX() end
function Sprite.Set:x(value) self.cobj:setPositionX(value) end

function Sprite.Get:y() return self.cobj:getPositionY() end
function Sprite.Set:y(value) self.cobj:setPositionY(value) end

local obj = Sprite.new()
obj.x = 30
obj.y = 40

obj.y = obj.x

这样做的好处就是，可以比较自然的访问和赋值一些变量，特别是UI布局中，可以少写很多setXX、getXX的方法。