Lua+元方法__index链：多重继承的查找链本质

setmetatable(Child, { __index = function(t, k) return Parent1[k] or Parent2[k] end })

graph TD
    A["Child.foo"] --> B{"Child有foo?"}
    B -->|否| C["查Parent1.foo"]
    C -->|否| D["查Parent2.foo"]
    D -->|否| E["返回nil"]
    C -->|有| F["返回Parent1.foo"]
    D -->|有| G["返回Parent2.foo"]

local function createClass(...)
    local parents = {...}
    local child = {}
    local childMT = {
        __index = function(t, k)
            for _, p in ipairs(parents) do
                local v = p[k]
                if v ~= nil then return v end
            end
        end
    }
    setmetatable(child, childMT)
    child.parents = parents
    return child
end

local Flyer = { fly = function() print("飞") end }
local Swimmer = { swim = function() print("游") end }
local Duck = createClass(Flyer, Swimmer)

Duck.fly()   -- 飞（来自Flyer）
Duck.swim()  -- 游（来自Swimmer）