【高级技巧】__eq与__len：比较与长度的元方法定制

local bagMeta = {
    __len = function(self)
        local n = 0
        for _ in pairs(self.items) do
            n = n + 1
        end
        return n
    end,
    __eq = function(a, b)
        for k, v in pairs(a.items) do
            if b.items[k] ~= v then
                return false
            end
        end
        for k in pairs(b.items) do
            if a.items[k] == nil then
                return false
            end
        end
        return true
    end,
}
local function makeBag()
    return setmetatable({ items = {} }, bagMeta)
end

local function bagDemo(actor)
    actor = getplayerbyname(actor)
    local bagA, bagB = makeBag(), makeBag()
    bagA.items["金创药"] = 3
    bagB.items["金创药"] = 3
    sendmsg(actor, 1, "背包占用 " .. #bagA .. " 格，两包内容一致：" ..
        tostring(bagA == bagB))
end