【进阶实战】建造者模式组装复杂邮件：流式封装

local MailBuilder = {}
MailBuilder.__index = MailBuilder
function MailBuilder.new()
    return setmetatable({ to = nil, title = "", body = "",
        rewards = "", expireH = 72 }, MailBuilder)
end
function MailBuilder:to(name)
    self.to = name
    return self
end
function MailBuilder:title(t)
    self.title = t
    return self
end
function MailBuilder:body(b)
    self.body = b
    return self
end
function MailBuilder:attach(item, num)
    if self.rewards ~= "" then
        self.rewards = self.rewards .. ";" .. item .. "," .. num
    else
        self.rewards = item .. "," .. num
    end
    return self
end
function MailBuilder:build()
    assert(self.to, "收件人未设置")
    assert(self.rewards ~= "", "补偿邮件必须带附件")
    local uid = "#" .. self.to
    sendmail(uid, 0, self.title, self.body, self.rewards)
    sendmsg(getplayerbyname(self.to), 1, "补偿邮件已送达，有效期 " .. self.expireH .. " 小时。")
    return true
end

MailBuilder.new()
    :to("无名小卒")
    :title("装备异常补偿")
    :body("3 月 12 日装备回滚补偿，请查收。")
    :attach("裁决之杖", 1)
    :attach("金币", 500000)
    :build()