Lua事件总线：元表驱动的分发中枢

-- 错误：发布方挨个调用界面刷新，耦合满屏
function setBadge(n)
    badgeNum = n
    refreshBadge(n)
    refreshTab(n)
end

local store, subs = {}, {}
local bus = setmetatable({}, {
    __index = store,
    __newindex = function(_, k, v)
        store[k] = v
        local list = subs[k]
        if list then
            for _, fn in ipairs(list) do
                fn(k, v)
            end
        end
    end
})
local function subscribe(key, fn)
    subs[key] = subs[key] or {}
    table.insert(subs[key], fn)
end
subscribe("badge", function(_, v)
    refreshBadge(v)
end)
bus.badge = 3