关注降价：收藏商品的价格提醒

-- 错误：收藏后无任何变化提醒
function watch(it)
    watchList[it.id] = it.price
end

local function checkPrice(it)
    local watch = getWatch(it.id)
    if watch == nil then return end
    if it.price < watch.price
        and not watch.notified then
        notify(watch.owner, it.id,
            watch.price, it.price)
        setWatchFlag(it.id, true)
    end
end
local function unwatch(it)
    delWatch(it.id)
end