Adding version numbers to a Clojure ref

Question

我想知道每次更改我的引用时是否添加时间戳元数据的版本号是否有意义，以便可以使用数据的新鲜度来确定是否由GUI组件更新了数据。

Answer 1

If you add metadata (or plain data) to the ref, then UI components will have to poll the ref to know whether to update. You might be better off to use an agent send within the ref update to notify interested parties.

Answer 2

Although metadata is an option, you could just put your ref data in a map. Either way you have to do a map lookup since Clojure's metadata lives in a map. Using metadata just makes you jump through the extra hurdle. So when you define/update your ref just make it a map and you will be functionally equivalent without having to use meta to get at the information.

Answer 3

you can use the answer to your question from yesterday for this purpose also.

(def my-ref (ref {}))
(def my-ref-version (atom 0))
(add-watch my-ref (fn [key ref old new] (swap! my-ref-version inc))) 

If you stick to immutable data structures then you can save a copy of the data you last served and compare it to the data you are considering serving. This would be a lot simpler and you would not resend data that had been updated to the same value. using time stamps it good when you cant directly compare the data to what you last sent. 使用时间戳 ，当您无法直接将数据与上次发送的数据进行比较时， 有用。 With languages that dont offer efficient copy functions for the collections this is necessary because you can't efficiently save a copy of your data before sending it. with clojure's collections saving a copy before you send is both easy and efficient.

Answer 4

I guess, you will need to remember the last time you updated the gui with the value of the ref, in order to know whether updating the gui is necessary or not. If the value is a big datastructure or the update is expensive this might make sense.