核心数据和Swift中的计算属性与持久存储

Question

I have a feeling this is a silly question, but I'm new to this and I can't find a clear answer.

I have an Activity entity and a Sessions entity. Each Activity can have many Sessions. Each Session has a duration attribute.

Now I want the average session duration for an activity. Should I add an averageSessionDuration attribute to my Activity entity and update it and persist it when a user adds or deletes a session, or can I just add a computed property to my Activity class and have it calculate the average session duration whenever it's called?

I guess using a computed property the average session duration would be calculated a little more often than necessary, but I don't know if that matters, does it make a difference in the end?

I don't know what goes on behind the curtains, is fetching every session duration and taking the average a lot more expensive than just fetching an averageSessionDuration property that is updated when necessary and persisted? What are the best practices in this case? Is there something else I'm missing?

Thanks in advance,

Daniel

Answer 1

It most likely doesn't make any difference in performance but it would be cleaner code (I think) if you use a computed property instead of trying to make sure it's always up to date when you save other values.

If you are scared of performance issues you can always cache the calculated value every session and just invalidate the cache when you change values in the app. That might be the best of two worlds.

This is a question of taste and I'm sure there are arguments supporting both sides.

Answer 2

If you are ever going to fetch on that property then you will want to persist it. This is assuming a SQLite store.

If you are never going to fetch from it then a computed property is going to be cleaner.

That is the only question that is important for making that decision.