Swift用枚举理解泛型

Question

I have following code written by other person:

public enum MSEntity<T : Metable> {
    case meta(MSMeta)
    case entity(T)

    public func value() -> T? {
        guard case let MSEntity.entity(value) = self else {
            return nil
        }
        return value
    }

    public func objectMeta() -> MSMeta {
        switch self {
        case .entity(let entity): return entity.meta
        case .meta(let meta): return meta
        }
    }
}

I have following questions:

1) what is the point of doing case entity(T)? What is "value" of that enum case and for what it may be used for?

2) I can't understand public func value() func. What is that guard checking? What is value? Can someone provide an example where similar code may be useful?

Answer 1

The enum seems to represent an object that can either only contain metadata or a entity with metadata.

It has 2 cases: meta represents an object with only metadata and that metadata is this case's associated value, entity represents an object that has a entity and metadata. This case's associated value is the object's entity, which contains metadata (so the constraint on T is Metable )

what is the point of doing case entity(T)? What is "value" of that enum case and for what it may be used for?

Well, for this you have to look at the docs or ask the person who wrote the code. As far as I'm concerned, MSEntity is an object that has 2 states:

• only metadata ( meta case)
• entity and metadata ( entity case)

So the entity case is there to represent that.

I can't understand public func value() func. What is that guard checking? What is value?

The value function seems to return the object's entity, but what if this object has no entity (ie self is actually in the meta case)? The author of this code decided to return nil . That is why there is a check: if self is in the meta case. The word value is part of the pattern matching. It might be clearer if I wrote it like this:

guard case let MSEntity.entity(let value) = self else {

Whatever is in the associated value of entity case is put into a variable called value , so that you can return it later.

Answer 2

The (MSMeta) and (T) associated values of the meta and entity cases, respectively.

The value() method (which IMO should be a computed property of type T? , rather than a function of type () -> T? ) returns the associated value of the entity case, if self is entity , otherwise it returns nil (when the self is meta ).