在通用 swift object 中约束 PAT

Question

Is there a way to constrain a protocol with associated type (PAT) within an object, ie not by adding the protocol to the generic list of the object.

My object already uses another generic protocol (T: ItemProtocol) that could be used to constrain the delegate PAT but I don't see how this has to be done.

protocol ItemProtocol {
    var title: String { get }
}

protocol FooDelegate: AnyObject {
    associatedtype Item: ItemProtocol
    func foobar(item: Item)
}

struct Foo<T: ItemProtocol> {
    // doesn't compile: where clause cannot be attached to a non-generic declaration
    typealias Delegate = FooDelegate where Delegate.Item == T

    let items: [T]
    weak var delegate: Delegate? // how to constrain Delegate.Item == T ?
}

I know something like this works, but in this case the object (Foo) depends on the delegate which I don't like.

struct Foo<Delegate: FooDelegate> {
    let items: [Delegate.Item]
    weak var delegate: Delegate?
}

Answer 1

It is not possible to do this "within" a type declaration. All of the generic placeholders and constraints need to exist in the type declaration (except for constrained extensions).

However, the compiler does not consider it to be redundant to define a generic placeholder that is the same type as an associated type of another generic placeholder.

struct Foo<Item, Delegate: FooDelegate> where Item == Delegate.Item {
  let items: [Item]
  var delegate: Delegate
}

Whether you go with that or this is a matter of preference.

struct Foo<Delegate: FooDelegate> {
  typealias Item = Delegate.Item

  let items: [Item]
  var delegate: Delegate
}