You need to declare the type of the protocol as AnyObject. protocol ProtocolNameDelegate: AnyObject { // Protocol stuff goes here } class SomeClass { weak var delegate: ProtocolNameDelegate? } Using AnyObject you say that only classes can conform to this protocol, whereas structs or enums can’t.
Protocol Observing inherits from protocol Hashable, which in turn inherits from protocol Equatable. Protocol Equatable has the following requirement: func ==(lhs: Self, rhs: Self) -> Bool And a protocol that contains Self somewhere inside it cannot be used anywhere except in a type constraint. Here is a similar question.
The ability to put superclass constraints on protocols declarations (that is, being able to define protocol P where Self : C where C is the type of a class) was a premature consequence of SE-0156, and the syntax should have been rejected in Swift 4.x until the feature was implemented. Attempting to use this feature … Read more
In Swift 4 it is now possible to declare a variable that is a subclass of a type and implements one or more protocols at the same time. var myVariable: MyClass & MyProtocol & MySecondProtocol To do an optional variable: var myVariable: (MyClass & MyProtocol & MySecondProtocol)? or as the parameter of a method: func … Read more
The problem, which David has already alluded to, is that once you constrain a protocol’s associatedtype to a specific (non @objc) protocol, you have to use a concrete type to satisfy that requirement. This is because protocols don’t conform to themselves – therefore meaning that you cannot use Address to satisfy the protocol’s associated type … Read more
Suppose for the moment we adjust your protocol to add a routine that uses the associated type: public protocol RequestType: class { associatedtype Model var path: String { get set } func frobulateModel(aModel: Model) } And Swift were to let you create an array of RequestType the way you want to. I could pass an … Read more
There’s no real reason why this shouldn’t be possible, a read-only property requirement can be covariant, as returning a ConformsToB instance from a property typed as ProtocolB is perfectly legal. Swift just currently doesn’t support it. In order to do so, the compiler would have to generate a thunk between the protocol witness table and … Read more
The problem is that you’re making a promise that the compiler can’t prove you’ll keep. So you created this promise: Calling copy() will return its own type, fully initialized. But then you implemented copy() this way: func copy() -> Self { return C() } Now I’m a subclass that doesn’t override copy(). And I return … Read more
Why don’t protocols conform to themselves? Allowing protocols to conform to themselves in the general case is unsound. The problem lies with static protocol requirements. These include: static methods and properties Initialisers Associated types (although these currently prevent the use of a protocol as an actual type) We can access these requirements on a generic … Read more