Defining protocols in Swift

Question

What are the differences between these three protocol definitions in Swift?

1. protocol FooDelegate {}

2. @objc protocol FooDelegate {} -- is this the same as protocol FooDelegate, NSObjectProtocol {} ?

3. protocol FooDelegate: class {}

Answer 1

I'll deal with the cases in a slightly different order:

1. A protocol can be adopted by an enum, a struct, or a class.

2. protocol FooDelegate: class {} means that this protocol can be adopted only by a class.

3. @objc protocol FooDelegate {} exposes this protocol to Objective-C - Objective-C can "see" it. It also implies (2), ie now this is a class protocol.

A good example of why you'd want to do (2) is when you want to declare a weak instance property typed as a protocol:

weak var delegate : MyDelegateProtocol

You can't say weak unless this thing is a class instance - not a struct or enum (since they do not have the same memory management). Declaring this protocol as a class protocol calms the fears of the compiler by telling it that whoever adopts this protocol will in fact be a class.

A good example of why you'd want to do (3) is that you want your protocol to have optional members. Only Objective-C can do that, so you effectively turn your protocol into an Objective-C protocol. And of course protocols defined in Objective-C are automatically seen by Swift as @objc protocols.

Answer 2

To add to matt's answer,

@objc protocol FooDelegate {} -- is this the same as protocol FooDelegate, NSObjectProtocol {}?

You mean protocol FooDelegate : NSObjectProtocol {} ? No. It doesn't extend NSObjectProtocol unless you explicitly declare it as such.