Declare class inside function.

PewPew.swift

protocol SomeProtocol: class {
    func pew() -> String
}

class SomeClass {
    
    func someFunc() -> SomeProtocol {
        let someClosure = { () -> SomeProtocol in
            class PewPew: SomeProtocol {
                func pew() -> String {
                    return "Pew-pew"
                }
            }
            
            return PewPew()
        }
        
        return someClosure()
    }
}

let obj = SomeClass()
let objb = obj.someFunc()
let resultingString = objb.pew()

PewSmew.swift

protocol PewShmewProtocol {
    func pew() -> String
    func shmew() -> String
}

class SomeClass {
    
    var pewShmewHandler: PewShmewProtocol?
    
    func pew() -> String?  {
        return pewShmewHandler?.pew()
    }
    
    func shmew() -> String? {
        return pewShmewHandler?.shmew()
    }
}

let obj = SomeClass()

obj.pewShmewHandler = {
    
    struct PewShmew: PewShmewProtocol {
        func pew() -> String {
            return "Pew-pew"
        }
        func shmew() -> String {
            return "Pew-shmew"
        }
    }
    
    return PewShmew()
}()


obj.pew()
obj.shmew()

obj.pewShmewHandler = {
    
    class PewShmew: PewShmewProtocol {
        var c = 0
        
        func pew() -> String {
            c += 1
            return "P-p \(c)"
        }
        func shmew() -> String {
            c += 1
            return "P-sh \(c)"
        }
    }
    
    return PewShmew()
    }()

obj.pew()
obj.shmew()