How to create generic protocols in Swift?

Question

I'd like to create a protocol with a method that takes a generic input and returns a generic value.

This is what I've tried so far, but it produces the syntax error.

Use of undeclared identifier T.

What am I doing wrong?

protocol ApiMapperProtocol {     func MapFromSource(T) -> U }  class UserMapper: NSObject, ApiMapperProtocol {     func MapFromSource(data: NSDictionary) -> UserModel {         var user = UserModel() as UserModel         var accountsData:NSArray = data["Accounts"] as NSArray              return user     }  } 

Answer 1

It's a little different for protocols. Look at "Associated Types" in Apple's documentation.

This is how you use it in your example

protocol ApiMapperProtocol {     associatedtype T     associatedtype U     func MapFromSource(_:T) -> U }  class UserMapper: NSObject, ApiMapperProtocol {     typealias T = NSDictionary     typealias U = UserModel      func MapFromSource(_ data:NSDictionary) -> UserModel {         var user = UserModel()         var accountsData:NSArray = data["Accounts"] as NSArray         // For Swift 1.2, you need this line instead         // var accountsData:NSArray = data["Accounts"] as! NSArray         return user     } } 

Answer 2

To expound on Lou Franco's answer a bit, If you wanted to create a method that used a particular ApiMapperProtocol, you do so thusly:

protocol ApiMapperProtocol {     associatedtype T     associatedtype U     func mapFromSource(T) -> U }  class UserMapper: NSObject, ApiMapperProtocol {     // these typealiases aren't required, but I'm including them for clarity     // Normally, you just allow swift to infer them     typealias T = NSDictionary      typealias U = UserModel      func mapFromSource(data: NSDictionary) -> UserModel {         var user = UserModel()         var accountsData: NSArray = data["Accounts"] as NSArray         // For Swift 1.2, you need this line instead         // var accountsData: NSArray = data["Accounts"] as! NSArray         return user     } }  class UsesApiMapperProtocol {     func usesApiMapperProtocol<         SourceType,         MappedType,         ApiMapperProtocolType: ApiMapperProtocol where           ApiMapperProtocolType.T == SourceType,           ApiMapperProtocolType.U == MappedType>(           apiMapperProtocol: ApiMapperProtocolType,            source: SourceType) -> MappedType {         return apiMapperProtocol.mapFromSource(source)     } } 

UsesApiMapperProtocol is now guaranteed to only accept SourceTypes compatible with the given ApiMapperProtocol:

let dictionary: NSDictionary = ... let uses = UsesApiMapperProtocol() let userModel: UserModel = uses.usesApiMapperProtocol(UserMapper()     source: dictionary)