How do I specify that a non-generic Swift type should comply to a protocol?

Question

I'd like to implement a Swift method that takes in a certain class type, but only takes instances of those classes that comply to a specific protocol. For example, in Objective-C I have this method:

- (void)addFilter:(GPUImageOutput<GPUImageInput> *)newFilter; 

where GPUImageOutput is a particular class, and GPUImageInput is a protocol. Only GPUImageOutput classes that comply to this protocol are acceptable inputs for this method.

However, the automatic Swift-generated version of the above is

func addFilter(newFilter: GPUImageOutput!) 

This removes the requirement that GPUImageOutput classes comply with the GPUImageInput protocol, which will allow non-compliant objects to be passed in (and then crash at runtime). When I attempt to define this as GPUImageOutput<GPUImageInput>, the compiler throws an error of

Cannot specialize non-generic type 'GPUImageOutput'

How would I do such a class and protocol specialization in a parameter in Swift?

Answer 1

Is swift you must use generics, in this way:

Given these example declarations of protocol, main class and subclass:

protocol ExampleProtocol {     func printTest()   // classes that implements this protocol must have this method }  // an empty test class class ATestClass {  }  // a child class that implements the protocol class ATestClassChild : ATestClass, ExampleProtocol {     func printTest()     {         println("hello")     } } 

Now, you want to define a method that takes an input parameters of type ATestClass (or a child) that conforms to the protocol ExampleProtocol. Write the method declaration like this:

func addFilter<T where T: ATestClass, T: ExampleProtocol>(newFilter: T) {     println(newFilter) } 

Your method, redefined in swift, should be

func addFilter<T where T:GPUImageOutput, T:GPUImageInput>(newFilter:T!) {     // ... } 

EDIT:

as your last comment, an example with generics on an Enum

    enum OptionalValue<T> {         case None         case Some(T)     }     var possibleInteger: OptionalValue<Int> = .None     possibleInteger = .Some(100) 

Specialized with protocol conformance:

    enum OptionalValue<T where T:GPUImageOutput, T:GPUImageInput> {         case None         case Some(T)     } 

EDIT^2:

you can use generics even with instance variables:

Let's say you have a class and an instance variable, you want that this instance variable takes only values of the type ATestClass and that conforms to ExampleProtocol

class GiveMeAGeneric<T: ATestClass where T: ExampleProtocol> {     var aGenericVar : T? } 

Then instantiate it in this way:

    var child = ATestClassChild()     let aGen = GiveMeAGeneric<ATestClassChild>()     aGen.aGenericVar = child 

If child doesn't conform to the protocol ExampleProtocol, it won't compile

Answer 2

this method header from ObjC:

- (void)addFilter:(GPUImageOutput<GPUImageInput> *)newFilter { ... } 

is identical to this header in Swift:

func addFilter<T: GPUImageOutput where T: GPUImageInput>(newFilter: T?) { ... } 

both method will accept the same set of classes

• which is based on GPUImageOutput class; and
• conforms GPUImageInput protocol; and
• the newFilter is optional, it can be nil;