CODEWITHSUNDEEP
swift
Brad Larson
Brad Larson

Reputation: 170309

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

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?

Upvotes: 42

Views: 13146

Answers (3)

janakmshah
janakmshah

Reputation: 151

From Swift 4 onwards you can do:

func addFilter(newFilter: GPUImageOutput & GPUImageInput)

Further reading: https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Protocols.html

http://braking.github.io/require-conformance-to-multiple-protocols/

Multiple Type Constraints in Swift

Upvotes: 1

LombaX
LombaX

Reputation: 17364

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

Upvotes: 22

holex
holex

Reputation: 24041

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;

Upvotes: 13

Related Questions