Mocking a generic class in Swift

Question

I have a class for that I want to write unit test. This class has a dependency, which is a generic class, that I want to mock.

This is the class I want to test.

class ClassToTest: SomeProtocol {
    
    private let dependency = DependencyClass<AnotherClass>()
    
    init() {
        //Some Code Here
    }
}

class DependencyClass<General: AnotherClassProtocol> {
    func getData(from general: General) -> AnyPublisher<Data, Error> {
        //Some Code Here
    }
}

What I tried

class ClassToTest: SomeProtocol {
    
    private let dependency: DependencyClassProtocol
    
    init(_ dependency: DependencyClassProtocol = DependencyClass<AnotherClass>()) {
        //Some Code Here
        self. dependency = dependency
    }
}

class DependencyClass<General: AnotherClassProtocol>: DependencyClassProtocol {
    func getData(from general: General) -> AnyPublisher<Data, Error> {
        //Some Code Here
    }
}

protocol DependencyClassProtocol {
    associatedtype General
    func getData(from general: General) -> AnyPublisher<Data, Error> 
}

This approach gives me error "Protocol can an only be used as a generic constraint because it has Self or associated type requirements".

How can I Mock the DependencyClass to test the exact behaviour of ClassToTest.

Answer 1

I have not removed the generic constraints from the DependencyClass<General: AnotherClassProtocol> and also not added the type alias in the class definition. As soon as I did these changes it worked.

import Combine
import Foundation

protocol DependencyClassProtocol {
    associatedtype General
    func getData(from general: General) -> AnyPublisher<Data, Error>?
}

class ClassToTest: SomeProtocol {
    
    private let dependency: any DependencyClassProtocol
    
    init(_ dependency: any DependencyClassProtocol = DependencyClass()) {
        //Some Code Here
        self.dependency = dependency
    }
}

class DependencyClass: DependencyClassProtocol {
    typealias General = AnotherClassProtocol
    
    func getData(from general: General) -> AnyPublisher<Data, Error>? {
        return nil
    }
}

class MockDependencyClass: DependencyClassProtocol {
    typealias General = AnotherClassProtocol
    
    func getData(from general: General) -> AnyPublisher<Data, Error>? {
        return nil
    }
}

protocol AnotherClassProtocol {
    
}

protocol SomeProtocol {
    
}

class AnotherClass: AnotherClassProtocol {}

let ctt = ClassToTest(MockDependencyClass())