Swinject: How to use delegate pattern with interface segregation (class vs interface)?

Question

I'm working my way into the injection dependency issue. Now the question arises how to use the interface segregation principle in the case of the delegate pattern? I am using the Swinject framework for dependency injection. How can I solve this?

class Client {
    private var interface: ParentInterface

    ...

    func execute() {
        interface = globalContainer.resolve(ParentInterface.self)
        interface?.parentMethod()
    }
}

protocol ParentInterface {
    func parentMethod()
}

class Parent: ParentInterface, ParentDelegate {

    // Dependency
    private var child: Child? //  I WANT TO USE AN PROTOCOL HERE, NOT THE CLASS

    init(childReference: Child) {
        self.child = childReference
        self.child?.delegate = self //  But if I use the protocol I cant access the delegate property
    }

    public func parentMethod() {
        let result = calcSomething()

        // Access child class interface method
        child?.childMethod(result)
    }
    ...
}

The child class, nothing unusual so far.

protocol ParentDelegate: class {
    func delagteMethod(value: Double)
}

protocol ChildInterface {
    func childMethod(_ result: Double)
}

class Child: ChildInterface {

    weak var delegate: ParentDelegate?

    ...

    private func delagteMethod() {
      delegate?.delagteMethod(value: someValue)
    }

}

But to inject the dependencies properly I need a protocol and not a direct class reference, right? Like this:


//  Would like to
container.register(ParentInterface.self) { r in
    Parent(childInterface: r.resolve(ChildInterface.self)!)
} 

//  Only way I get it working without interface
container.register(ParentDelegate.self) { r in
    Parent(childClass: Child())
}

container.register(ChildInterface.self) { _ in Child() }
    .initCompleted { r, c in
        let child = c as! Child
        child.delegate = r.resolve(ParentDelegate.self) 
}

In short, I'm going around in circles. If I use an interface for the child class I can't access the delegate property, if I use a class reference I can't reasonably to mock/stub the interface methods.

What am I missing? Thanks a lot in advance!

score 1 · Answer 1 · answered Sep 19 '19 at 14:27

I would recommend removing self.child?.delegate = self from the Parent constructor for two reasons:

Shouldn't the DI framework be responsible for wiring all the dependencies?
In Swinject specifically, this may lead to incorrect dependencies, see the remark in docs

Pattern that I tend to use is more or less what you came up with:

class Parent: ParentInterface, ParentDelegate { 
    init(child: ChilInterface) { self.child = child }
}

class Child: ChildInterface {
    weak var parent: ParentDelegate?
}

container.register(ParentInterface.self) { Parent(child: $0.resolve(ChildInterface.self)!) }
    .implements(ParentDelegate.self)

container.register(ChildInterface.self) { _ in Child() }
    .initCompleted { ($1 as! Child).delegate = $0.resolve(ParentDelegate.self) }

Additionally, if you wanted to get rid of force casting, you could do something like:

container.register(Child.self) { _ in Child() }
   .implements(ChildInterface.self)
   .initCompleted { $1.delegate = $0.resolve(ParentDelegate.self) }

Swinject: How to use delegate pattern with interface segregation (class vs interface)?

1 Answers1