swift-type-erasure.swift

import UIKit

// Type erasure example

protocol Joint {
    associatedtype T
    var count: T { get }
}

class SingleJoint: Joint {

    var count: Float {
        1
    }
}

class MultipleJoints: Joint {

    var count: Int {
        10
    }
}

protocol ViewModelProtocol {
    var joints: String { get }
}

class ViewModel2<T: Joint> {

    private let joint: T

    init(joint: T) {
        self.joint = joint
    }
}

// Here we do type erasing
// To be able to assign ViewModel2 instance to a property that confirms to ViewModelProtocol later
extension ViewModel2: ViewModelProtocol {
    var joints: String {
        "\(joint.count)"
    }
}

// Introducing generic types to the view controller is not an option
class ViewController: UIViewController {
    let model: ViewModelProtocol = ViewController.modelInstance()

    func test() {
        // Here we refer to a non generic protocol
        // that delegates a call to a corresponding generic implementation i.e. SingleJoint or MultipleJoints
        print("Joints \(model.joints)")
    }
}

extension ViewController {

    private static var decisionMakeCondition: Bool {
        true
    }

    // This is where the magic happens
    // We can return an instance of a generic type assigning it to non generic one
    private static func modelInstance() -> ViewModelProtocol {
        if self.decisionMakeCondition {
            return ViewModel2(joint: SingleJoint())
        } else {
            return ViewModel2(joint: MultipleJoints())
        }
    }
}

let controller = ViewController()
controller.test()