如何在Swift的泛型中声明基类？

Question

Example of code which doesn't work, but I would like to have it compiled with success. Idea is described very nicely in following source code. You can paste it into Swift playground easily.

import UIKit

// MARK: - Base classes with generics

class FlowController {

}

class ViewModel<FlowControllerType: FlowController> {
    let flowController: FlowControllerType
    init (flowController: FlowControllerType) {
        self.flowController = flowController
    }
}

class ViewController<ViewModelType: ViewModel<FlowController>> {

    var viewModel: ViewModelType!
}

// MARK: - My implementation

class MyFlowController: FlowController {

}

class MyViewModel: ViewModel<MyFlowController> {
    override init (flowController: MyFlowController) {
        super.init(flowController: flowController)
    }
}

// ERROR: 'ViewController' requires that 'MyModelView' inherit from 'ViewModel<FlowController>'
class MyViewController: ViewController<MyViewModel> {

}

// MARK: - Code

let viewController = MyViewController()
viewController.viewModel = MyViewModel(flowController: MyFlowController())

Problem is that I would like to declare MyViewController , but I get error:

'ViewController' requires that 'MyModelView' inherit from 'ViewModel<FlowController>'

Question is how to declare these classes to make it working? Should I use where clause for this? I want to have my own types for ViewController.viewModel and ViewModel.flowController in base classes keeping these dependencies between classes. Thank you in advance.

Related question: Swift generic inheritance

Answer 1

I have probably answer, but it could look better than this solution. Trick is in following line

class ViewController<FlowControllerType: FlowController, ViewModelType: ViewModel<FlowControllerType>> {

which should be used instead of

class ViewController<ViewModelType: ViewModel<FlowController>> {

I hope somebody else will do it better, but for now only this works. Full playground example:

import UIKit

// MARK: - Base classes with generics

class FlowController {

}

class ViewModel<FlowControllerType: FlowController> {
    let flowController: FlowControllerType
    init (flowController: FlowControllerType) {
        self.flowController = flowController
    }
}

class ViewController<FlowControllerType: FlowController, ViewModelType: ViewModel<FlowControllerType>> {

    var viewModel: ViewModelType!
}

// MARK: - My implementation

class MyFlowController: FlowController {

}

class MyViewModel: ViewModel<MyFlowController> {
    override init (flowController: MyFlowController) {
        super.init(flowController: flowController)
    }
}

class MyViewController: ViewController<MyFlowController, MyViewModel> {

}

// MARK: - Code

let viewController = MyViewController()
viewController.viewModel = MyViewModel(flowController: MyFlowController())