符合协议的协议相关类型

Question

Code

I have the following protocols:

protocol BaseViewController {
    typealias ViewModelType: BaseViewModel

    var viewModel: ViewModelType? { get set }
}

protocol BaseViewModel {
}

I also have the following protocol for a view model:

protocol MainViewModel: BaseViewModel {
}

Then in my MainViewController:

class MainViewController: UIViewController, BaseViewController {
    typealias ViewModelType = MainViewModel

    var viewModel: ViewModelType?

    ...
}

Errors

On the MainViewController I get the error

Type 'MainViewController' does not conform to protocol 'BaseViewController'

Below this are two related errors:

1. On BaseViewController:

Unable to infer associated type 'ViewModelType' for protocol 'BaseViewController'

2. In MainViewController on the viewModel property

Inferred type 'BaseViewModel' (by matching requirement 'viewModel') is invalid: does not conform to 'BaseViewModel'

Desired Result

I would like to restrict the value of ViewModelType to conform to protocol BaseViewModel. If this can be accomplished another way, then that will answer my question. But I would like to know what I'm doing wrong here.

Answer 1

我认为您需要协议BaseViewModel的具体实现，因此可以将MainViewModel定义为结构或类。

Answer 2

The documentation for typealias indicates it has the following format:

typealias-assignment → = type

And type is defined as:

type → array-type dictionary-type function-type type-identifier tuple-type optional-type implicitly-unwrapped-optional-type protocol-composition-type metatype-type

And protocol-composition-type is defined as:

protocol-composition-type → protocol<protocol-identifier-listopt>

So listing a protocol does not seem to be supported, so it's not clear why you're not getting a compilation error there. If you change it to that format, it seems to work (compiles but I have not tested you get the desired result):

protocol BaseViewController {
    typealias ViewModelType = protocol<BaseViewModel>

    var viewModel: ViewModelType? { get set }
}

And:

class ViewController: UIViewController, BaseViewController {
    typealias ViewModelType = protocol<MainViewModel>