>我正在尝试实现具有关联类型的数据源协议
protocol DataSourceCompatible {
associatedtype CellModel
func cellModelForItem(at indexPath: IndexPath) -> CellModel
}
协议AddressBookViewModelType继承自基本协议，并将关联的值约束为另一个协议
protocol AddressBookViewModelType: class, DataSourceCompatible where CellModel == AddressBookCellModelType {
}
AddressBookViewModel是AddressBookViewModelType协议的具体实现
class AddressBookViewModel: AddressBookViewModelType {
func cellModelForItem(at indexPath: IndexPath) -> AddressBookCellModelType {
let contact = sectionedContacts[indexPath.section][indexPath.row]
return AddressBookCellModel(contact: contact)
}
}
代码编译良好，但是当我在视图控制器上将视图模型声明为属性时，编译器失败并显示Protocol 'AddressBookViewModelType' can only be used as a generic constraint because it has Self or associated type requirements。
class AddressBookViewController: UIViewController {
private var viewModel: AddressBookViewModelType!
func configure(viewModel: AddressBookViewModelType) {
self.viewModel = viewModel
}
...
}
我记得看到类型擦除可能会解决问题，但我对类型擦除概念并不熟悉。有没有办法解决这个问题？
更新：
AddressBookCellModelType和AddressBookCellModel在这里有什么关系？
它是实现协议的结构。
protocol AddressBookCellModelType {
var name: String { get }
var photo: UIImage? { get }
var isInvited: Bool { get }
}
struct AddressBookCellModel: AddressBookCellModelType {
....
}
您是否尝试过仅将其用作泛型，例如警告/错误：
class AddressBookViewController<T: AddressBookViewModelType> : UIViewController {
private var viewModel: T!
func configure(viewModel: T) {
self.viewModel = viewModel
}
...
}
您需要使用变量T的属性初始化控制器，以便可以推断类型。
为了扩展我在评论中的问题，查看此代码，它看起来在不添加AddressBookCellModelType或AddressBookViewModelType的情况下将完全相同灵活，这也将摆脱头痛，同时仍然在DataSourceCompatible上通用。
// This protocol is fine and very useful for making reusable view controllers. Love it.
protocol DataSourceCompatible {
associatedtype CellModel
func cellModelForItem(at indexPath: IndexPath) -> CellModel
}
// No need for a protocol here. The struct is its own interface.
// This ensures value semantics, which were being lost behind the protocol
// (since a protocol does not promise value semantics)
struct AddressBookCellModel {
var name: String
var photo: UIImage?
var isInvited: Bool
}
// AddressBookViewModel conforms to DataSourceCompatible
// Its conformance sets CellModel to AddressBookCellModel without needing an extra protocol
class AddressBookViewModel: DataSourceCompatible {
let sectionedContacts: [[AddressBookCellModel]] = []
func cellModelForItem(at indexPath: IndexPath) -> AddressBookCellModel {
return sectionedContacts[indexPath.section][indexPath.row]
}
}
class AddressBookViewController: UIViewController {
private var viewModel: AddressBookViewModel!
func configure(viewModel: AddressBookViewModel) {
self.viewModel = viewModel
}
}
这样做允许通用 VC，而无需引入更多需要的部分：
class DataSourceViewController<DataSource: DataSourceCompatible>: UIView {
private var viewModel: DataSource.CellModel!
func configure(viewModel: DataSource.CellModel) {
self.viewModel = viewModel
}
}
let vc = DataSourceViewController<AddressBookViewModel>()
这只是 Swift 规范，你不能使用"具有关联类型的协议"作为类型声明。原因是编译器在编译时无法知道关联的类型实际上是什么，这违反了 Swift 的"类型安全"。
解决方案是像您所说的那样使用类型橡皮擦，或者使类型通用。