使用具有约束关联类型的协议作为 属性 in Swift
Use protocol with constrained associated type as property in Swift
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值约束到另一个协议
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)
}
}
代码编译正常,但是当我在 viewcontroller 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,但我对类型擦除的概念不是很熟悉。有办法解决这个问题吗?
更新:
How are AddressBookCellModelType and AddressBookCellModel related here?
它是一个实现协议的结构体。
protocol AddressBookCellModelType {
var name: String { get }
var photo: UIImage? { get }
var isInvited: Bool { get }
}
struct AddressBookCellModel: AddressBookCellModelType {
....
}
你有没有试过把它当作一个通用的,就像warning/error说的:
class AddressBookViewController<T: AddressBookViewModelType> : UIViewController {
private var viewModel: T!
func configure(viewModel: T) {
self.viewModel = viewModel
}
...
}
您需要使用 属性 变量 T
初始化您的控制器,以便推断类型。
这只是 Swift 规范,您不能使用 'protocol with associated type' 作为类型声明。原因是编译器无法在编译时知道关联的类型实际上是什么,这违反了Swift的"type-safety"。
解决方案是像你说的那样使用类型橡皮擦,或者使类型通用。
为了在评论中扩展我的问题,看看这段代码,它看起来就像不添加 AddressBookCellModelType
或 AddressBookViewModelType
一样灵活,而且这也可以消除令人头疼的问题,同时在 DataSourceCompatible
.
上仍然是通用的
// 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
}
}
这样做可以实现通用 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>()
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值约束到另一个协议
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)
}
}
代码编译正常,但是当我在 viewcontroller 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,但我对类型擦除的概念不是很熟悉。有办法解决这个问题吗?
更新:
How are AddressBookCellModelType and AddressBookCellModel related here?
它是一个实现协议的结构体。
protocol AddressBookCellModelType {
var name: String { get }
var photo: UIImage? { get }
var isInvited: Bool { get }
}
struct AddressBookCellModel: AddressBookCellModelType {
....
}
你有没有试过把它当作一个通用的,就像warning/error说的:
class AddressBookViewController<T: AddressBookViewModelType> : UIViewController {
private var viewModel: T!
func configure(viewModel: T) {
self.viewModel = viewModel
}
...
}
您需要使用 属性 变量 T
初始化您的控制器,以便推断类型。
这只是 Swift 规范,您不能使用 'protocol with associated type' 作为类型声明。原因是编译器无法在编译时知道关联的类型实际上是什么,这违反了Swift的"type-safety"。
解决方案是像你说的那样使用类型橡皮擦,或者使类型通用。
为了在评论中扩展我的问题,看看这段代码,它看起来就像不添加 AddressBookCellModelType
或 AddressBookViewModelType
一样灵活,而且这也可以消除令人头疼的问题,同时在 DataSourceCompatible
.
// 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
}
}
这样做可以实现通用 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>()