[英]Protocols with associated type and optional methods
I would like to make a data source protocol with: 我想做一个数据源协议:
eg(currently, with Swift 4.1 compilation for code snipped below fails) 例如(当前,使用Swift 4.1编译下面的代码失败了)
@objc protocol DataSourceNotWrkng {
associatedtype Item
var array: [Item] { get set }
@objc optional func titleForHeader(in section: Int) -> String
}
class ComponentsDataSourceNotWrkng: DataSourceNotWrkng {
typealias Item = Int
var array: [Int]
init(with array: [Int]) {
self.array = array
}
}
This code fails with unreadable error: 此代码失败,并出现无法读取的错误:
0x0x7fdede80c9a0 Module name=tabbedApp
0x0x7fdede1a8020 FileUnit file="/Users/gvr/Developer/freelance/tabbedApp (1.2)/tabbedApp/Utilities/DataSource.swift"
0x0x7fdede1a83e0 ProtocolDecl name=DataSourceNotWrkng
0x0x7fdede1a8a30 AbstractFunctionDecl name=_ : <<error type>>
(error_type)
You can't use both associated types and Objective-C protocols. 您不能同时使用关联的类型和Objective-C协议。 Protocol with associated types are not visible into Objective-C, and Objective-C protocols can't have associated types.
具有关联类型的协议在Objective-C中不可见,并且Objective-C协议不能具有关联类型。
The reason is associated types is a static dispatch feature, while Objective-C protocol rely on runtime features, so they can't live together in the same protocol. 原因是关联的类型是静态调度功能,而Objective-C协议依赖于运行时功能,因此它们不能在同一协议中一起使用。
The compiler should not allow you to do this, however seems there's a bug that makes it crash it before it can give you a proper error message. 编译器不应允许您执行此操作,但是似乎有一个错误使它崩溃,然后才能给您适当的错误消息。
Note that a feature like an optional function can be achieved by other means than an @objc
protocol, by using default implementations. 请注意,可以使用默认实现,通过
@objc
协议以外的其他方式来实现诸如可选功能之类的功能。 This will allow you to keep using the protocol and it's associated type: 这将允许您继续使用该协议及其关联的类型:
protocol MyDataSource {
associatedtype Item
var array: [Item] { get set }
// moved the optionality to the method result level
func titleForHeader(in section: Int) -> String?
}
extension MyDataSource {
// conforming types no longer need to implement this method,
// if they don't want to, they will get the default implementation
func titleForHeader(in section: Int) -> String? {
return nil
}
}
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