我为具有关联类型的协议(protocol)添加了一个协议(protocol)扩展名为 PickerType .我写了一个函数的重新实现，refresh(:,completion:)在协议(protocol)中定义并在不同的协议(protocol)扩展中实现。

但是当我调用 refresh(:,completion:) 时，我的新扩展中的函数没有被调用除非编译器知道 PickerType 是什么类型是。我写了以下内容:

extension PickerItemProvider where PickerType: Equatable & SyncableEntity {
    func refresh(_ sender: Any, completion: (() -> Void)?) {
        print("we're trying to have this implementation called")
        PickerType.startSync()
    }
}

如果我调用 refresh(:,completion:)，它会像我预期的那样被调用在 PickerSectionProvider<ObservationType> 上(请参阅下面我的 Playground 中的代码)但不是在我调用 refresh(:,completion:) 时在 ItemProvider 上，这是一个必须符合 PickerItemProvider 的通用类型(再次查看下面的代码)。

// We are trying to get our pickers to sync their provided type when you pull to refresh. But the implementation of refresh(:, completion:) that we added doesn’t get called.

import Foundation

protocol PickerItemProvider: class {
    associatedtype PickerType
    func findItem(by identifier: NSNumber) -> PickerType?
    func itemAt(_ indexPath: IndexPath) -> PickerType?
    func refresh(_ sender: Any, completion: (() -> Void)?)
}

extension PickerItemProvider {
    func findItem(by identifier: NSNumber) -> PickerType? {
        return nil
    }

    public func refresh(_ sender: Any, completion: (() -> Void)?) {
        print("the default refresh implementation")
    }
}

public class PickerSectionProvider<ProvidedType: Equatable> : PickerItemProvider {
    func itemAt(_ indexPath: IndexPath) -> ProvidedType? {
        return nil
    }
}

extension PickerItemProvider where PickerType: Equatable & SyncableEntity {
    func refresh(_ sender: Any, completion: (() -> Void)?) {
        print("we’re trying to have this implementation called instead of the above implementation of refresh")
        PickerType.startSync()
    }
}

protocol SyncableEntity {
    static func startSync()
}

extension SyncableEntity {
    static func startSync() {

    }
}

class ObservationType: Equatable, SyncableEntity {

}

func ==(lhs: ObservationType, rhs: ObservationType) -> Bool {
    return false
}

class GenericPickerViewController<PickerType: Equatable, ItemProvider: PickerItemProvider> where ItemProvider.PickerType == PickerType {
    var itemProvider: ItemProvider?

    init() {

    }

    func foo() {
        // Why doesn’t the implementation of refresh(:,completion:) we added get called here?
        itemProvider?.refresh("dummy sender") {

        }
    }
}

class PopupPickerRow<T: Equatable, ItemProvider: PickerItemProvider> where ItemProvider.PickerType == T {
    var pickerController = GenericPickerViewController<T, ItemProvider>()
}

let pickerSectionProvider = PickerSectionProvider<ObservationType>()

let row = PopupPickerRow<ObservationType, PickerSectionProvider<ObservationType>>()

row.pickerController.itemProvider = pickerSectionProvider

row.pickerController.foo()

最佳答案

请参阅下面更正的实现，

import Foundation

protocol PickerItemProvider: class {
    associatedtype PickerType
    func findItem(by identifier: NSNumber) -> PickerType?
    func itemAt(_ indexPath: IndexPath) -> PickerType?
}

extension PickerItemProvider {
    func findItem(by identifier: NSNumber) -> PickerType? {
        return nil
    }

    func refresh(_ sender: Any, completion: (() -> Void)?) {
        print("the default refresh implementation")
    }
}

public class PickerSectionProvider<ProvidedType: Equatable> : PickerItemProvider {
    func itemAt(_ indexPath: IndexPath) -> ProvidedType? {
        return nil
    }
}

extension PickerItemProvider where PickerType: Equatable & SyncableEntity {

    func refresh(_ sender: Any, completion: (() -> Void)?) {
        print("we’re trying to have this implementation called instead of the above implementation of refresh")
        PickerType.startSync()
    }
}

protocol SyncableEntity {
    static func startSync()
}

extension SyncableEntity {
    static func startSync() {

    }
}

class ObservationType: Equatable, SyncableEntity {

}

func ==(lhs: ObservationType, rhs: ObservationType) -> Bool {
    return false
}

class GenericPickerViewController<PickerType: Equatable & SyncableEntity, ItemProvider: PickerItemProvider> where ItemProvider.PickerType == PickerType {
    var itemProvider: ItemProvider?

    init() {

    }

    func foo() {
        // Why doesn’t the implementation of refresh(:,completion:) we added get called here?
        itemProvider?.refresh("dummy sender") {

        }
    }
}

class PopupPickerRow<T: Equatable & SyncableEntity, ItemProvider: PickerItemProvider> where ItemProvider.PickerType == T {
    var pickerController = GenericPickerViewController<T, ItemProvider>()
}

let pickerSectionProvider = PickerSectionProvider<ObservationType>()

let row = PopupPickerRow<ObservationType, PickerSectionProvider<ObservationType>>()

row.pickerController.itemProvider = pickerSectionProvider

row.pickerController.foo()

首先，当你想override一个method在 protocol 中声明为要求的实现并在 protocol 中提供了一些默认实现extension它将始终调用在 extension 中实现的方法丢弃 where条款要求。您可以在 this 中看到类似的问题问题和this .

所以要制作 protocol寻找 method满足 where 中的约束您需要删除 method 的子句来自 protocol 的签名并将其仅保留在 extension 中正如我上面所做的。

其次你错过了PickerType要求为 Equatable & SyncableEntity在定义 GenericPickerViewController 时和 PopupPickerRow所以我也更新了。

关于swift - 不调用协议(protocol)扩展中添加的函数的重新实现，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/53328770/