如何将具有关联类型的协议与 ObservableObject 一起使用？

Question

我正在尝试为还必须符合ObservableObject的东西创建一个协议，该协议将存在于另一个 object Services中：

protocol ThumbnailServiceProtocol: ObservableObject where ObjectWillChangePublisher == ObservableObjectPublisher {
    var processingThumbnails: [String] { get }
}

final class ThumbnailService: ThumbnailServiceProtocol {
    var processingThumbnails: [String] = []
}

struct Services {
    var thumbnailService: ThumbnailServiceProtocol
}

var services = Services(thumbnailService: ThumbnailService())

当我尝试换出服务以在我的测试中使用模拟时，我的问题就出现了，如下所示：

final class MockThumbnailService: ThumbnailServiceProtocol {
    var processingThumbnails: [String] = []
}

services = Services(thumbnailService: MockThumbnailService()) /// Cannot convert value of type 'MockThumbnailService' to expected argument type 'ThumbnailServiceProtocol'

我认为问题出在ObservableObject是一个具有关联类型（ associatedtype ObjectWillChangePublisher ）的协议，但我认为我会通过指定where ObjectWillChangePublisher == ObservableObjectPublisher来“填补漏洞”。

请问我错过了什么吗？

编辑：澄清一下， services是一个全局 object ，我想在我的单元测试中用一个包含模拟的新Services实例来替换它，这就是我试图让它工作的原因：

var services = Services(thumbnailService: ThumbnailService())
services = Services(thumbnailService: MockThumbnailService()) /// Cannot convert value of type 'MockThumbnailService' to expected argument type 'ThumbnailServiceProtocol'

Answer 1

您的示例中的问题是Services.processingThumbnails的类型是ThumbnailService ，而不是ThumbnailServiceProtocol 。 这些类型并不相同。 您需要使Services通用。 尝试这个：

protocol ThumbnailServiceProtocol: ObservableObject {
    var processingThumbnails: [String] { get }
}

final class ThumbnailService: ThumbnailServiceProtocol {
    var processingThumbnails: [String] = []
}

final class MockThumbnailService: ThumbnailServiceProtocol {
    var processingThumbnails: [String] = []
}

struct Services<T> where T: ThumbnailServiceProtocol, T.ObjectWillChangePublisher == ObservableObjectPublisher {
    var thumbnailService: T
}

let servicesA = Services(thumbnailService: ThumbnailService())
let servicesB = Services(thumbnailService: MockThumbnailService())