如何將具有關聯類型的協議與 ObservableObject 一起使用？

Question

我正在嘗試為還必須符合ObservableObject的東西創建一個協議，該協議將存在於另一個 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())

當我嘗試換出服務以在我的測試中使用模擬時，我的問題就出現了，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'

我認為問題出在ObservableObject是一個具有關聯類型（ associatedtype ObjectWillChangePublisher ）的協議，但我認為我會通過指定where ObjectWillChangePublisher == ObservableObjectPublisher來“填補漏洞”。

請問我錯過了什么嗎？

編輯：澄清一下， services是一個全局 object ，我想在我的單元測試中用一個包含模擬的新Services實例來替換它，這就是我試圖讓它工作的原因：

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

Answer 1

您的示例中的問題是Services.processingThumbnails的類型是ThumbnailService ，而不是ThumbnailServiceProtocol 。 這些類型並不相同。 您需要使Services通用。 嘗試這個：

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())