對協議關聯類型的可選轉換失敗（返回 nil）

Question

我的開源庫中有一個協議（ 鏈接以防代碼示例不夠）。

所述協議有兩種關聯類型，一種用於函數“發送”類型，一種用於“返回類型”，定義如下：

/// A protocol which can be adopted to define a function that can be performed on any given camera
public protocol CameraFunction {

    /// An associated type which is taken as an input when calling this function on a `Camera` instance
    associatedtype SendType

    /// An associated type which is returned from the camera when calling this function on the `Camera`
    associatedtype ReturnType

    /// The enum representation of this given function
    var function: _CameraFunction { get }
}

在函數實現中（在另一個協議上聲明的performFunction ）：

func performFunction<T>(_ function: T, payload: T.SendType?, callback: @escaping ((Error?, T.ReturnType?) -> Void)) where T : CameraFunction {

        switch function.function {
        case .getEvent:
            let packet = Packet.commandRequestPacket(code: .getAllDevicePropData, arguments: [0], transactionId: ptpIPClient?.getNextTransactionId() ?? 0)
            ptpIPClient?.awaitDataFor(transactionId: packet.transactionId, callback: { (dataResult) in

                switch dataResult {
                case .success(let data):
                    guard let numberOfProperties = data.data[qWord: 0] else { return }
                    var offset: UInt = UInt(MemoryLayout<QWord>.size)
                    var properties: [PTPDeviceProperty] = []
                    for _ in 0..<numberOfProperties {
                        guard let property = data.data.getDeviceProperty(at: offset) else { break }
                        properties.append(property)
                        offset += property.length
                    }
                    let event = CameraEvent(sonyDeviceProperties: properties)
                    callback(nil, event as? T.ReturnType)
                case .failure(let error):
                    callback(error, nil)
                }
            })
            ptpIPClient?.sendCommandRequestPacket(packet, callback: nil)

我創建了一個CameraEvent對象並嘗試向下轉換（不確定這是否是正確的術語）到T.ReturnType 。 在調用時， event non-nil ，但是將其轉換為T.ReturnType會給出nil結果，即使它應該匹配！

Event.get (.getEvent) 定義如下：

/// Functions for interacting with the event API on the camera
public struct Event: CameraFunction {

    public var function: _CameraFunction

    public typealias SendType = Bool

    public typealias ReturnType = CameraEvent

    /// Gets the current event
    public static let get = Event(function: .getEvent)
}

同樣重要的是要注意，我談到的第二個協議的另一個實現（定義performFunction函數的那個​​）成功地執行了非常相似的邏輯，我沒有得到 nil ！ 一定是我在這里做錯了什么，但我不知道它可能是什么！

截圖以防萬一！

調用協議函數

協議功能的實現

Answer 1

這是...只是調試器是一堆垃圾！ 決定添加一個print語句以確保它是nil 。 事實證明並非如此！ 打印正常，但lldb似乎認為無論出於何種原因都為零。