Question

我的开源库（link in-case code samples aren't enough）中有一个协议。

所述协议具有两种关联类型，一种用于函数“发送”类型，另一种用于“返回类型”，如下所示：

/// 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 }
}

在函数实现中（在另一协议上声明的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)

我创建了一个CameraEvent对象，并尝试向下转换（不确定这是否是正确的术语）到T.ReturnType。在调用点，event是non-nil，但是将其强制转换为T.ReturnType会得到nil结果，即使它应该匹配！

Event.get（.getEvent）的定义如下：

/// 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)
}

还必须注意，我所讨论的第二种协议的另一种实现（定义{{1}}函数的一种）成功地实现了非常相似的逻辑，而且我不会退缩！这里肯定有我做错了什么，但我不知道这可能是什么！

屏幕截图，以防万一！

调用协议的功能

协议功能的实现

Answer 1

这是...只是调试器是一堆垃圾！决定添加print语句以确保绝对是nil。事实并非如此！可以正常打印，但是lldb似乎出于任何原因都认为它没有用。

可选转换为协议的关联类型失败（返回nil）

调用协议的功能

协议功能的实现

1 个答案: