尝试归档符合 NSCoder 的类的实例

Question

我正在开发我的第一个 Swift iOS 应用程序，在序列化和保存我从服务器获取其 JSON 的对象时遇到问题。我正在使用Gloss，这是一个轻量级的 JSON 解析库，它定义了一个可解码协议，通过该协议可以从 JSON 实例化一个实例。我的意图是通过首先提取其 id 从JSON（[String : AnyObject] 的类型别名）加载一个东西，然后检查我是否已经有一个本地存档副本。如果我这样做，请取消存档并获取图像。如果没有，请对图像文件进行异步请求。

问题是Thing.localArchiveExists(id) 总是返回false。事物已成功实例化，但它们总是重新获取图像。我检查了文件系统并确认没有写入存档文件。但是，我没有看到“错误。无法存档”，这表明保存成功。我是否缺少有关如何存档和保存 NSCoder 对象的信息？谢谢！

这是我对 Decodable 协议的实现：

// MARK: Decodable protocol
// When a thing is loaded from JSON, we load its image from archive if possible.
required init?(json: JSON) {
    guard let id: Int = "id" <~~ json else { return nil}

    if Thing.localArchiveExists(id) {
        guard let savedThing = NSKeyedUnarchiver.unarchiveObjectWithFile(Thing.archiveFilePath(id)) as? Thing else { return nil }
        self.id = savedThing.id
        self.name = savedThing.name
        self.image = savedThing.image
        self.imageUrl = savedThing.imageUrl
        super.init()
        print("Loaded Thing \(self.name) from archive")
    }
    else {
        guard let name: String = "name" <~~ json else { return nil}
        guard let imageUrl: NSURL = "url" <~~ json else { return nil}
        self.id = id
        self.name = name
        self.imageUrl = imageUrl
        super.init()
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {
            let data = NSData(contentsOfURL: imageUrl)
            dispatch_async(dispatch_get_main_queue(), {
                self.image = UIImage(data: data!)
                guard self.save() else {
                    print("ERROR. Could not archive")
                    return
                }
                print("Loaded Thing \(self.name) from server")
            })
        }
    }
}

以下是 Thing 类的相关部分：

// MARK: Properties
var id: Int?
var name: String
var imageUrl: NSURL?
var image: UIImage?

// MARK: Archiving Paths
static let DocumentsDirectory = NSFileManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask).first!
static let ArchiveURL = DocumentsDirectory.URLByAppendingPathComponent("things")

// MARK: Types
struct PropertyKey {
    static let nameKey = "name"
    static let imageKey = "image"
    static let imageUrlKey = "imageUrl"
    static let idKey = "id"
}

// Returns the file URL at which a Thing with the given ID should be saved.
class func archiveFilePath(id: Int) -> String {
    return Thing.ArchiveURL.URLByAppendingPathComponent("thing\(id)").absoluteString
}

// Checks whether an archived copy of a Thing with the given ID exists.
class func localArchiveExists(id: Int) -> Bool {
    let fileManager = NSFileManager.defaultManager()
    return fileManager.fileExistsAtPath(Thing.archiveFilePath(id))
}

// MARK: NSCoding
func encodeWithCoder(coder: NSCoder) {
    coder.encodeObject(name, forKey: PropertyKey.nameKey)
    if image != nil {
        coder.encodeObject(image!, forKey: PropertyKey.imageKey)
    }
    if imageUrl != nil {
        coder.encodeObject(imageUrl!, forKey: PropertyKey.imageUrlKey)
    }
    coder.encodeInteger(id!, forKey: PropertyKey.idKey)
}

required convenience init?(coder aDecoder: NSCoder) {
    let name = aDecoder.decodeObjectForKey(PropertyKey.nameKey) as! String
    let image = aDecoder.decodeObjectForKey(PropertyKey.imageKey) as? UIImage
    let imageUrl = aDecoder.decodeObjectForKey(PropertyKey.imageUrlKey) as? NSURL
    let id = aDecoder.decodeIntegerForKey(PropertyKey.idKey) 

    // Must call designated initializer.
    self.init(name: name, image: image, imageUrl: imageUrl, id: id)
}

func save() -> Bool {        
    // For some reason I can't archive to file.
    return NSKeyedArchiver.archiveRootObject(self, toFile: Thing.archiveFilePath(self.id!))
}  

Answer 1

我发现了我的问题：保存失败，因为我还没有创建我试图保存我的东西的目录。

func save() -> Bool {        
    let archivedData = NSKeyedArchiver.archivedDataWithRootObject(self)
    do {
        try NSFileManager.defaultManager().createDirectoryAtURL(Thing.ArchiveURL, withIntermediateDirectories: true, attributes: [:])
        try archivedData.writeToFile(Thing.archiveFilePath(self.id!), options: [])
        return true
    } catch {
        print(error)
        return false
    }
}