使用 JSONDecoder 解码的对象的打印输出出现问题

我正在尝试快速解码 JSON 字符串，但在解码后访问属性时遇到一些奇怪的问题。

这是我从本地存储的 JSON 文件检索的 JSON 文件的内容

[
  {
    "word": "a",
    "usage": [
      {
        "partOfSpeech": "determiner"
      }
    ]
  }
]

这是访问 JSON 文件属性的代码

struct WordDictionary : Codable {
    var word: String
    var usage: [Usage]
}

struct Usage: Codable {
    var partOfSpeech: String
}
                
if let url = Bundle.main.url(forResource: FILE_NAME, withExtension: "json") {
    do {
        let data = try Data(contentsOf: url)
        let decoder = JSONDecoder()
        
        let jsonData = try decoder.decode([WordDictionary].self, from: data)
        print(jsonData[0].word) //Outputs "a"
        print(jsonData[0].usage) //Outputs "[MyApp.AppDelegate.(unknown context at $102a37f00).(unknown context at $102a38038).Usage(partOfSpeech: "determiner")]"

    } catch {
        print("error:\(error)")
    }
}

正如你所看到的，当我尝试print(jsonData[0].usage)在获得“使用情况”属性之前，我收到了一系列未知的数据消息。当我打印这一行时我只想看看determiner，我不确定序言中关于“未知背景”的内容到底是什么。

我也在运行这段代码didFinishLaunchingWithOptions的功能AppDelegate.

我不确定我错过了什么。几天来我一直在尝试寻找解决方案并尝试不同的方法，但仍然无法获得所需的输出，任何帮助将不胜感激。

tl;dr

您在类型的描述中看到“未知上下文”，因为它是在函数内部定义的。您可以通过将这些类型定义移到函数之外或实现您自己的来解决此问题CustomStringConvertible https://developer.apple.com/documentation/swift/customstringconvertible/一致性。

这是你在哪里定义类型的问题。

考虑：

class AppDelegate: UIResponder, UIApplicationDelegate {

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        struct WordDictionary: Codable {
            var word: String
            var usage: [Usage]
        }

        struct Usage: Codable {
            var partOfSpeech: String
        }

        do {
            let url = Bundle.main.url(forResource: "test", withExtension: "json")!
            let data = try Data(contentsOf: url)
            let words = try JSONDecoder().decode([WordDictionary].self, from: data)
            print(words[0].usage)
        } catch {
            print(error)
        }

        return true
    }

    ...
}

这会产生：

[MyApp.AppDelegate。（$ 102bac454 处未知上下文）。（$ 102bac58c 处未知上下文）。Usage（partOfSpeech：“确定符”）]

也就是说Usage是在某些未知的上下文中定义的AppDelegate within MyApp。简而言之，它不知道如何表示函数内定义的类型的层次结构。

对比一下：

class AppDelegate: UIResponder, UIApplicationDelegate {

    struct WordDictionary: Codable {
        var word: String
        var usage: [Usage]
    }

    struct Usage: Codable {
        var partOfSpeech: String
    }

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        do {
            let url = Bundle.main.url(forResource: "test", withExtension: "json")!
            let data = try Data(contentsOf: url)
            let words = try JSONDecoder().decode([WordDictionary].self, from: data)
            print(words[0].usage)
        } catch {
            print(error)
        }

        return true
    }

    ...
}

其产生：

[MyApp.AppDelegate.Usage(partOfSpeech: "限定符")]

您也可以添加自己的CustomStringConvertible https://developer.apple.com/documentation/swift/customstringconvertible/一致性：

struct WordDictionary: Codable {
    var word: String
    var usage: [Usage]
}

struct Usage: Codable {
    var partOfSpeech: String
}

extension Usage: CustomStringConvertible {
    var description: String { "Usage(partOfSpeech: \"\(partOfSpeech)\")" }
}

@main
class AppDelegate: UIResponder, UIApplicationDelegate {

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

        do {
            let url = Bundle.main.url(forResource: "test", withExtension: "json")!
            let data = try Data(contentsOf: url)
            let words = try JSONDecoder().decode([WordDictionary].self, from: data)
            print(words[0].usage)
        } catch {
            print(error)
        }

        return true
    }

    ...
}

其产生：

[用法（词性：“限定词”）]

Through CustomStringConvertible，你可以使print按照您想要的方式格式化它。