Swift JSON解码提示'id'键缺失但实际存在的解决方法

错误信息 #

Fatal error: Couldn't parse classification.json as AnimalGroup:
keyNotFound(CodingKeys(stringValue: "id", intValue: nil), Swift.DecodingError.Context(codingPath: [], debugDescription: "No value associated with key CodingKeys(stringValue: \"id\", intValue: nil) (\"id\").", underlyingError: nil))

相关代码与JSON样本 #

AnimalGroup结构体定义 #

struct AnimalGroup: Identifiable, Decodable, Hashable {
    let id: Int
    let code: Int
    let form: String?
    
    let name: String
    let shortName: String
    
    func hash(into hasher: inout Hasher) {
            hasher.combine(id)
            hasher.combine(name)
        }
}

JSON加载与解码函数 #

var previewAnimal: Animal = load("animal.json")
var previewGroup: AnimalGroup = load("classification.json")

// Function that loads JSON data into a T type and returns the decoded data
func load<T: Decodable>(_ filename: String) -> T {
    let data: Data

    guard let file = Bundle.main.url(forResource: filename, withExtension: nil)
        else {
            fatalError("Couldn't find \(filename) in main bundle.")
    }

    do {
        data = try Data(contentsOf: file)
    } catch {
        fatalError("Couldn't load \(filename) from main bundle:\n\(error)")
    }

    do {
        let decoder = JSONDecoder()
        decoder.keyDecodingStrategy = .convertFromSnakeCase // Remember to add this line to avoid running into a crash
        return try decoder.decode(T.self, from: data)
    } catch {
        fatalError("Couldn't parse \(filename) as \(T.self):\n\(error)")
    }
}

classification.json样本 #

{
    "group": [
        {
            "code": 3,
            "form": null,
            "id": 1,
            
            "name": "Vertebrates",
            "short_name": "CRS"
        },
        {
            "code": 4,
            "form": null,
            "id": 2,
            
            "name": "Mammal",
            "short_name": "CRS"
        },
        {
            "code": 5,
            "form": null,
            "id": 3,
            
            "name": "Reptile",
            "short_name": "CRS"
        },
        {
            "code": 6,
            "form": null,
            "id": 4,
            
            "name": "Avians",
            "short_name": "CRS"
        },
        {
            "code": 7,
            "form": null,
            "id": 5,
            
            "name": "Amphibians",
            "short_name": "CRS"
        }
    ]
}

原因分析 #

问题核心是JSON顶层结构与解码目标不匹配：

• 你的JSON顶层是一个包含group键的对象，group的值才是AnimalGroup类型的数组
• 但你直接尝试将整个JSON解码为单个AnimalGroup实例，解码器会在JSON顶层寻找id、code等键，自然找不到，因此抛出keyNotFound错误

解决办法 #

方法1：定义顶层响应结构体（推荐） #

新增一个对应JSON顶层结构的结构体，再通过它获取AnimalGroup数组：

struct AnimalGroupResponse: Decodable {
    let group: [AnimalGroup]
}

修改加载代码的类型：

var previewGroup: AnimalGroupResponse = load("classification.json")
// 访问数组：previewGroup.group

方法2：直接解码数组（需调整解码逻辑） #

如果不想新增结构体，可以在load函数中先解析顶层对象，提取group数组的数据后再解码：

func loadAnimalGroups(from filename: String) -> [AnimalGroup] {
    let data: Data

    guard let file = Bundle.main.url(forResource: filename, withExtension: nil) else {
        fatalError("Couldn't find \(filename) in main bundle.")
    }

    do {
        data = try Data(contentsOf: file)
    } catch {
        fatalError("Couldn't load \(filename) from main bundle:\n\(error)")
    }

    do {
        let decoder = JSONDecoder()
        decoder.keyDecodingStrategy = .convertFromSnakeCase
        // 先解码为字典提取group数组
        let topLevel = try decoder.decode([String: [AnimalGroup]].self, from: data)
        guard let groups = topLevel["group"] else {
            fatalError("No 'group' key found in \(filename)")
        }
        return groups
    } catch {
        fatalError("Couldn't parse \(filename) as [AnimalGroup]:\n\(error)")
    }
}

// 使用时：
var previewGroup: [AnimalGroup] = loadAnimalGroups(from: "classification.json")

注意：你的shortName与JSON中的short_name对应，已经设置了.convertFromSnakeCase，这部分是正确的，无需修改。

内容的提问来源于stack exchange，提问作者chairman