Swift解析Menu.json报错：期望解码数组却发现字典的问题排查

问题背景 #

尝试解析名为Menu.json的JSON数据，结构如下：

[{
    "day": "Monday",
    "locationName": "Memorial Union Quad",
    "coordinate": [38.54141, -121.74845],
    "menu": {
        "Penne Pasta w/ Bolognese" : ["🌾","🐷"],
        "Penne Pasta w/ Roasted Mushrooms" : ["🌾","V"]
    }
},{
    "day": "Tuesday",
    "locationName": "International Center",
    "coordinate": [38.54540, -121.75494],
    "menu": {
        "Beef & Lamb Gyro" : ["🌾","🫛", "🥛"],
        "Edamame Hummus w/ Veggies" : ["🫛","🥛", "SE", "VE"]
    }
},{
    "day": "Wednesday",
    "locationName": "Storer Hall",
    "coordinate": [38.54114, -121.75461],
    "menu": {
        "Seafood Salad Tostada" : ["🥚","🦐", "🫛", "🐟"],
        "Southwest Black Bean Tostada" : ["V"]
    }
},{
    "day": "Thursday",
    "locationName": "Orchard Park",
    "coordinate": [38.544828, -121.765170],
    "menu": {
        "Teriyaki Beef w/ Stir Fry Noodles" : ["🌾","🫛", "SE"],
        "Teriyaki Tofu w/ Veggie Stir Fry" : ["🌾","🫛", "SE","V"]
    }
},{
    "day": "Friday",
    "locationName": "Memorial Union Quad",
    "coordinate": [38.54141, -121.74845],
    "menu": {
        "Soy Ciltrano Lime Chicken" : ["🌾","🫛"],
        "Southwest Tofu" : ["🫛","V"]
    }
}]

对应的Swift数据模型：

import Foundation
import OrderedCollections

struct Menu : Codable, Hashable {
    var id: UUID { UUID() }
    let day: String
    let locationName: String
    let coordinate: [Double]
    let menu: OrderedDictionary<String, [String]>
    
    func getTodaysLocation(_ today: String)-> String{
        if today == day{
            return locationName
        }
        return ""
    }
}

使用Bundle扩展的decode函数解码：

import Foundation

extension Bundle {
    func decode(_ file: String) -> [Menu] {
        guard let url = self.url(forResource: file, withExtension: nil) else {
            fatalError("Failed to locate \(file) in bundle.")
        }

        guard let data = try? Data(contentsOf: url) else {
            fatalError("Failed to load \(file) from bundle.")
        }

        let decoder = JSONDecoder()

        do {
            return try decoder.decode([Menu].self, from: data)
        } catch DecodingError.keyNotFound(let key, let context) {
            fatalError("Failed to decode \(file) from bundle due to missing key '\(key.stringValue)' – \(context.debugDescription)")
        } catch DecodingError.typeMismatch(_, let context) {
            fatalError("Failed to decode \(file) from bundle due to type mismatch – \(context.debugDescription)")
        } catch DecodingError.valueNotFound(let type, let context) {
            fatalError("Failed to decode \(file) from bundle due to missing \(type) value – \(context.debugDescription)")
        } catch DecodingError.dataCorrupted(_) {
            fatalError("Failed to decode \(file) from bundle because it appears to be invalid JSON.")
        } catch {
            fatalError("Failed to decode \(file) from bundle: \(error.localizedDescription)")
        }
    }
}

运行后出现错误：

"Failed to decode Menu.json from bundle due to type mismatch – Expected to decode Array but found a dictionary instead."

错误原因 #

问题出在OrderedDictionary<String, [String]>的默认Codable实现上。OrderedDictionary默认会将自身编码/解码为键值对数组（格式类似[{"key": "...", "value": "..."}]），但你的JSON中menu字段是标准的JSON对象（键值对直接嵌套），两者格式不匹配，导致解码器期望数组却收到字典，触发类型不匹配错误。

解决方案 #

方案1：改用普通Dictionary（无需保持顺序） #

如果不需要保留菜单条目的顺序，直接将menu字段类型改为Dictionary<String, [String]>即可，Swift标准Dictionary默认支持JSON对象的解码：

import Foundation

struct Menu : Codable, Hashable {
    var id: UUID { UUID() }
    let day: String
    let locationName: String
    let coordinate: [Double]
    // 替换OrderedDictionary为普通Dictionary
    let menu: [String: [String]]
    
    func getTodaysLocation(_ today: String)-> String{
        if today == day{
            return locationName
        }
        return ""
    }
}

方案2：自定义OrderedDictionary的Codable实现（需保持顺序） #

如果必须保留菜单条目的顺序，需要为OrderedDictionary扩展自定义的Codable逻辑，使其能从JSON对象解码：

import Foundation
import OrderedCollections

// 扩展OrderedDictionary，支持从JSON对象解码/编码
extension OrderedDictionary: Codable where Key: CodingKey & Hashable, Value: Codable {
    public init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: Key.self)
        self.init()
        for key in container.allKeys {
            let value = try container.decode(Value.self, forKey: key)
            self.updateValue(value, forKey: key)
        }
    }
    
    public func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: Key.self)
        for (key, value) in self {
            try container.encode(value, forKey: key)
        }
    }
}

// 原模型保持不变
struct Menu : Codable, Hashable {
    var id: UUID { UUID() }
    let day: String
    let locationName: String
    let coordinate: [Double]
    let menu: OrderedDictionary<String, [String]>
    
    func getTodaysLocation(_ today: String)-> String{
        if today == day{
            return locationName
        }
        return ""
    }
}

注意：此扩展要求Key类型遵循CodingKey，String默认符合该协议，所以OrderedDictionary<String, [String]>可以正常使用。

验证修改 #

修改完成后，重新调用Bundle.decode("Menu.json")即可正常解码JSON数据。

内容的提问来源于stack exchange，提问作者user28773992