Swift中解码[String:Int]字典时蛇形键转驼峰策略失效问题

问题场景 #

原始JSON数据如下：

{"message": "The data were fetched successfully.","data": {"date": "2022-07-22","day": 149,"resource": "https://www.someWebsite.com","stats": {"personnel_units": 39000,"tanks": 1704,"armoured_fighting_vehicles": 3920,"artillery_systems": 863,"mlrs": 251,"aa_warfare_systems": 113,"planes": 221,"helicopters": 188,"vehicles_fuel_tanks": 2803,"warships_cutters": 15,"cruise_missiles": 167,"uav_systems": 713,"special_military_equip": 72,"atgm_srbm_systems": 4},"increase": {"personnel_units": 150,"tanks": 0,"armoured_fighting_vehicles": 8,"artillery_systems": 4,"mlrs": 0,"aa_warfare_systems": 0,"planes": 0,"helicopters": 0,"vehicles_fuel_tanks": 22,"warships_cutters": 0,"cruise_missiles": 0,"uav_systems": 3,"special_military_equip": 0,"atgm_srbm_systems": 0}}}

通过quicktype生成的结构体如下：

struct DataModel: Codable {
    var message: String
    var data: ObjectsModel
}

struct ObjectsModel: Codable {
    var date: String
    var day: Int
    var resource: String
    var stats, increase: [String: Int]
}

当前遇到的问题：设置decoder.keyDecodingStrategy = .convertFromSnakeCase后，stats和increase内部的蛇形键（比如personnel_units）无法转换为驼峰格式，也没办法给这些内部键设置CodingKeys做映射。

问题原因 #

因为stats和increase被定义成了[String: Int]字典类型，而keyDecodingStrategy的转换逻辑只会作用于结构体的属性名，不会处理字典内部的键值对，所以蛇形键转驼峰的策略对字典里的键无效，也没法用CodingKeys来映射这些字典键。

解决方案 #

把stats和increase对应的字典转换成具体的Codable结构体，这样就能利用Swift的自动键解码策略，或者手动设置CodingKeys来实现蛇形键到驼峰属性的映射。

步骤1：定义统一的统计数据结构体

因为stats和increase的键结构完全一致，可以复用同一个结构体：

struct MilitaryStats: Codable {
    let personnelUnits: Int
    let tanks: Int
    let armouredFightingVehicles: Int
    let artillerySystems: Int
    let mlrs: Int
    let aaWarfareSystems: Int
    let planes: Int
    let helicopters: Int
    let vehiclesFuelTanks: Int
    let warshipsCutters: Int
    let cruiseMissiles: Int
    let uavSystems: Int
    let specialMilitaryEquip: Int
    let atgmSrbmSystems: Int
}

步骤2：修改ObjectsModel的属性类型

把原来的[String: Int]替换成上面定义的MilitaryStats：

struct ObjectsModel: Codable {
    let date: String
    let day: Int
    let resource: String
    let stats: MilitaryStats
    let increase: MilitaryStats
}

struct DataModel: Codable {
    let message: String
    let data: ObjectsModel
}

步骤3：解码时设置键转换策略

现在设置convertFromSnakeCase就能自动把JSON里的蛇形键转换成结构体的驼峰属性名：

let jsonData = // 你的JSON数据Data对象
let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase

do {
    let model = try decoder.decode(DataModel.self, from: jsonData)
    // 访问驼峰格式的属性，比如model.data.stats.personnelUnits
    print(model.data.stats.personnelUnits)
} catch {
    print("解码失败：\(error)")
}

可选：手动设置CodingKeys（如果不想用自动转换）

如果需要手动映射键，也可以给MilitaryStats添加CodingKeys：

struct MilitaryStats: Codable {
    let personnelUnits: Int
    let tanks: Int
    // ... 其他属性
    
    enum CodingKeys: String, CodingKey {
        case personnelUnits = "personnel_units"
        case tanks
        case armouredFightingVehicles = "armoured_fighting_vehicles"
        // ... 其他键的映射
    }
}

这种方式不需要设置keyDecodingStrategy，直接通过CodingKeys完成蛇形键到驼峰属性的映射。

内容的提问来源于stack exchange，提问作者Ozzzie