Swift自定义Decoder如何处理嵌套Container不存在时的nil值问题

你当前代码崩溃的核心原因是nestedContainer(keyedBy:forKey:)方法在对应key不存在时会直接抛出解码错误，只需将这一步改为可选判断逻辑即可兼容name字段完全缺失的场景，修改后的完整代码如下：

struct UserModel: Decodable {
    var gender: String
    var title: String?
    var first: String?
    var last: String?
    
    // 顶层编码键
    enum CodingKeys: String, CodingKey {
        case name, gender
    }
    
    enum NameKeys: CodingKey {
        case title, first, last
    }
    
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        gender = try container.decode(String.self, forKey: .gender)
        // 新增判断：只有name字段存在时才解析内部字段
        if container.contains(.name) {
            let name = try container.nestedContainer(keyedBy: NameKeys.self, forKey: .name)
            title = try name.decodeIfPresent(String.self, forKey: .title)
            first = try name.decodeIfPresent(String.self, forKey: .first)
            last = try name.decodeIfPresent(String.self, forKey: .last)
        }
        // 如果name字段不存在，三个可选属性默认保持nil即可
    }
}

可选替代写法 #

如果你不想用contains判断，也可以用try?捕获获取嵌套容器的错误，效果完全一致：

init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    gender = try container.decode(String.self, forKey: .gender)
    // 获取嵌套容器失败时直接跳过解析
    if let name = try? container.nestedContainer(keyedBy: NameKeys.self, forKey: .name) {
        title = try name.decodeIfPresent(String.self, forKey: .title)
        first = try name.decodeIfPresent(String.self, forKey: .first)
        last = try name.decodeIfPresent(String.self, forKey: .last)
    }
}

两种写法都可以实现：就算顶层name字段完全缺失，解码也能正常完成，title、first、last三个属性会自动为nil，不会抛出崩溃错误。

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