如何将递归数据映射到结构中？

Question

这篇文章与previous post I made相关。我希望映射以下嵌套字典：

["A": [["A1": ["A11", "A12"]], ["A2": ["A21", "A22"]]],
   "B": [["B1": ["B11", "B12"]], ["B2": ["B21", "B22"]]]
    ]

进入递归结构：

Item(title:"",children:
    [Item(title:"A",children:
        [Item(title:"A1", children:
            [Item(title:"A11"),Item(title:"A12")]
            )]),
          Item(title:"B",children:
            [Item(title:"B1"),Item(title:"B2")]
        )]
)

与

struct Item: Identifiable {
    let id = UUID()
    var title: String
    var children: [Item] = []
}

为了实验，我从 ["A": [["A1": ["A11"]]] 开始，做了一个 json 字符串：

let json1: String = """
            {"title": "", "children":[{"title": "A",
                                      "children": [{"title": "A1",
                                                    "children": [{"title": "A11"}]
                                                    }]
                                     }]
            }
"""



let decoder = JSONDecoder()
let info = try decoder.decode(Item.self, from: json.data(using: .utf8)!)
print(info)

只有当我在最后一个节点中包含 "children": [] 时它才有效，如下所示：

   let json2: String =  """
                {"title": "", "children":[{"title": "A",
                                          "children": [{"title": "A1",
                                                        "children": [{"title": "A11", "children": []}]
                                                    }]
                                     }]
            }
"""

我需要做什么才能使json1字符串工作，这样即使没有children的输入，它也会采用[]的默认值？

Answer 1

您必须提供自定义init(decoder:) 来处理这种情况。处理此问题需要您使用JSONDecoder 的容器的decodeIfPresent API，并仅在值存在时尝试解码，并在条件失败时提供默认值。方法如下：

extension Item: Codable {
    enum CodingKeys: String, CodingKey {
        case title, children
    }
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        title = try container.decode(String.self, forKey: .title)
        children = try container.decodeIfPresent([Item].self, forKey: .children) ?? []
    }
}

Answer 2

一种方法是将children 设为可选并使用计算属性来访问它。在children 为nil 的情况下，计算属性可以返回一个空数组。

struct Item: Identifiable, Decodable {
    let id = UUID()
    var title: String
    private var privateChildren: [Item]?
    var children: [Item] {
        return self.privateChildren ?? []
    }

    enum CodingKeys: String,CodingKey {
        case title
        case privateChildren = "children"
    }
}

我使用了CodingKeys 枚举，以便您可以在 JSON 和代码中保持相同的名称，但如果您在 JSON 或代码中更改了属性名称，则可以避免这种情况。