使用字典/数组初始化符合 Codable 的对象

Question

我的主要用例是使用字典创建一个对象：例如

struct Person: Codable { let name: String }    
let dictionary = ["name": "Bob"]
let person = Person(from: dictionary)    

我想避免编写自定义实现，并希望尽可能高效。

Answer 1

目前我最好的解决方案是这个，但它有编码/解码的开销。

extension Decodable {
  init(from: Any) throws {
    let data = try JSONSerialization.data(withJSONObject: from, options: .prettyPrinted)
    let decoder = JSONDecoder()
    self = try decoder.decode(Self.self, from: data)
  }
}

根据问题中的示例，结果将是

let person = Person(from: dictionary)

如果您有兴趣采取另一种方式，那么这可能会有所帮助https://stackoverflow.com/a/46329055/1453346

Answer 2

基于Chris Mitchelmore answer

详情

• Xcode 版本 10.3 (10G8)，Swift 5

解决方案

import Foundation

extension Decodable {

    init(from value: Any,
         options: JSONSerialization.WritingOptions = [],
         decoder: JSONDecoder) throws {
        let data = try JSONSerialization.data(withJSONObject: value, options: options)
        self = try decoder.decode(Self.self, from: data)
    }

    init(from value: Any,
         options: JSONSerialization.WritingOptions = [],
         decoderSetupClosure: ((JSONDecoder) -> Void)? = nil) throws {
        let decoder = JSONDecoder()
        decoderSetupClosure?(decoder)
        try self.init(from: value, options: options, decoder: decoder)
    }

    init?(discardingAnErrorFrom value: Any,
          printError: Bool = false,
          options: JSONSerialization.WritingOptions = [],
          decoderSetupClosure: ((JSONDecoder) -> Void)? = nil) {
        do {
            try self.init(from: value, options: options, decoderSetupClosure: decoderSetupClosure)
        } catch {
            if printError { print("\(Self.self) decoding ERROR:\n\(error)") }
            return nil
        }
    }
}

用法

struct Item: Decodable {
    let id: Int
    let name: String
    let isActive: Bool
    var date: Date
}

let dictionary = ["id": 1, "name": "Item", "is_active": false,
                  "date": "2019-08-06T06:55:00.000-04:00"] as [String : Any]
do {
    let item1 = try Item(from: dictionary) { decoder in
        decoder.keyDecodingStrategy = .convertFromSnakeCase
        let dateFormatter = DateFormatter()
        dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSZ"
        decoder.dateDecodingStrategy = .formatted(dateFormatter)
    }
    print(item1)
} catch {
    print("Error: \(error)")
}

print("\n========================")
let item2 = Item(discardingAnErrorFrom: dictionary)
print(String(describing: item2))

print("\n========================")
let item3 = Item(discardingAnErrorFrom: dictionary, printError: true)
print(String(describing: item3))

print("\n========================")
let item4 = Item(discardingAnErrorFrom: dictionary){ decoder in
    decoder.keyDecodingStrategy = .convertFromSnakeCase
    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSZ"
    decoder.dateDecodingStrategy = .formatted(dateFormatter)
}
print(String(describing: item4))

使用记录

Item(id: 1, name: "Item", isActive: false, date: 2019-08-06 10:55:00 +0000)

========================
nil

========================
Item decoding ERROR:
keyNotFound(CodingKeys(stringValue: "isActive", intValue: nil), Swift.DecodingError.Context(codingPath: [], debugDescription: "No value associated with key CodingKeys(stringValue: \"isActive\", intValue: nil) (\"isActive\").", underlyingError: nil))
nil

========================
Optional(__lldb_expr_5.Item(id: 1, name: "Item", isActive: false, date: 2019-08-06 10:55:00 +0000))

Answer 3

我改编了Chris Mitchelmore 的答案，使其成为一个可失败的初始化程序，而不是抛出代码。在某些情况下让它更方便一些。

extension Decodable {
    init?(from: Any) {
        guard let data = try? JSONSerialization.data(withJSONObject: from, options: .prettyPrinted) else { return nil }
        let decoder = JSONDecoder()
        guard let decoded = try? decoder.decode(Self.self, from: data) else { return nil }
        self = decoded
    }
}