Init an object conforming to Codable with a dictionary/array

Question

Primarily my use case is to create an object using a dictionary: e.g.

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

I would like to avoid writing custom implementations and want to be as efficient as possible.

Answer 1

At the moment the best solution I have is this but it has the overhead of encoding/decoding.

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)
  }
}

Following from the example in the question the result would be

let person = Person(from: dictionary)

If you're interested in going the other way then this might help https://stackoverflow.com/a/46329055/1453346

Answer 2

based on Chris Mitchelmore answer

Details

• Xcode Version 10.3 (10G8), Swift 5

Solution

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
        }
    }
}

Usage

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))

Usage log

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

I adapted Chris Mitchelmore's answer so that it is a failable initializer instead of throwing code. Makes it a bit handier in some cases.

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
    }
}