Swift 4 JSON Decodable simplest way to decode type change

Unfortunately, I don't believe such an option exists in the current JSONDecoder API. There only exists an option in order to convert exceptional floating-point values to and from a string representation.

Another possible solution to decoding manually is to define a Codable wrapper type for any LosslessStringConvertible that can encode to and decode from its String representation:

struct StringCodableMap<Decoded : LosslessStringConvertible> : Codable {

    var decoded: Decoded

    init(_ decoded: Decoded) {
        self.decoded = decoded
    }

    init(from decoder: Decoder) throws {

        let container = try decoder.singleValueContainer()
        let decodedString = try container.decode(String.self)

        guard let decoded = Decoded(decodedString) else {
            throw DecodingError.dataCorruptedError(
                in: container, debugDescription: """
                The string \(decodedString) is not representable as a \(Decoded.self)
                """
            )
        }

        self.decoded = decoded
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(decoded.description)
    }
}

Then you can just have a property of this type and use the auto-generated Codable conformance:

struct Example : Codable {

    var name: String
    var age: Int
    var taxRate: StringCodableMap<Float>

    private enum CodingKeys: String, CodingKey {
        case name, age
        case taxRate = "tax_rate"
    }
}

Although unfortunately, now you have to talk in terms of taxRate.decoded in order to interact with the Float value.

However you could always define a simple forwarding computed property in order to alleviate this:

struct Example : Codable {

    var name: String
    var age: Int

    private var _taxRate: StringCodableMap<Float>

    var taxRate: Float {
        get { return _taxRate.decoded }
        set { _taxRate.decoded = newValue }
    }

    private enum CodingKeys: String, CodingKey {
        case name, age
        case _taxRate = "tax_rate"
    }
}

Although this still isn't as a slick as it really should be – hopefully a later version of the JSONDecoder API will include more custom decoding options, or else have the ability to express type conversions within the Codable API itself.

However one advantage of creating the wrapper type is that it can also be used in order to make manual decoding and encoding simpler. For example, with manual decoding:

struct Example : Decodable {

    var name: String
    var age: Int
    var taxRate: Float

    private enum CodingKeys: String, CodingKey {
        case name, age
        case taxRate = "tax_rate"
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)

        self.name = try container.decode(String.self, forKey: .name)
        self.age = try container.decode(Int.self, forKey: .age)
        self.taxRate = try container.decode(StringCodableMap<Float>.self,
                                            forKey: .taxRate).decoded
    }
}

Using Swift 5.1, you may choose one of the three following ways in order to solve your problem.

---

#1. Using Decodable init(from:) initializer

Use this strategy when you need to convert from String to Float for a single struct, enum or class.

import Foundation

struct ExampleJson: Decodable {

    var name: String
    var age: Int
    var taxRate: Float

    enum CodingKeys: String, CodingKey {
        case name, age, taxRate = "tax_rate"
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)

        name = try container.decode(String.self, forKey: CodingKeys.name)
        age = try container.decode(Int.self, forKey: CodingKeys.age)
        let taxRateString = try container.decode(String.self, forKey: CodingKeys.taxRate)
        guard let taxRateFloat = Float(taxRateString) else {
            let context = DecodingError.Context(codingPath: container.codingPath + [CodingKeys.taxRate], debugDescription: "Could not parse json key to a Float object")
            throw DecodingError.dataCorrupted(context)
        }
        taxRate = taxRateFloat
    }

}

Usage:

import Foundation

let jsonString = """
{
  "name": "Bob",
  "age": 25,
  "tax_rate": "4.25"
}
"""

let data = jsonString.data(using: String.Encoding.utf8)!
let decoder = JSONDecoder()
let exampleJson = try! decoder.decode(ExampleJson.self, from: data)
dump(exampleJson)
/*
 prints:
 ▿ __lldb_expr_126.ExampleJson
   - name: "Bob"
   - age: 25
   - taxRate: 4.25
 */

---

#2. Using an intermediate model

Use this strategy when you have many nested keys in your JSON or when you need to convert many keys (e.g. from String to Float) from your JSON.

import Foundation

fileprivate struct PrivateExampleJson: Decodable {

    var name: String
    var age: Int
    var taxRate: String

    enum CodingKeys: String, CodingKey {
        case name, age, taxRate = "tax_rate"
    }

}

struct ExampleJson: Decodable {

    var name: String
    var age: Int
    var taxRate: Float

    init(from decoder: Decoder) throws {
        let privateExampleJson = try PrivateExampleJson(from: decoder)

        name = privateExampleJson.name
        age = privateExampleJson.age
        guard let convertedTaxRate = Float(privateExampleJson.taxRate) else {
            let context = DecodingError.Context(codingPath: [], debugDescription: "Could not parse json key to a Float object")
            throw DecodingError.dataCorrupted(context)
        }
        taxRate = convertedTaxRate
    }

}

Usage:

import Foundation

let jsonString = """
{
  "name": "Bob",
  "age": 25,
  "tax_rate": "4.25"
}
"""

let data = jsonString.data(using: String.Encoding.utf8)!
let decoder = JSONDecoder()
let exampleJson = try! decoder.decode(ExampleJson.self, from: data)
dump(exampleJson)
/*
 prints:
 ▿ __lldb_expr_126.ExampleJson
   - name: "Bob"
   - age: 25
   - taxRate: 4.25
 */

---

#3. Using a KeyedDecodingContainer extension method

Use this strategy when converting from some JSON keys' types to your model's property types (e.g. String to Float) is a common pattern in your application.

import Foundation

extension KeyedDecodingContainer  {

    func decode(_ type: Float.Type, forKey key: Key) throws -> Float {
        if let stringValue = try? self.decode(String.self, forKey: key) {
            guard let floatValue = Float(stringValue) else {
                let context = DecodingError.Context(codingPath: codingPath, debugDescription: "Could not parse json key to a Float object")
                throw DecodingError.dataCorrupted(context)
            }
            return floatValue
        } else {
            let doubleValue = try self.decode(Double.self, forKey: key)
            return Float(doubleValue)
        }
    }

}

struct ExampleJson: Decodable {

    var name: String
    var age: Int
    var taxRate: Float

    enum CodingKeys: String, CodingKey {
        case name, age, taxRate = "tax_rate"
    }

}

Usage:

import Foundation

let jsonString = """
{
    "name": "Bob",
    "age": 25,
    "tax_rate": "4.25"
}
"""

let data = jsonString.data(using: String.Encoding.utf8)!
let decoder = JSONDecoder()
let exampleJson = try! decoder.decode(ExampleJson.self, from: data)
dump(exampleJson)
/*
 prints:
 ▿ __lldb_expr_126.ExampleJson
 - name: "Bob"
 - age: 25
 - taxRate: 4.25
 */