Customize json parsing of a Swift Decodable

custom_json_parsing.swift

// https://gist.github.com/jonathan-beebe/a631492c6589b14d0c06fe846d266ff7

import UIKit

// The vendor sandbox returns the following values for an `undefined` string value:
//
// - `""`
// - `"None"`
// - `null`
//
// Can we normalizing these into a Swift `nil` when parsing the json?

// MARK: Sample JSON

let jsonString = """
{
    "name": "Alice",
    "subnet_id": "None",
    "transaction_id": "",
    "to": null
}
"""

// MARK: Swift Model

struct TransactionModel: Decodable {
    let name: String?
    let subnet_id: String?
    let transaction_id: String?
    let to: String?
}

// MARK: Custom Decoding

extension KeyedDecodingContainer {
    /// Wraps native `decodeIfPresent` to additionally map the value, if present.
    func decodeIfPresent<T>(_ type: T.Type, forKey key: K, map: ((T?) -> T?)) throws -> T? where T : Decodable {
        return map(try decodeIfPresent(type, forKey: key))
    }
}

extension Array where Element : Equatable {
    /// Wraps native `contains` function to allow for optional input.
    func contains(_ element: Element?) -> Bool {
        guard let element = element else { return false }
        return contains(element)
    }
}

// Mark: Vendor specific extensions for custom decoding of strings

struct Vendor {
    static let undefinedStrings = ["None", ""]
}

extension KeyedDecodingContainer {
    func decodeVendorString(forKey key: Key) throws -> String? {
        return try decodeIfPresent(String.self, forKey: key, map: { Vendor.undefinedStrings.contains($0) ? nil : $0 })
    }
}

extension TransactionModel {
    
    enum TransactionModelKey: String, CodingKey {
        case name
        case subnet_id
        case transaction_id
        case to
    }
    
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: TransactionModelKey.self)
        self.init(
            name           : try container.decodeVendorString(forKey: .name),
            subnet_id      : try container.decodeVendorString(forKey: .subnet_id),
            transaction_id : try container.decodeVendorString(forKey: .transaction_id),
            to             : try container.decodeVendorString(forKey: .to)
        )
    }
}

// MARK: Try it out

do {
    let decoder = JSONDecoder()
    let parsed = try decoder.decode(TransactionModel.self, from: jsonString.data(using: .utf8)!)
    print(parsed) // -> TransactionModel(name: Optional("Alice"), subnet_id: nil, transaction_id: nil, to: nil)
} catch {
    print(error)
}