Custom Swift Encoder/Decoder for the Strings Resource Format

Question

I have been playing around with Codable and reading and writing JSON from and to a file. Now I would like to write a custom Coder that can read and write iOS .strings files. Can anyone help me with this? I found the protocols Encoder and Decoder, but I have no idea what I should implement here:

class StringsEncoder {}

extension StringsEncoder: Encoder {
    var codingPath: [CodingKey?] {
        return []
    }

    var userInfo: [CodingUserInfoKey : Any] {
        return [:]
    }

    func container<Key>(keyedBy type: Key.Type) -> KeyedEncodingContainer<Key> where Key : CodingKey {

    }

    func unkeyedContainer() -> UnkeyedEncodingContainer {

    }

    func singleValueContainer() -> SingleValueEncodingContainer {

    }
}

extension StringsEncoder: Decoder {
    func container<Key>(keyedBy type: Key.Type) throws -> KeyedDecodingContainer<Key> where Key : CodingKey {

    }

    func unkeyedContainer() throws -> UnkeyedDecodingContainer {

    }

    func singleValueContainer() throws -> SingleValueDecodingContainer {

    }
}

Homologize

Have you looked into the Codable protocols and what they can do? I'm not sure the .strings format is a right fit for this — the Codable protocols have to support general-purpose formats which have arrays, dictionaries, numbers, null values, etc. .strings files don't support any of that... It's a very single-purpose format. – Hammy

You probably can, though it seems a bit overkill for the strings format. Check out the JSONEncoder source for example, which is well documented. swift unboxed will have a post similar to what you are looking for soon I think – Gaudreau

I know you want to understand Codable, but on the off-chance you just want to read and write to strings file checkout String.propertyListFromStringsFileFormat() and Dictionary.descriptionInStringsFileFormat – Phebephedra

mikeash has a really good post about building a custom codable object too: mikeash.com/pyblog/… – Phebephedra

You might want to look at Swift’s implementation of JSONEncoder here. There is probably room for Swift to provide some more support for custom Encoders and Decoders, but for now a good idea is to customize that implementation to suit your requirements. – Calculating

Strings files are more like a dictionary, and Codable usually works best with fixed layout data structures. – Terminate

Implementation of JSONEncoder has moved here – Isabelleisac

Answer 1

A bit late to the party here but I feel this might be helpful/informative to others given the question high vote count. (But I won't really get into the actual usefulness of such code in practice—please check the comments above for that.)

Unfortunately, given the coding stack flexibility and type-safeness, implementing a new encoding and decoding solution, for an alternative external representation, is far from a trivial task... so let's get started:

Encoding

Let's start by implementing the encoding part for the desired strings file external representation. (The necessary types will be introduced in a top-down approach.)

Like the standard JSONEncoder class we need to introduce a class to expose/drive our new encoding API. Let's call that StringsEncoder:

/// An object that encodes instances of a data type 
/// as strings following the simple strings file format.
public class StringsEncoder {
    
    /// Returns a strings file-encoded representation of the specified value. 
    public func encode<T: Encodable>(_ value: T) throws -> String {
        let stringsEncoding = StringsEncoding()
        try value.encode(to: stringsEncoding)
        return dotStringsFormat(from: stringsEncoding.data.strings)
    }
    
    private func dotStringsFormat(from strings: [String: String]) -> String {
        var dotStrings = strings.map { "\"\($0)\" = \"\($1)\";" }
        dotStrings.sort()
        dotStrings.insert("/* Generated by StringsEncoder */", at: 0)
        return dotStrings.joined(separator: "\n")
    }
}

Next, we need to provide a type (e.g., a struct) conforming to the core Encoder protocol:

fileprivate struct StringsEncoding: Encoder {
    
    /// Stores the actual strings file data during encoding.
    fileprivate final class Data {
        private(set) var strings: [String: String] = [:]
        
        func encode(key codingKey: [CodingKey], value: String) {
            let key = codingKey.map { $0.stringValue }.joined(separator: ".")
            strings[key] = value
        }
    }
    
    fileprivate var data: Data
    
    init(to encodedData: Data = Data()) {
        self.data = encodedData
    }

    var codingPath: [CodingKey] = []
    
    let userInfo: [CodingUserInfoKey : Any] = [:]
    
    func container<Key: CodingKey>(keyedBy type: Key.Type) -> KeyedEncodingContainer<Key> {
        var container = StringsKeyedEncoding<Key>(to: data)
        container.codingPath = codingPath
        return KeyedEncodingContainer(container)
    }
    
    func unkeyedContainer() -> UnkeyedEncodingContainer {
        var container = StringsUnkeyedEncoding(to: data)
        container.codingPath = codingPath
        return container
   }
    
    func singleValueContainer() -> SingleValueEncodingContainer {
        var container = StringsSingleValueEncoding(to: data)
        container.codingPath = codingPath
        return container
    }
}

Finally, we need to handle all 3 encoding containers types:

• KeyedEncodingContainer
• UnkeyedEncodingContainer
• SingleValueEncodingContainer

fileprivate struct StringsKeyedEncoding<Key: CodingKey>: KeyedEncodingContainerProtocol {

    private let data: StringsEncoding.Data
    
    init(to data: StringsEncoding.Data) {
        self.data = data
    }
    
    var codingPath: [CodingKey] = []
    
    mutating func encodeNil(forKey key: Key) throws {
        data.encode(key: codingPath + [key], value: "nil")
    }
    
    mutating func encode(_ value: Bool, forKey key: Key) throws {
        data.encode(key: codingPath + [key], value: value.description)
    }
    
    mutating func encode(_ value: String, forKey key: Key) throws {
        data.encode(key: codingPath + [key], value: value)
    }
    
    mutating func encode(_ value: Double, forKey key: Key) throws {
        data.encode(key: codingPath + [key], value: value.description)
    }
    
    mutating func encode(_ value: Float, forKey key: Key) throws {
        data.encode(key: codingPath + [key], value: value.description)
    }
    
    mutating func encode(_ value: Int, forKey key: Key) throws {
        data.encode(key: codingPath + [key], value: value.description)
    }
    
    mutating func encode(_ value: Int8, forKey key: Key) throws {
        data.encode(key: codingPath + [key], value: value.description)
    }
    
    mutating func encode(_ value: Int16, forKey key: Key) throws {
        data.encode(key: codingPath + [key], value: value.description)
    }
    
    mutating func encode(_ value: Int32, forKey key: Key) throws {
        data.encode(key: codingPath + [key], value: value.description)
    }
    
    mutating func encode(_ value: Int64, forKey key: Key) throws {
        data.encode(key: codingPath + [key], value: value.description)
    }
    
    mutating func encode(_ value: UInt, forKey key: Key) throws {
        data.encode(key: codingPath + [key], value: value.description)
    }
    
    mutating func encode(_ value: UInt8, forKey key: Key) throws {
        data.encode(key: codingPath + [key], value: value.description)
    }
    
    mutating func encode(_ value: UInt16, forKey key: Key) throws {
        data.encode(key: codingPath + [key], value: value.description)
    }
    
    mutating func encode(_ value: UInt32, forKey key: Key) throws {
        data.encode(key: codingPath + [key], value: value.description)
    }
    
    mutating func encode(_ value: UInt64, forKey key: Key) throws {
        data.encode(key: codingPath + [key], value: value.description)
    }
    
    mutating func encode<T: Encodable>(_ value: T, forKey key: Key) throws {
        var stringsEncoding = StringsEncoding(to: data)
        stringsEncoding.codingPath.append(key)
        try value.encode(to: stringsEncoding)
    }
    
    mutating func nestedContainer<NestedKey: CodingKey>(
        keyedBy keyType: NestedKey.Type,
        forKey key: Key) -> KeyedEncodingContainer<NestedKey> {
        var container = StringsKeyedEncoding<NestedKey>(to: data)
        container.codingPath = codingPath + [key]
        return KeyedEncodingContainer(container)
    }
    
    mutating func nestedUnkeyedContainer(forKey key: Key) -> UnkeyedEncodingContainer {
        var container = StringsUnkeyedEncoding(to: data)
        container.codingPath = codingPath + [key]
        return container
    }
    
    mutating func superEncoder() -> Encoder {
        let superKey = Key(stringValue: "super")!
        return superEncoder(forKey: superKey)
    }
    
    mutating func superEncoder(forKey key: Key) -> Encoder {
        var stringsEncoding = StringsEncoding(to: data)
        stringsEncoding.codingPath = codingPath + [key]
        return stringsEncoding
    }
}

fileprivate struct StringsUnkeyedEncoding: UnkeyedEncodingContainer {

    private let data: StringsEncoding.Data
    
    init(to data: StringsEncoding.Data) {
        self.data = data
    }
    
    var codingPath: [CodingKey] = []

    private(set) var count: Int = 0
    
    private mutating func nextIndexedKey() -> CodingKey {
        let nextCodingKey = IndexedCodingKey(intValue: count)!
        count += 1
        return nextCodingKey
    }
    
    private struct IndexedCodingKey: CodingKey {
        let intValue: Int?
        let stringValue: String

        init?(intValue: Int) {
            self.intValue = intValue
            self.stringValue = intValue.description
        }

        init?(stringValue: String) {
            return nil
        }
    }

    mutating func encodeNil() throws {
        data.encode(key: codingPath + [nextIndexedKey()], value: "nil")
    }
    
    mutating func encode(_ value: Bool) throws {
        data.encode(key: codingPath + [nextIndexedKey()], value: value.description)
    }
    
    mutating func encode(_ value: String) throws {
        data.encode(key: codingPath + [nextIndexedKey()], value: value)
    }
    
    mutating func encode(_ value: Double) throws {
        data.encode(key: codingPath + [nextIndexedKey()], value: value.description)
    }
    
    mutating func encode(_ value: Float) throws {
        data.encode(key: codingPath + [nextIndexedKey()], value: value.description)
    }
    
    mutating func encode(_ value: Int) throws {
        data.encode(key: codingPath + [nextIndexedKey()], value: value.description)
    }
    
    mutating func encode(_ value: Int8) throws {
        data.encode(key: codingPath + [nextIndexedKey()], value: value.description)
    }
    
    mutating func encode(_ value: Int16) throws {
        data.encode(key: codingPath + [nextIndexedKey()], value: value.description)
    }
    
    mutating func encode(_ value: Int32) throws {
        data.encode(key: codingPath + [nextIndexedKey()], value: value.description)
    }
    
    mutating func encode(_ value: Int64) throws {
        data.encode(key: codingPath + [nextIndexedKey()], value: value.description)
    }
    
    mutating func encode(_ value: UInt) throws {
        data.encode(key: codingPath + [nextIndexedKey()], value: value.description)
    }
    
    mutating func encode(_ value: UInt8) throws {
        data.encode(key: codingPath + [nextIndexedKey()], value: value.description)
    }
    
    mutating func encode(_ value: UInt16) throws {
        data.encode(key: codingPath + [nextIndexedKey()], value: value.description)
    }
    
    mutating func encode(_ value: UInt32) throws {
        data.encode(key: codingPath + [nextIndexedKey()], value: value.description)
    }
    
    mutating func encode(_ value: UInt64) throws {
        data.encode(key: codingPath + [nextIndexedKey()], value: value.description)
    }
    
    mutating func encode<T: Encodable>(_ value: T) throws {
        var stringsEncoding = StringsEncoding(to: data)
        stringsEncoding.codingPath = codingPath + [nextIndexedKey()]
        try value.encode(to: stringsEncoding)
    }
    
    mutating func nestedContainer<NestedKey: CodingKey>(
        keyedBy keyType: NestedKey.Type) -> KeyedEncodingContainer<NestedKey> {
        var container = StringsKeyedEncoding<NestedKey>(to: data)
        container.codingPath = codingPath + [nextIndexedKey()]
        return KeyedEncodingContainer(container)
    }
    
    mutating func nestedUnkeyedContainer() -> UnkeyedEncodingContainer {
        var container = StringsUnkeyedEncoding(to: data)
        container.codingPath = codingPath + [nextIndexedKey()]
        return container
    }
    
    mutating func superEncoder() -> Encoder {
        var stringsEncoding = StringsEncoding(to: data)
        stringsEncoding.codingPath.append(nextIndexedKey())
        return stringsEncoding
    }
}

fileprivate struct StringsSingleValueEncoding: SingleValueEncodingContainer {
    
    private let data: StringsEncoding.Data
    
    init(to data: StringsEncoding.Data) {
        self.data = data
    }

    var codingPath: [CodingKey] = []
    
    mutating func encodeNil() throws {
        data.encode(key: codingPath, value: "nil")
    }
    
    mutating func encode(_ value: Bool) throws {
        data.encode(key: codingPath, value: value.description)
    }
    
    mutating func encode(_ value: String) throws {
        data.encode(key: codingPath, value: value)
    }
    
    mutating func encode(_ value: Double) throws {
        data.encode(key: codingPath, value: value.description)
    }
    
    mutating func encode(_ value: Float) throws {
        data.encode(key: codingPath, value: value.description)
    }
    
    mutating func encode(_ value: Int) throws {
        data.encode(key: codingPath, value: value.description)
    }
    
    mutating func encode(_ value: Int8) throws {
        data.encode(key: codingPath, value: value.description)
    }
    
    mutating func encode(_ value: Int16) throws {
        data.encode(key: codingPath, value: value.description)
    }
    
    mutating func encode(_ value: Int32) throws {
        data.encode(key: codingPath, value: value.description)
    }
    
    mutating func encode(_ value: Int64) throws {
        data.encode(key: codingPath, value: value.description)
    }
    
    mutating func encode(_ value: UInt) throws {
        data.encode(key: codingPath, value: value.description)
    }
    
    mutating func encode(_ value: UInt8) throws {
        data.encode(key: codingPath, value: value.description)
    }
    
    mutating func encode(_ value: UInt16) throws {
        data.encode(key: codingPath, value: value.description)
    }
    
    mutating func encode(_ value: UInt32) throws {
        data.encode(key: codingPath, value: value.description)
    }
    
    mutating func encode(_ value: UInt64) throws {
        data.encode(key: codingPath, value: value.description)
    }
    
    mutating func encode<T: Encodable>(_ value: T) throws {
        var stringsEncoding = StringsEncoding(to: data)
        stringsEncoding.codingPath = codingPath
        try value.encode(to: stringsEncoding)
    }
}

Obviously, I made some design decisions regarding how to encode nested types using the (very!) simple strings file format. Hopefully, my code is clear enough that it should be easy to tweak the encoding details if so desired.

Tests

A simple test for a trivial Codable type:

struct Product: Codable {
    var name: String
    var price: Float
    var info: String
}

let iPhone = Product(name: "iPhone X", price: 1_000, info: "Our best iPhone yet!")

let stringsEncoder = StringsEncoder()
do {
    let stringsFile = try stringsEncoder.encode(iPhone)
    print(stringsFile)
} catch {
    print("Encoding failed: \(error)")
}

Output:

/* Generated by StringsEncoder */
"info" = "Our best iPhone yet!";
"name" = "iPhone X";
"price" = "1000.0";

A more complex test with nested structs and arrays:

struct Product: Codable {
    var name: String
    var price: Float
    var info: String
}

struct Address: Codable {
    var street: String
    var city: String
    var state: String
}

struct Store: Codable {
    var name: String
    var address: Address // nested struct
    var products: [Product] // array
}

let iPhone = Product(name: "iPhone X", price: 1_000, info: "Our best iPhone yet!")
let macBook = Product(name: "Mac Book Pro", price: 2_000, info: "Early 2019")
let watch = Product(name: "Apple Watch", price: 500, info: "Series 4")

let appleStore = Store(
    name: "Apple Store",
    address: Address(street: "300 Post Street", city: "San Francisco", state: "CA"),
    products: [iPhone, macBook, watch]
)

let stringsEncoder = StringsEncoder()
do {
    let stringsFile = try stringsEncoder.encode(appleStore)
    print(stringsFile)
} catch {
    print("Encoding failed: \(error)")
}

Output:

/* Generated by StringsEncoder */
"address.city" = "San Francisco";
"address.state" = "CA";
"address.street" = "300 Post Street";
"name" = "Apple Store";
"products.0.info" = "Our best iPhone yet!";
"products.0.name" = "iPhone X";
"products.0.price" = "1000.0";
"products.1.info" = "Early 2019";
"products.1.name" = "Mac Book Pro";
"products.1.price" = "2000.0";
"products.2.info" = "Series 4";
"products.2.name" = "Apple Watch";
"products.2.price" = "500.0";

Decoding

Given how big this answer already is, I'm gonna to leave the decoding part (i.e., creating the StringsDecoder class, conforming to the Decoder protocol, etc) as an exercise to the reader... please let me know if you guys need any help with that and I will post a complete solution later ;)

Hali

this is brilliant. Any suggestions on how to add custom Date format encoding? – Osteoplastic

Also to note, String(describing:) should be used instead of .description – Osteoplastic

@Osteoplastic Thanks ;) You could add a custom Date formatting in the method that encodes an Encodable object (i.e., check value is Date). You would then convert the Date to string—that’s your custom date formatting!—and then finally call encode(to:) on that. You would need to add this logic in the three containers types to cover all Date usages. Finally, be sure to also check JSONEncoder.DateEncodingStrategy for a nice way to expose date formatting as an extensible API for your encoder. – Hali

So are you distantly related to Donald Knuth? 😃 For a personal project I'm writing a parser for mail headers. As an exercise in learning more I would like to use Decodable. I know I could easily do this in other ways but as I said I'd like to lean to do this with Decodable. I think I understand the usual things that are described in articles talking about custom types using JSONDecode but I don't understand the part that JSONDeocde does? – Edmee

Wouldn't naming the private class Data be an issue for custom Encoders where the output format is the basic Data type ? – Kirkcudbright

I have a question : Inside your SingleValueContainer, when encode receives a Codable object of an unknown type, it doesn't mutate it's own ```strings`` at all. Isn't the data lost then ? Or am I missing something ? – Kirkcudbright