JSON Parsing in Swift Anat Gilboa 

Inspiration 

The Proposal: Argo Argo: - type-safe JSON decoding library that extracts models from JSON 

Currying struct User { let id: Int let name: String let age: Int } static func create(id: Int)(name: String)(age: Int) -> User { return User(id: id, name: name, age: age) } 

The Proposal: Result Result typealias JSONObject = [String:AnyObject] enum JSONError : ErrorType { case NoSuchKey(String) case TypeMismatch } func stringForKey(json: JSONObject, key: String) -> Result { guard let value = json[key] else { return .Failure(.NoSuchKey(key)) } if let value = value as? String { return .Success(value) } else { return .Failure(.TypeMismatch) } } 

The Proposal: Result Result typealias JSONObject = [String:AnyObject] enum JSONError : ErrorType { case NoSuchKey(String) case TypeMismatch } func stringForKey(json: JSONObject, key: String) -> Result { guard let value = json[key] else { return .Failure(.NoSuchKey(key)) } if let value = value as? String { return .Success(value) } else { return .Failure(.TypeMismatch) } } 

What's so special about parsing JSON in Swift?... 

No Library, No Problem? struct User { let id: Int let name: String let age: Int } 

if-let it up let user: User? if let user = json["user"] as? [String: String] { if let id = user["id"] { if let name = user["name"] { if let age = user["age"] { user = User(id, name: name, age: age) } } } } 

guard? let user: User? guard let user = json["user"] as? [String: String] { if let id = user["id"], name = user["name"], age = user["age"] { user = User(id, name: name, age: age) } } else { return nil } 

How ‘bout do-catch? 4 Good for many trys in sequence 4 Don't forget to mark the function that is throwing 

The Goal 1. Look into different mapping libraries to see their interfaces 2. Take into account the simplicity of the respective APIs 

The Subjects Alembic Argo Arrow CaesarParser Decodable Elevate Freddy Genome Gloss JSON mapper ObjectMapper SwiftyJSON Tailor Tyro Unbox 

Example: SwiftyJSON import SwiftyJSON struct User { let id: Int let name: String let age: Int } let json = JSON(["id":2378, "name":"Jack", "age": 23]) if let name = json["name"].string { // Do a thing } else { print(json["name"].error) } 

Example: SwiftyJSON public let ErrorUnsupportedType: Int = 999 public let ErrorIndexOutOfBounds: Int = 900 public let ErrorWrongType: Int = 901 public let ErrorNotExist: Int = 500 public let ErrorInvalidJSON: Int = 490 

Example: Mapper import Mapper struct User { ... } extension User : Mappable { init(map: Mapper) throws { try id = map.from("id") try name = map.from("name") age = map.optionalFrom("age") } } 

Example: Mapper public enum MapperError: ErrorType { case ConvertibleError(value: AnyObject?, type: Any.Type) case CustomError(field: String?, message: String) case InvalidRawValueError(field: String, value: Any, type: Any.Type) case MissingFieldError(field: String) case TypeMismatchError(field: String, value: AnyObject, type: Any.Type) } 

Example: Freddy import Freddy struct User { ... } extension User: JSONDecodable { public init(json value: JSON) throws { id = try value.int("id") name = try value.string("name") age = try value.int("age") } } 

Example: Freddy public enum Error: ErrorType { case IndexOutOfBounds(index: Swift.Int) case KeyNotFound(key: Swift.String) case UnexpectedSubscript(type: JSONPathType.Type) case ValueNotConvertible(value: JSON, to: Any.Type) } 

Example: Decodable import Decodable struct User { ... } extension User: Decodable { static func decode(j: Any) throws -> User { return try User( id: j => "id", name: j => "name", age: j => "age" ) } } 

Example: Decodable public enum DecodingError: ErrorProtocol, Equatable { case typeMismatch(expected: Any.Type, actual: Any.Type, Metadata) case missingKey(String, Metadata) case rawRepresentableInitializationError(rawValue: Any, Metadata) case other(ErrorProtocol, Metadata) } 

The Findings 

Protocol that throws 4 Decodable 4 Distillable 4 JSONDecodable 4 JSONConvertible 

Forms of Decoding 4 custom operator 4 subscript 

Custom Operators E.g. - <- - <-! - <-? - <--! - <--? - <~~ - => 

Subscript json["people"][0]["name"] or with a path json[path: "people", 0, "name"] 

Error Handling 4 Result 4 Implementation of ErrorType 

Conclusion 

Conclusion 4 Type-safe 4 Generic-friendly 4 Error-informative 

Example: Argo import Argo import Curry import Result struct User { let id: Int let name: String let age: Int? } extension User: Decodable { static func decode(j: JSON) -> Decoded { return curry(User.init) <^> j <| "id" <*> j <| "name" <*> j <|? "age" } } 

Further Reading http://bit.ly/tryJSON 1. Efficient JSON in Swift with Functional Concepts and Generics 2. Real World JSON Parsing with Swift 3. Parsing Embedded JSON and Arrays in Swift (H/T Tony DiPasquale) 

Thanks! 4 Community members: Matt Diephouse, Soroush Khanlou, Matt Bischoff 4 Amex friends 4 Your awesome JSON decoding libraries! 

@anat_gilboa