Rebuilding the Parse SDK in Swift 4 with protocols and codables

1. Values, Protocols and Codables
Florent Vilmart

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What we’ll cover

3. Disclaimer
I have no idea what I’m doin’

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Parse SDK
not your average SDK

5. An API, to build API’s

6. Parse SDK iOS/OSX Parse Swift
Runtime Dynamism Compile-time Dynamism
Object Oriented PoP
NSObject struct
async (Bolts) sync
iOS/macOS/tvOS/watchOS Just Swift
Objective-C vs Swift
The past, the present…

7. o

9. An REST API is a contract
… but …

10. Object
Data
Data
Object
Serialize ⚠️💣
Send
Responds
💣 ⚠️ Deserialize
💥
💥
Any API Client…
Till today

11. T:
Encodable
Data
Data
U:
Decodable
JSONEncoder
Send
Responds
JSONDecoderFor Free!
For Free!
URLSession
REST with Swift 4

12. T: Encodable
URLSession
U: Decodable
Great spot to fake / mock your server!
Or datastore
Or anything really…
REST abstract

13. Encodable / Decodable
Strings, Numbers, enum, Arrays, Dictionaries (of Codables) …
Objects / Structs with only Codable’s
Custom *(through protocol conformance)
Implement CodingKeys for encoding subset
Composability
Codable’s

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Encoders / Decoders

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JSONEncoder / ParseEncoder
JSON and friends

17. Protocols
protocol Saving: Encodable {
associatedtype SavingType
func save(options: Options) throws -> SavingType
func save() throws -> SavingType
}
protocol Fetching: Decodable {
associatedtype FetchingType
func fetch(options: Options) throws -> FetchingType
func fetch() throws -> FetchingType
}

18. ObjectType Protocol
public protocol ObjectType: Fetching, Saving {
static var className: String { get }
var objectId: String? { get set }
var createdAt: Date? { get set }
var updatedAt: Date? { get set }
var ACL: ACL? { get set }
}
public extension ObjectType {
public func save(options: Options = []) throws -> Self {
return try Command<Self, SaveResponse>(method: .POST,
path: object.endpoint,
params: nil,
body: object)
.execute(options: options)
.map { $0.apply(self) }
}
}

19. Extensible
extension Saving {
typealias Callback = (Self.SavingType?, Error?) -> Void
func save(options: API.Options = [],
callback: @escaping Callback) {
runAsync(options: options,
function: self.save,
callback: callback)
}
}

20. runAsync
let queue = DispatchQueue(label: "org.parse-community.ParseSwift.async")
private func runAsync<T>(options: API.Options,
function: @escaping (API.Options) throws -> T?,
callback: @escaping (T?, Error?) -> Void) {
queue.async {
do {
callback(try function(options), nil)
} catch let e {
callback(nil, e)
}
}
}

21. As a SDK user
struct GameScore: ParseSwift.ObjectType {
//: Those are required for ObjectType
var objectId: String?
var createdAt: Date?
var updatedAt: Date?
var ACL: ACL?
//: Your own properties
var score: Int
//: a custom initializer
init(score: Int) {
self.score = score
}
}
let score = GameScore(score: 10)
guard let savedScore = try? score.save() else { fatalError() }

22. public struct Command<T, U>: Encodable where T: Encodable, U: Decodable {
let method: Method
let path: Endpoint
let params: [String: String?]?
let body: T?
public func data() throws -> Data? {
return try JSONEncoder().encode(body)
}
public func execute(options: Options) throws -> Response<U> {
return try options.executor.execute(command: self, options: options)
}
internal func decode(responseData: Data) throws -> Response<U> {
do {
return Response(object: try Decoder.json.decode(U.self, from: responseData))
} catch _ {
throw try Decoder.json.decode(ParseError.self, from: responseData)
}
}
}
Command

23. Commands
extension Command where T: ObjectType {
// MARK: custom encoding
func data() throws -> Data? {
return try API.Encoder.parse.encode(body)
}
}
struct Response<T> where T: Decodable {
let object: T
func map<U>(_ mapper: (T) throws -> U) rethrows -> U {
return try mapper(object)
}
}

24. Executor
extension API.Command {
internal func getURLRequest(options: API.Options) throws -> URLRequest {
/* build a URLRequest from the command */
}
}
extension URLSession: APIExecutor {
public func execute<T, U>(command: API.Command<T, U>,
options: API.Options) throws -> API.Response<U> {
let urlRequest = try command.getURLRequest(options: options)
let responseData = try self.syncDataTask(with: urlRequest)
return try command.decode(responseData: responseData)
}
}

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About Async

26. A Quick Word on sync code
no async is the best async
extension URLSession {
internal func syncDataTask(with request: URLRequest) throws -> Data {
let semaphore = DispatchSemaphore(value: 0)
var data: Data?
var error: Error?
var response: URLResponse?
dataTask(with: request) { (responseData, urlResponse, responseError) in
data = responseData; error = responseError; response = urlResponse;
semaphore.signal()
}.resume()
semaphore.wait()
guard let responseData = data else {
guard let error = error else { // no err no res?
throw NSError(domain: "unknown", code: -1, userInfo: ["response": response!])
}
throw error
}
return responseData
}
}
🤦♀️

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What’s next
Because all is not green!

28. Values, Protocols and Codables
Florent Vilmart