iOS Conf Singapore October 2015

Swift Sync and Async Error Handling
iOS Conf Singapore - October 2015
— @Javi
1 — "Swift Sync and Async Error Handling" - Javier Soto. iOS Conf Singapore - October 2015

View Slide

Agenda
→ Traditional Asynchronous Code
→ Problems with Traditional Error Handling
→ Result vs throws
→ Modeling asynchronous computation: Future
→ Making Result and throws play together

View Slide

Traditional asynchronous code

View Slide

struct User { let avatarURL: NSURL }
func requestUserInfo(userID: String, completion: (User?, NSError?) -> ())
func downloadImage(URL: NSURL, completion: (UIImage?, NSError?) -> ())
func loadAvatar(userID: String, completion: (UIImage?, NSError?) -> ()) {
requestUserInfo(userID) { user, error in
if let user = user {
downloadImage(user.avatarURL) { avatar, error in
if let avatar = avatar { completion(avatar, nil) }
else { completion(nil, error!) }
}
} else { completion(nil, error!) }
}
}

View Slide


View Slide

(.Some, .None)
(.None, .Some)
(.Some, .Some)
(.None, .None)

View Slide

(.Some, .Some)
(.None, .None)

View Slide

Error Handling
downloadImage(url) { imageOrNil, errorOrNil in
if error = errorOrNil {
// What if image is also not nil??
}
else if image = imageOrNil {
}
}

View Slide

Error Handling
var error: NSError?
let string = NSString(contentsOfFile:path encoding:NSUTF8Encoding error:&error)
if string == nil {
// Oops:
if error.code == NSFileReadNoSuchFileError {
// ...
}
else if ...
}

View Slide


View Slide

NSError Alternative
protocol ErrorType { }
enum UserInfoErrorDomain: ErrorType {
case UserDoesNotExist
case UserRequestFailure(reason: String)
case NetworkRequestFailure(underlyingError: NSError)
}
extension NSError: ErrorType { }

View Slide

NSError Alternative
enum NoError: ErrorType { }
let error = NoError(?)

View Slide

Result
enum Result {
case Success(T)
case Failure(E)
}
Result by Rob Rix:
https://github.com/antitypical/Result

View Slide

Swift's throws

View Slide

Swift's throws
func readFile(path: String) throws -> NSData

View Slide

Swift's throws
// throws... what exactly?

View Slide

Swift's throws
Functionally equivalent to
func readFile(path: String) -> Result

View Slide

Swift's throws
do {
let contents = try readFile("path/to/my/file")
}
catch(SomeTypeOfError) {
// How can I know which errors I should match..?
}
catch {
// catch-all block.
}

View Slide

Swift's throws
let contents: NSData? = try? readFile("path/to/my/file")

View Slide

Swift's throws
func readFile(path: String) -> Result
let file = readFile("path/to/my/file")
switch file {
case let .Succcess(value):
// ...
case let .Failure(error):
// This `error` variable has type information!
}

View Slide

Swift's throws
Hopefully in a future version of Swift ...
func readFile(path: String) throws FileIOError -> NSData

View Slide

Error handling in
Asynchronous APIs

View Slide

Future

View Slide

Futures
→ Encapsulate a deferred computation.
→ Treat values that incur a delay to be retrieved as
if they were regular values.
→ Allow us to treat errors as ﬁrst class citizens.
→ Easily composable.

View Slide

Future
struct Future {
typealias ResultType = Result
typealias Completion = ResultType -> ()
typealias AsyncOperation = Completion -> ()
private let operation: AsyncOperation
}

View Slide

Future
struct Future {
init(operation: AsyncOperation) {
self.operation = operation
}
func start(completion: Completion) {
self.operation() { result in
completion(result)
}
}
}

View Slide

Future.map(): transforming the computed value

View Slide

struct User { let avatarURL: NSURL }
func requestUserInfo(userID: String) -> Future
func requestUserAvatarURL(userID: String) -> Future {
return requestUserInfo(userID)
.map { $0.avatarURL }
}

View Slide

struct Future {
func map(f: T -> U) -> Future
}

View Slide

map() in other types
struct Array {
func map(f: T -> U) -> [U]
}
enum Optional {
func map(f: T -> U) -> U?
}
struct Future {
func map(f: T -> U) -> Future
}

View Slide

func map(f: T -> U) -> Future {
// Return a new Future w/ a new operation...
return Future(operation: { completion in
})
}

View Slide

// Retrieve the value from self...
self.start { result in
}
})
}

View Slide

// Consider .Success and .Failure...
switch result {
}
}
})
}

View Slide

case let .Success(value):
// Call completion with the transformed value
completion(Result.Success(f(value)))

View Slide

// We didn't get a value: no transformation
completion(Result.Failure(error))

View Slide

switch result {
completion(Result.Success(f(value)))
}
}
})
}

View Slide

Future.andThen(): concatenating async work

View Slide

func requestUserAvatarURL(userID: String) -> Future
func downloadImage(URL: NSURL) -> Future
func downloadUserAvatar(userID: String) -> Future {
return requestUserAvatarURL(userID)
.andThen(downloadImage)
}

View Slide

struct Future {
func andThen(f: T -> Future) -> Future
}

View Slide

func andThen(f: T -> Future) -> Future {
}
}

View Slide

self.start { firstFutureResult in
}
})
}

View Slide

switch firstFutureResult {
case let .Success(value): // ...
case let .Failure(error): // ...
}
}
})
}

View Slide

let nextFuture = f(value)

View Slide

let nextFuture = f(value)
nextFuture.start { finalResult in
completion(finalResult)
}

View Slide


View Slide

switch firstFutureResult {
case let .Success(value): f(value).start(completion)
case let .Failure(error): completion(Result.Failure(error))
}
}
})
}

View Slide

We can go from this...
func loadAvatar(userID: String, completion: (UIImage?, NSError?) -> ()) {
requestUserInfo(userID) { user, error in
if let user = user {
downloadImage(user.avatarURL) { avatar, error in
if let avatar = avatar {
completion(avatar, nil)
} else {
completion(nil, error)
}
}
}
else { completion(nil, error) }
}
}

View Slide

... To this
func requestUserInfo(userID: String) -> Future
func downloadImage(URL: NSURL) -> Future
func loadAvatar(userID: String) -> Future {
return requestUserInfo(userID)
.map { $0.avatarURL }
.andThen(downloadImage)
}

View Slide

Mixing it all up
Result and throws

View Slide

Result and throws
throws => Result
extension Result {
public init(@autoclosure _ f: () throws -> T) {
do {
self = .Success(try f())
} catch {
self = .Failure(error as! Error)
}
}
}

View Slide

Result and throws
Result => throws
extension Result {
public func dematerialize() throws -> T {
switch self {
case .Success(value):
return value
throw error
}
}
}

View Slide

Result and throws
Future => throws
let avatarFuture = loadAvatar("4815162342")
avatarFuture.start { result in
do {
let avatar = try result.materialize()
}
catch {
// handle `error`
}
}

View Slide

Limitations of Futures
→ Only represent computation to retrieve one value.
→ Can't encapsulate streams of values.
→ Only consumer (pull) driven, not producer (push)
driven.

View Slide

ReactiveCocoa
Signals > Futures

View Slide

Follow-up Resources
→ ReactiveCocoa: https://github.com/
ReactiveCocoa/ReactiveCocoa
→ Railway Oriented Programming: http://
fsharpforfunandproﬁt.com/posts/recipe-part2/

View Slide

Thanks!
Questions?

View Slide

iOS Conf Singapore October 2015

iOS Conf Singapore October 2015

Javier Soto

More Decks by Javier Soto

Other Decks in Programming

Featured

Transcript