Guarantees in Swift for novices

Sync vs async execution

Writing asynchronous code is likely one of the hardest a part of constructing an app.

What precisely is the distinction between a synchronous and an asynchronous execution? Nicely, I already defined this in my Dispatch framework tutorial, however here’s a fast recap. A synchoronousoperate normally blocks the present thread and returns some worth afterward. An asynchronousoperate will immediately return and passes the outcome worth right into a completion handler. You need to use the GCD framework to carry out duties sync on async on a given queue. Let me present you a fast instance:

func aBlockingFunction() -> String {
    sleep(.random(in: 1...3))
    return "Whats up world!"
}

func syncMethod() -> String {
    return aBlockingFunction()
}

func asyncMethod(completion block: @escaping ((String) -> Void)) {
    DispatchQueue.world(qos: .background).async {
        block(aBlockingFunction())
    }
}

print(syncMethod())
print("sync technique returned")
asyncMethod { worth in
    print(worth)
}
print("async technique returned")

As you’ll be able to see the async technique runs fully on a background queue, the operate will not block the present thread. For this reason the async technique can return immediately, so you may at all times see the return output earlier than the final hey output. The async technique’s completion block is saved for later execution, that is the explanation why is it attainable to call-back and return the string worth approach after the unique operate have returned.

What occurs when you do not use a distinct queue? The completion block shall be executed on the present queue, so your operate will block it. It should be considerably async-like, however in actuality you are simply transferring the return worth right into a completion block.

func syncMethod() -> String {
    return "Whats up world!"
}

func fakeAsyncMethod(completion block: ((String) -> Void)) {
    block("Whats up world!")
}

print(syncMethod())
print("sync technique returned")
fakeAsyncMethod { worth in
    print(worth)
}
print("pretend async technique returned")

I do not actually wish to deal with completion blocks on this article, that may very well be a standalone submit, however in case you are nonetheless having bother with the concurrency mannequin or you do not perceive how duties and threading works, it is best to learn do some analysis first.

Callback hell and the pyramid of doom

What is the downside with async code? Or what’s the results of writing asynchronous code? The brief reply is that it’s important to use completion blocks (callbacks) as a way to deal with future outcomes.

The lengthy reply is that managing callbacks sucks. It’s a must to watch out, as a result of in a block you’ll be able to simply create a retain-cycle, so it’s important to cross round your variables as weak or unowned references. Additionally if it’s important to use a number of async strategies, that’ll be a ache within the donkey. Pattern time! 🐴

struct Todo: Codable {
    let id: Int
    let title: String
    let accomplished: Bool
}

let url = URL(string: "https://jsonplaceholder.typicode.com/todos")!

URLSession.shared.dataTask(with: url) { knowledge, response, error in
    if let error = error {
        fatalError("Community error: " + error.localizedDescription)
    }
    guard let response = response as? HTTPURLResponse else {
        fatalError("Not a HTTP response")
    }
    guard response.statusCode <= 200, response.statusCode > 300 else {
        fatalError("Invalid HTTP standing code")
    }
    guard let knowledge = knowledge else {
        fatalError("No HTTP knowledge")
    }

    do {
        let todos = strive JSONDecoder().decode([Todo].self, from: knowledge)
        print(todos)
    }
    catch {
        fatalError("JSON decoder error: " + error.localizedDescription)
    }
}.resume()

The snippet above is an easy async HTTP knowledge request. As you’ll be able to see there are many optionally available values concerned, plus it’s important to do some JSON decoding if you wish to use your individual varieties. This is only one request, however what when you’d have to get some detailed information from the primary aspect? Let’s write a helper! #no 🤫

func request(_ url: URL, completion: @escaping ((Knowledge) -> Void)) {
    URLSession.shared.dataTask(with: url) { knowledge, response, error in
        if let error = error {
            fatalError("Community error: " + error.localizedDescription)
        }
        guard let response = response as? HTTPURLResponse else {
            fatalError("Not a HTTP response")
        }
        guard response.statusCode <= 200, response.statusCode > 300 else {
            fatalError("Invalid HTTP standing code")
        }
        guard let knowledge = knowledge else {
            fatalError("No HTTP knowledge")
        }
        completion(knowledge)
    }.resume()
}


let url = URL(string: "https://jsonplaceholder.typicode.com/todos")!
request(url) { knowledge in
    do {
        let todos = strive JSONDecoder().decode([Todo].self, from: knowledge)
        guard let first = todos.first else {
            return
        }
        let url = URL(string: "https://jsonplaceholder.typicode.com/todos/(first.id)")!
        request(url) { knowledge in
            do {
                let todo = strive JSONDecoder().decode(Todo.self, from: knowledge)
                print(todo)
            }
            catch {
                fatalError("JSON decoder error: " + error.localizedDescription)
            }
        }
    }
    catch {
        fatalError("JSON decoder error: " + error.localizedDescription)
    }
}

See? My downside is that we’re slowly transferring down the rabbit gap. Now what if we’ve got a third request? Hell no! It’s a must to nest all the things one stage deeper once more, plus it’s important to cross across the crucial variables eg. a weak or unowned view controller reference as a result of sooner or later in time it’s important to replace the whole UI based mostly on the end result. There should be a greater method to repair this. 🤔

Outcomes vs futures vs guarantees?

The outcome kind was launched in Swift 5 and it is extraordinarily good for eliminating the optionally available issue from the equation. This implies you do not have to take care of an optionally available knowledge, and an optionally available error kind, however your result’s both of them.

Futures are mainly representing a worth sooner or later. The underlying worth could be for instance a outcome and it ought to have one of many following states:

• pending – no worth but, ready for it…
• fulfilled – success, now the outcome has a worth
• rejected – failed with an error

By definition a futures should not be writeable by the end-user. Which means builders shouldn’t be in a position to create, fulfill or reject one. But when that is the case and we comply with the foundations, how will we make futures?

We promise them. It’s a must to create a promise, which is mainly a wrapper round a future that may be written (fulfilled, rejected) or reworked as you need. You do not write futures, you make guarantees. Nonetheless some frameworks lets you get again the longer term worth of a promise, however you should not be capable of write that future in any respect.

Sufficient principle, are you able to fall in love with guarantees? ❤️

Guarantees 101 – a newbie’s information

Let’s refactor the earlier instance through the use of my promise framework!

extension URLSession {

    enum HTTPError: LocalizedError {
        case invalidResponse
        case invalidStatusCode
        case noData
    }

    func dataTask(url: URL) -> Promise<Knowledge> {
        return Promise<Knowledge> { [unowned self] fulfill, reject in
            self.dataTask(with: url) { knowledge, response, error in
                if let error = error {
                    reject(error)
                    return
                }
                guard let response = response as? HTTPURLResponse else {
                    reject(HTTPError.invalidResponse)
                    return
                }
                guard response.statusCode <= 200, response.statusCode > 300 else {
                    reject(HTTPError.invalidStatusCode)
                    return
                }
                guard let knowledge = knowledge else {
                    reject(HTTPError.noData)
                    return
                }
                fulfill(knowledge)
            }.resume()
        }
    }
}

enum TodoError: LocalizedError {
    case lacking
}

let url = URL(string: "https://jsonplaceholder.typicode.com/todos")!
URLSession.shared.dataTask(url: url)
.thenMap { knowledge in
    return strive JSONDecoder().decode([Todo].self, from: knowledge)
}
.thenMap { todos -> Todo in
    guard let first = todos.first else {
        throw TodoError.lacking
    }
    return first
}
.then { first in
    let url = URL(string: "https://jsonplaceholder.typicode.com/todos/(first.id)")!
    return URLSession.shared.dataTask(url: url)
}
.thenMap { knowledge in
    strive JSONDecoder().decode(Todo.self, from: knowledge)
}
.onSuccess { todo in
    print(todo)
}
.onFailure(queue: .important) { error in
    print(error.localizedDescription)
}

What simply occurred right here? Nicely, I made form of a promisified model of the information job technique carried out on the URLSession object as an extension. In fact you’ll be able to return the HTTP outcome or simply the standing code plus the information when you want additional information from the community layer. You need to use a brand new response knowledge mannequin or perhaps a tuple. 🤷‍♂️

Anyway, the extra attention-grabbing half is the underside half of the supply. As you’ll be able to see I am calling the model new dataTask technique which returns a Promise<Knowledge> object. As I discussed this earlier than a promise could be reworked. Or ought to I say: chained?

Chaining guarantees is the largest benefit over callbacks. The supply code just isn’t wanting like a pyramid anymore with loopy indentations and do-try-catch blocks, however extra like a series of actions. In each single step you’ll be able to rework your earlier outcome worth into one thing else. In case you are acquainted with some practical paradigms, it will be very easy to know the next:

• thenMap is an easy map on a Promise
• then is mainly flatMap on a Promise
• onSuccess solely will get known as if all the things was advantageous within the chain
• onFailure solely will get known as if some error occurred within the chain
• at all times runs at all times whatever the consequence

If you wish to get the principle queue, you’ll be able to merely cross it by way of a queue parameter, like I did it with the onFailure technique, nevertheless it works for each single aspect within the chain. These features above are simply the tip of the iceberg. You may as well faucet into a series, validate the outcome, put a timeout on it or get better from a failed promise.

There may be additionally a Guarantees namespace for different helpful strategies, like zip, which is able to zipping collectively 2, 3 or 4 totally different form of guarantees. Identical to the Guarantees.all technique the zip operate waits till each promise is being accomplished, then it offers you the results of all the guarantees in a single block.

Guarantees.all(guarantees)
.thenMap { arrayOfResults in
    
}

Guarantees.zip(promise1, promise2)
.thenMap { result1, result2 in
    
}

It is also value to say that there’s a first, delay, timeout, race, wait and a retry technique beneath the Guarantees namespace. Be at liberty to mess around with these as properly, generally they’re extremly helpful and highly effective too. 💪

There are solely two issues with guarantees

The primary challenge is cancellation. You may’t merely cancel a operating promise. It is doable, nevertheless it requires some superior or some say “hacky” methods.

The second is async / await. If you wish to know extra about it, it is best to learn the concurrency manifesto by Chis Lattner, however since this can be a newbie’s information, let’s simply say that these two key phrases can add some syntactic sugar to your code. You will not want the additional (then, thenMap, onSuccess, onFailure) traces anymore, this fashion you’ll be able to focus in your code. I actually hope that we’ll get one thing like this in Swift 6, so I can throw away my Promise library for good. Oh, by the best way, libraries…

Promise libraries value to test

My promise implementation is much from good, nevertheless it’s a fairly easy one (~450 traces of code) and it serves me very well. This weblog submit by @khanlou helped me lots to know guarantees higher, it is best to learn it too! 👍

There are many promise libraries on github, but when I had to select from them (as a substitute my very own implementation), I would positively go together with one of many following ones:

• PromiseKit – The preferred one
• Guarantees by Google – characteristic wealthy, fairly in style as properly
• Promise by Khanlou – small, however based mostly on on the JavaScript Guarantees/A+ spec
• SwiftNIO – not an precise promise library, nevertheless it has a superbly written occasion loop based mostly promise implementation beneath the hood

Professional tip: do not attempt to make your individual Promise framework, as a result of multi-threading is extraordinarily onerous, and you do not wish to fiddle with threads and locks.

Guarantees are actually addictive. When you begin utilizing them, you’ll be able to’t merely return and write async code with callbacks anymore. Make a promise as we speak! 😅