Microfutures
Lightweight implementation of Futures that shares a similar subscription interface with RxSwift.
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Microfutures
Introduction
Microfutures is a very small library (60 LOCs) that implements a simple Futures/Promises flow. It also has a similar public interface to RxSwift.
What is a future?
A future is a representation of a value that hasn’t been already generated. The best use case of Futures is to simplify an asynchronous flow. Instead of writing nested callbacks, you can chain futures, turning that awful callback hell into a beautiful functional pipeline.
In a glance
Microfutures lets you turn from this:
getUser(withID: 3) { user, error in
if let error = error {
print("An error ocurred")
} else if let user = user {
self.getPosts(forUserID: user.id) { posts, error in
if let error = error {
print("An error ocurred")
} else if let posts = posts {
if let firstPost = posts.first {
self.getComments(forPostID: firstPost.id) { error, comments in
if let error = error {
print("An error ocurred")
} else if let comments = comments {
print("Comments count: \(comments.count)")
}
}
}
}
}
}
}
(And I am not exaggerating here, this is a pretty common scenario, and everyone is guilty for writing something like this at least once.)
To this:
getUser(withID: 3)
.flatMap(getPosts)
.map { posts in return posts.first?.id }
.flatMap(getComments)
.map { comments in return comments.count }
.subscribe (
onNext: { commentsCount in
print("Comments count: \(commentsCount)")
},
onError: { error in
print("An error ocurred.")
}
)
Much cleaner.
Creating a future
Creating a future couldn’t be simpler.
Lets compare how you write a callback based async function vs a future based async function.
// Callback based func:
func getUser(withID id: Int, completion: (Error?, User?) -> Void) {
APIClient
// This api client also works using callbacks
.get("https://somecoolapi.com/users/\(id)") { error, json in
if let error = error {
completion(error, nil)
return
} else {
guard let json = json else {
completion(NetworkingError.emptyResponse, nil)
return
}
guard let user = User(json: json) else {
completion(NetworkingError.invalidReponse, nil)
return
}
completion(nil, user)
}
}
}
Here is the future based async function:
func getUser(withID id: Int) -> Future<User> {
return Future { completion in
APIClient
// Imagine that this api client still uses a callback based approach.
.get("https://somecoolapi.com/users/\(id)") { error, json in
if let error = error {
completion(.failure(error))
return
} else {
guard let json = json else {
completion(.failure(NetworkingError.emptyResponse))
return
}
guard let user = User(json: json) else {
completion(.failure(NetworkingError.invalidReponse))
return
}
completion(.success(user))
}
}
}
}
Yes, it’s exactly the same. But that’s because our API Client still uses a callback based approach.
One of the strong points of using a Futures based approach is that you can use functional tools. Compare the previous snippet with this:
func getUser(withID id: Int) -> Future<User> {
return APIClient
.get("https://somecoolapi.com/users/\(id)")
.map { json in
guard let user = User(json: json) else {
throw NetworkingError.invalidReponse
}
completion(.success(user))
}
}
As I will explain later, the map function transforms the Future value to another value. In this case, map transforms the json value to a User object.
Transforming Future’s value using map
map transforms the output of a Future in another value.
For example:
getAlbum(withID: 3)
.map { album in
return album.title
}
.map { albumTitle in
return "THe album title is \(albumTitle)"
}
// ...
An important thing about map is that it won’t be executed if the future contains an error.
map can also throw an error if it’s necessary:
getUser(withID: 3)
.map { user in
guard let mobile = user.mobilePhone else {
throw UserError.noMobileNumber
}
return mobile
}
Chaining Futures using flatMap
Sometimes you want to perform an async function after another async function. This often results in a callback hell.
Futures has a solution for that, and it’s using flatMap
flatMap receives a function that transforms the output value of the future and returns another future.
For example:
func getPosts(forUserID userID: Int) -> Future<[Post]> {
return APIClient
.get("https://somecoolapi.com/posts?userID=\(userID)")
.map { json in
guard let jsonArray = json as? [JSON] else {
throw NetworkingError.invalidReponse
}
return jsonArray.map(Post.init)
}
}
getUser(withID: 3)
.flatMap { user in
return getPosts(forUserID: user.id)
}
// Or doing this
getUser(withID: 3)
.map { user in return user.id }
.flatMap (getPosts)
Resolving Futures
The last step is subscribing to the Future. It’s achieved by using the subscribe method.
For example:
getUser(withID: 3)
.map { user in return user.id }
.flatMap (getPosts)
.subscribe(
onNext: { posts in
// Do something with posts...
},
onError: { error in
// Handle this error.
}
)
Subscribe receives two functions, one for the happy path, and another for the wrong case.
onNext is a function that receives the Future value and performs something with that value.
onError is a function that receives an error and handles it.
Similarity with RxSwift
Future methods names has been chosen following RxSwift names. map, flatMap, and subscribe are names that RxSwift uses, and this library can be used as an introduction for somebody to RxSwift terms.
Example
To run the example project, clone the repo, and run pod install from the Example directory first.
Installation
Microfutures is available through CocoaPods. To install it, simply add the following line to your Podfile:
pod "Microfutures"
Or you can just copy and paste Microfutures.swift into your project.
Author
Fernando Ortiz, ortizfernandomartin@gmail.com
License
Microfutures is available under the MIT license. See the LICENSE file for more info.