Promise

A Promise library for Swift, based partially on Javascript's A+ spec.

What is a Promise?

A Promise is a way to represent a value that will exist (or will fail with an error) at some point in the future. This is similar to how an Optional represents a value that may or may not be there.

Using a special type to represent values that will exist in the future means that those values can be combined, transformed, and built in systematic ways. If the system knows what success and what failure look like, composing those asynchronous operations becomes much easier. For example, it becomes trivial to write reusable code that can:

• perform a chain of dependent asynchronous operations with one completion block at the end
• perform many independent asynchronous operations simultaneously with one completion block
• race many asynchronous operations and return the value of the first to complete
• retry asynchronous operations
• add a timeout to asynchronous operations

Promises are suited for any asynchronous action that can succeed or fail exactly once, such as HTTP requests. If there is an asynchronous action that can "succeed" more than once, or delivers a series of values over time instead of just one, take a look at Signals or Observables.

Basic Usage

To access the value once it arrives, you call the then method with a block.

let usersPromise = fetchUsers() // Promise<[User]>
usersPromise.then({ users in
    self.users = users
})

All usage of the data in the users Promise is gated through the then method.

In addition to performing side effects (like setting the users instance variable on self), then enables you do two other things. First, you can transform the contents of the Promise, and second, you can kick off another Promise, to do more asynchronous work. To do either of these things, return something from the block you pass to then. Each time you call then, the existing Promise will return a new Promise.

let usersPromise = fetchUsers() // Promise<[User]>
let firstUserPromise = usersPromise.then({ users in // Promise<User>
    return users[0]
})
let followersPromise = firstUserPromise.then({ firstUser in //Promise<[Follower]>
    return fetchFollowers(of: firstUser)
})
followersPromise.then({ followers in
    self.followers = followers
})

Based on whether you return a regular value or a promise, the Promise will determine whether it should transform the internal contents, or fire off the next promise and await its results.

As long as the block you pass to then is one line long, its type signature will be inferred, which will make Promises much easier to read and work with.

Since each call to then returns a new Promise, you can write them in a big chain. The code above, as a chain, would be written:

fetchUsers()
    .then({ users in
        return users[0]
    })
    .then({ firstUser in
        return fetchFollowers(of: firstUser)
    })
    .then({ followers in
        self.followers = followers
    })

To catch any errors that are created along the way, you can add a catch block as well:

fetchUsers()
    .then({ users in
        return users[0]
    })
    .then({ firstUser in
        return fetchFollowers(of: firstUser)
    })
    .then({ followers in
        self.followers = followers
    })
    .catch({ error in
        displayError(error)
    })

If any step in the chain fails, no more then blocks will be executed. Only failure blocks are executed. This is enforced in the type system as well. If the fetchUsers() promise fails (for example, because of a lack of internet), there's no way for the promise to construct a valid value for the users variable, and there's no way that block could be called.

Creating Promises

To create a promise, there is a convenience initializer that takes a block and provides functions to fulfill or reject the promise:

let promise = Promise<Data>(work: { fulfill, reject in
    try fulfill(Data(contentsOf: someURL)
})

It will automatically run on a global background thread.

You can use this initializer to wrap a completion block-based API, like URLSession.

let promise = Promise<(Data, HTTPURLResponse)>(work: { fulfill, reject in
    self.dataTask(with: request, completionHandler: { data, response, error in
        if let error = error {
            reject(error)
        } else if let data = data, let response = response as? HTTPURLResponse {
            fulfill((data, response))
        } else {
            fatalError("Something has gone horribly wrong.")
        }
    }).resume()
})

If the API that you're wrapping is sensitive to which thread it's being run on, like any UIKit code, be sure to pass add a queue: .main parameter to the work: initializer, and it will be executed on the main queue.

For delegate-based APIs, you can can create a promise in the .pending state with the default initializer.

let promise = Promise()

and use the fulfill and reject instance methods to change its state.

Advanced Usage

Because promises formalize how success and failure blocks look, it's possible to build behaviors on top of them.

always

For example, if you want to execute code when the promise fulfills — regardless of whether it succeeds or fails – you can use always.

activityIndicator.startAnimating()
fetchUsers()
    .then({ users in
        self.users = users
    })
    .always({
        self.activityIndicator.stopAnimating()
    })

Even if the network request fails, the activity indicator will stop. Note that the block that you pass to always has no parameters. Because the Promise doesn't know if it will succeed or fail, it will give you neither a value nor an error.

ensure

ensure is a method that takes a predicate, and rejects the promise chain if that predicate fails.

URLSession.shared.dataTask(with: request)
    .ensure({ data, httpResponse in
        return httpResponse.statusCode == 200
    })
    .then({ data, httpResponse in
        // the statusCode is valid
    })
    .catch({ error in 
        // the network request failed, or the status code was invalid
    })

Static methods

Static methods, like zip, race, retry, all, kickoff live in a namespace called Promises. Previously, they were static functions in the Promise class, but this meant that you had to specialize them with a generic before you could use them, like Promise<()>.all. This is ugly and hard to type, so as of v2.0, you now write Promises.all.

all

Promises.all is a static method that waits for all the promises you give it to fulfill, and once they have, it fulfills itself with the array of all fulfilled values. For example, you might want to write code to hit an API endpoint once for each item in an array. map and Promises.all make that super easy:

let userPromises = users.map({ user in
    APIClient.followUser(user)
})
Promises.all(userPromises)
    .then({
        //all the users are now followed!
    })
    .catch  ({ error in
        //one of the API requests failed
    })

kickoff

Because the then blocks of promises are a safe space for functions that throw, its sometimes useful to enter those safe spaces even if you don't have asynchronous work to do. Promises.kickoff is designed for that.

Promises
	.kickoff({
		return try initialValueFromThrowingFunction()
	})
	.then({ value in
		//use the value from the throwing function
	})
	.catch({ error in
		//if there was an error, you can handle it here.
	})

This (coupled with Optional.unwrap()) is particularly useful when you want to start a promise chain from an optional value.

Others behaviors

These are some of the most useful behaviors, but there are others as well, like race (which races multiple promises), retry (which lets you retry a single promise multiple times), and recover (which lets you return a new Promise given an error, allowing you to recover from failure), and others.

You can find these behaviors in the Promises+Extras.swift file.

Invalidatable Queues

Each method on the Promise that accepts a block accepts an execution context with the parameter name on:. Usually, this execution context is a queue.

Promise<Void>(queue: .main, work: { fulfill, reject in
    viewController.present(viewControllerToPresent, animated: flag, completion: {
        fulfill()
    })
}).then(on: DispatchQueue.global(), {
	return try Data(contentsOf: someURL)
})

Because ExecutionContext is a protocol, other things can be passed here. One particularly useful one is InvalidatableQueue. When working with table cells, often the result of a promise needs to be ignored. To do this, each cell can hold on to an InvalidatableQueue. An InvalidatableQueue is an execution context that can be invalidated. If the context is invalidated, then the block that is passed to it will be discarded and not executed.

To use this with table cells, the queue should be invalidated and reset on prepareForReuse().

class SomeTableViewCell: UITableViewCell {
    var invalidatableQueue = InvalidatableQueue()
        
    func showImage(at url: URL) {
        ImageFetcher(url)
            .fetch()
            .then(on: invalidatableQueue, { image in
                self.imageView.image = image
            })
    }
    
    override func prepareForReuse() {
        super.prepareForReuse()
        invalidatableQueue.invalidate()
        invalidatableQueue = InvalidatableQueue()
    }

}

Warning: don't chain blocks off anything that is executing on an invalidatable queue. then blocks that return Void won't stop the chain, but then blocks that return values or promises will stop the chain. Because the block can't be executed, the result of the next value in the chain won't be calculable, and the next promise will remain in the pending state forever, preventing resources from being released.

Ease of Use

I made several design decisions when writing this Promise library, erring towards making the library as easy to use as possible.

Simplified Naming

Other promise libraries use the functional names for then, such as map and flatMap. The benefit to using these monadic functional terms is minor, but cost, in terms of understanding, is high. In this library, you call then, and return anything you need to, and the library figures out how to handle it.

Error Parameterization

Other promise libraries allow you to define what type the error each promise will return. In theory, this a useful feature, allowing you to know what type the error will be in catch blocks.

In practice, this is stifling. In practice, if you're using errors from two different domains, you have to either a) use a lowest common denominator error, like NSError, or b) call a method like mapError to convert the error from one domain to another.

Also note that Swift's built-in error handling system doesn't have typed errors, opting for pattern matching instead.

Throwing

Lastly, you can use try and throw from within all the blocks, and the library will automatically translate it to a promise rejection. This makes working with APIs that throw much more easily. To extend our URLSession example, we can use the throwing JSONSerialization API easily.

URLSession.shared.dataTask(with: request)
    .ensure({ data, httpResponse in httpResponse.statusCode == 200 })
    .then({ data, httpResponse in
        return try JSONSerialization.jsonObject(with: data)
    })
    .then({ json in
        // use the json
    })

Working with optionals can be made simpler with a little extension.

struct NilError: Error { }

extension Optional {
    func unwrap() throws -> Wrapped {
        guard let result = self else {
            throw NilError()
        }
        return result
    }
}

Because you're in an environment where you can freely throw and it will be handled for you (in the form of a rejected Promise), you can now easily unwrap optionals. For example, if you need a specific key out of a json dictionary:

.then({ json in
    return try (json["user"] as? [String: Any]).unwrap()
})

And you will transform your optional into a non-optional.

Threading Model

The threading model for this library is dead simple. init(work:) happens on a background queue by default, and every other block-based method (then, catch, always, etc) executes on the main thread. These can be overridden by passing in a DispatchQueue object for the first parameter.

Promise<Void>(work: { fulfill, reject in
    viewController.present(viewControllerToPresent, animated: flag, completion: {
        fulfill()
    })
}).then(on: DispatchQueue.global(), {
	return try Data(contentsOf: someURL)
}).then(on: DispatchQueue.main, {
	self.data = data
})

Installation

CocoaPods

1. Add the following to your Podfile:

   pod 'Promises'

2. Integrate your dependencies using frameworks: add use_frameworks! to your Podfile.

3. Run pod install.

Playing Around

To get started playing with this library, you can use the included Promise.playground. Simply open the .xcodeproj, build the scheme, and then open the playground (from within the project) and start playing.