A lightweight implementation of Promises in Swift
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README.md

When

CI Status Version Carthage Compatible License Platform

Description

When is a lightweight implementation of Promises in Swift. It doesn't include any helper functions for iOS and OSX and it's intentional, to remove redundant complexity and give you more freedom and flexibility in your choices. It is type safe, thanks Swift generics, so you could create promises with whatever type you want.

When can easily be integrated into your projects and libraries to move your asynchronous code up to the next level.

Table of Contents

When Icon

Why?

To make asynchronous code more readable and standardized:

fetchJSON().then({ data: NSData -> [[String: AnyObject]] in
  // Convert to JSON
  return json
}).then({ json: [[String: AnyObject]] -> [Entity] in
  // Map JSON
  // Save to database
  return items
}).done({ items: [Entity] in
  self.items = items
  self.tableView.reloadData()
}).error({ error in
  print(error)
})

Usage

Promise

A promise represents the future value of a task. Promises start in a pending state and then could be resolved with a value or rejected with an error.

// Creates a new promise that could be resolved with a String value
let promise = Promise<String>()
// Resolves the promise
promise.resolve("String")
// Or rejects the promise
promise.reject(Error.notFound)
// Creates a new promise that is resolved with a String value
let promise = Promise({
  return "String"
})
// Creates a new promise that is rejected with an Error
let promise = Promise({
  //...
  throw Error.notFound
})

Callbacks of the current promise and all the chained promises (created with then) are executed on the main queue by default, but you can always specify the needed queue in init:

let promise = Promise<String>(queue: dispatch_get_main_queue())

Done

Adds a handler to be called when the promise object is resolved with a value:

// Create a new promise in a pending state
let promise = Promise<String>()
// Add done callback
promise.done({ value in
  print(value)
})
// Resolve the promise
promise.resolve("String")

Fail

Adds a handler to be called when the promise object is rejected with an Error:

// Create a new promise in a pending state
let promise = Promise<String>()
// Add fail callback
promise.fail({ error in
  print(error)
})
// Reject the promise
promise.reject(Error.notFound)

It's also possible to cancel a promise, which means it will be rejected with PromiseError.cancelled error. FailurePolicy can be used if you want to ignore this error in your fail handler:

// Create a new promise in a pending state
let promise = Promise<String>()
// This callback will not be called when a promise is cancelled
promise.fail({ error in
  print(error)
})
// This callback will be called when a promise is cancelled
promise.fail(policy: .allErrors, { error in
  print(error)
})
// Cancel the promise
promise.cancel()

Always

Adds a handler to be called when the promise object is either resolved or rejected. This callback will be called after done or fail handlers.

// Create a new promise in a pending state
let promise = Promise<String>()
// Add always callback
promise.always({ result in
  switch result {
  case let .success(value):
    print(value)
  case let .failure(error):
    print(error)
  }
})
// Resolve or reject the promise
promise.resolve("String") // promise.reject(Error.notFound)

Then

Returns a new promise that can use the result value of the current promise. It means that you could easily create chains of promises to simplify complex asynchronous operations into clear and simple to understand logic.

A new promise is resolved with the value returned from the provided closure:

let promise = Promise<NSData>()

promise
  .then({ data -> Int in
    return data.length
  }).then({ length -> Bool in
    return length > 5
  }).done({ value in
    print(value)
  })

promise.resolve("String".dataUsingEncoding(NSUTF8StringEncoding)!)

A new promise is resolved when the promise returned from the provided closure resolves:

struct Networking {
  static func GET(url: NSURL) -> Promise<NSData> {
    let promise = Promise<NSData>()
    //...
    return promise
  }
}

Networking.GET(url1)
  .then({ data -> Promise<NSData> in
    //...
    return Networking.GET(url2)
  }).then({ data -> Int in
    return data.length
  }).done({ value in
    print(value)
  })

then closure is executed on the main queue by default, but you can pass a needed queue as a parameter:

promise.then(on: dispatch_get_global_queue(QOS_CLASS_UTILITY, 0))({ data -> Int in
  //...
})

If you want to use background queue there are the helper methods for this case:

promise1.thenInBackground({ data -> Int in
  //...
})

promise2.thenInBackground({ data -> Promise<NSData> in
  //...
})

Recover

Returns a new promise that can be used to continue the chain when an error was thrown.

let promise = Promise<String>()
// Recover the chain
promise
  .recover({ error -> Promise<String> in
    return Promise({
      return "Recovered"
    })
  })
  .done({ string in
    print(string) // Recovered
  })
// Reject the promise
promise.reject(Error.notFound)

When

Provides a way to execute callback functions based on one or more promises. The when method returns a new "master" promise that tracks the aggregate state of all the passed promises. The method will resolve its "master" promise as soon as all the promises resolve, or reject the "master" promise as soon as one of the promises is rejected. If the "master" promise is resolved, the done callback is executed with resolved values for each of the promises:

let promise1 = Promise<Int>()
let promise2 = Promise<String>()
let promise3 = Promise<Int>()

when(promise1, promise2, promise3)
  .done({ value1, value2, value3 in
    print(value1)
    print(value2)
    print(value3)
  })

promise1.resolve(1)
promise2.resolve("String")
promise3.resolve(3)

Installation

When is available through CocoaPods. To install it, simply add the following line to your Podfile:

pod 'When'

For RxSwift extensions you can use CocoaPods subspecs:

pod 'When/RxSwift'

When is also available through Carthage. To install just write into your Cartfile:

github "vadymmarkov/When"

Author

Vadym Markov, markov.vadym@gmail.com

Credits

Credits for inspiration go to PromiseKit and Then.

Contributing

Check the CONTRIBUTING file for more info.

License

When is available under the MIT license. See the LICENSE file for more info.