Tomorrowland

Tomorrowland is an implementation of Promises for Swift and Objective-C. A Promise is a wrapper around an asynchronous task that provides a standard way of subscribing to task resolution as well as chaining promises together.

Features:

• It uses atomics internally instead of creating a separate DispatchQueue for each promise. This means it’s faster and uses fewer resources.
• It provides full support for cancellable promises. PromiseKit supports detection of “cancelled” errors but has no way to request cancellation of a promise. Hydra supports cancelling a promise, but it can’t actually stop any work being done by the promise unless the promise body itself polls for the cancellation status (so e.g. a promise wrapping a network task can’t reasonably cancel the network task). Tomorrowland improves on this by allowing the promise body to observe the cancelled state, and allows linking cancellation of a child promise to its parent.
• Its Obj-C support makes use of generics for improved type safety and better documentation.
• Like Hydra but unlike PromiseKit, it provides a way to suppress a registered callback (e.g. because you don’t care about the result anymore and don’t want stale data affecting your UI). This is distinct from promise cancellation.
• Tomorrowland promises are fully generic over the error type, whereas both PromiseKit and Hydra only support using Error as the error type. This may result in more typing to construct a promise but it allows for much more powerful error handling. Tomorrowland also has some affordances for working with promises that use Error as the error type.
• Tomorrowland is fully thread-safe. I have no reason to believe PromiseKit isn’t, but (at the time of this writing) there are parts of Hydra that are incorrectly implemented in a non-thread-safe manner.