Obligation is Promise library in Swift language

Promise represents the result of an asynchronous operation. A promise can be in one of three different states:

• pending - Operation is not completed, result is unknown
• fulfilled - Operation was successful, result can be obtained from the promise
• rejected - Operation was unsuccessful, error can be obtained from the promise

Once a promise is fulfilled or rejected, it is immutable (i.e. it can never change its state again). This is called a settled promise.

Each promise has an associated Context on which promised operation is executed.

The key pattern for working with promises is promise chaining. You chain promises by calling then, catch, recover methods and providing callbacks to be executed when the promise is fulfilled or rejected. Each of these methods returns a new promise that is fulfilled or rejected after provided callbacks are finished.

    Promise { fulfill, reject in
        fulfill(someLongOperationReturningInt())
    }.then { value in
        // value is Int
        return "\(value)"
    }.then { value in
        throw SomeError()
    }.recover { error in
        return "recovered"
    }.then { value in
        // value is String
        print(value)
    }.done()