promise with ReactiveSwift #3

promise3.swift

...

class Promise<Value, Error: Swift.Error> {
  ...
  
    @discardableResult func started(_ then: @escaping () -> Void) -> Promise<Value, Error> {
        sp.on(started: then)
        return self
    }
    
    @discardableResult func value(_ then: @escaping (Value) -> Void) -> Promise<Value, Error> {
        sp.on(value: then)
        return self
    }

    @discardableResult func completed(_ then: @escaping () -> Void) -> Promise<Value, Error> {
        sp.on(completed: then)
        return self
    }
    
    @discardableResult func failed(_ then: @escaping (Error) -> Void) -> Promise<Value, Error> {
        sp.on(failed: then)
        return self
    }
}