PromiseKit with optional promises

Question

I am currently investigating if I should integrate PromiseKit in an existing project.

My main issue is that I need to implement a business logic that can call up to 5 web-services. Some of them are called depending on the result of previous ones.

My current architecture is based on breaking up the code in several functions with closures that call each other.

I am trying to find out if I could write an easier to manage code with PromiseKit (or anything else).

Here is some pseudo-code of what I need done:

        // if true, the phone validation is skipped
        let forceRequest = false
        // true if a 3rd party web-service has checked the phone number
        let isVerified = true
        // true if the 3rd party checked the phone number and it is valid
        var isValid = false

        if !isVerified {
            // update value from 3rd party web-service
            isValid = isValidPhoneNumberPromise()
        }

        // If the phone no is invalid stop execution (unless forced)
        if !isValid && !force {
            throw MyError.error1
        }

        // web request to create order
        createOrderPromise()

        // if we have a valid phone number, first send an SMS, then update log
        if isValid {
            sendSmsPromise()
            updateLogPromise()
        }

Based on totiG's answer, I came with the following variation:

var isValid = isValid

        firstly
        {
            return Controller.verify(isVerified: isVerified, isValid: isValid)
        }
        .then { _isValid -> Promise<Int> in
            isValid = _isValid
            return Controller.createOrder()
        }
        .then
        { _ -> Promise<Bool> in
            if isValid {
                return Controller.isSendSms()
            }

            return Promise (value: true)
        }
        .then
        { _ -> Promise<Bool> in
            if isValid {
                return Controller.updateLog()
            }

            return Promise (value: true)
        }
        .catch
        { error in
            print (error)
        }

score 5 · Accepted Answer · answered Apr 19 '17 at 15:08

Yes you can do this using PromiseKit. I have written a basic example showing what you might need. Remember that you would throw an Error when a step fails and handle those failures in the catch block. In my example the verify step passes, but if isValidPhoneNumber was called it would stop the other steps from running. Where I have put the Promise(value: ) you would put your actual web service calls. If the final step to update log is always required to run, you could put this in a .always

enum Errors: Error {
    case invalidPhone
    case orderFailed
}

func orderPromise() {
    firstly {
        self.verify(isVerified: false, force: true)
    }.then { _ in
        self.createOrder()
    }.then { orderNumber in
        self.sendSms(orderNumber: orderNumber)
    }.then { smsSent in
        self.updateLog(smsSent: smsSent)
    }.catch { error in
        //Do something with the error
    }
}

private func verify(isVerified: Bool, force: Bool) -> Promise<Bool> {
    if isVerified || force {
        return Promise(value: true)
    }
    return isValidPhoneNumber()
}

private func isValidPhoneNumber() -> Promise<Bool> {
    return Promise(error: Errors.invalidPhone) //Assume that this fails, then catch in 'orderPromise' will be run
}

private func createOrder() -> Promise<String> {
    //Assume an order number is being passed back to use in the message
    return Promise(value: "Order100")
}

private func sendSms(orderNumber: String) -> Promise<Bool> {
    return Promise(value: true)
}

private func updateLog(smsSent: Bool) -> Promise<Bool> {
    return Promise(value: true)
}

Thank you for your help. Hi, I ended up using a variation of your code (see question edit), please let me know if you see any issues with my approach. — Cristian-Leonard Miehs, Apr 20 '17 at 13:34
While your code does work, you are checking the isValid property is true in every then function. Promises should work so that the next step only executes if the previous one is valid. What you should do, as in my example is return an error (or you could throw an exception) when the verification fails - this would then cause the catch function to run and you dont need to worry about checking isValid anywhere else. — totiDev, Apr 20 '17 at 13:53
The problem is that the last 2 promises are optional. I need to execute them only if the phone number is valid. If I don't call them, it is not considered an error. — Cristian-Leonard Miehs, Apr 21 '17 at 15:12

PromiseKit with optional promises

1 Answers1