Payments in the Mobile Payments SDK are handled by the PaymentManager and must conform to the PaymentManagerDelegate protocol. The PaymentManagerDelegate is called when a payment starts and finishes successfully, fails, or is canceled. Your application should include logic to handle each of these scenarios. To learn about error messages produced by the Mobile Payments SDK and ways to resolve them, see Handling Errors. If the payment completes successfully, a Payment object is returned, describing details of the payment. This can be either an OnlinePayment or an OfflinePayment.

extension <#YourViewController#>: PaymentManagerDelegate {
    func paymentManager(_ paymentManager: PaymentManager, didFinish payment: Payment) {
        // You can further inspect each payment once cast
        if let onlinePayment = payment as? OnlinePayment {
            print("Online Payment - ID: \(onlinePayment.id!), status: \(onlinePayment.status.description)")
        } else if let offlinePayment = payment as? OfflinePayment {
            print("Offline Payment - ID: \(offlinePayment.localID), status: \(offlinePayment.status.description)")
        }
    }

    func paymentManager(_ paymentManager: PaymentManager, didFail payment: Payment, withError error: Error) {
        // Handle this the same way as `didFinish`
    }

    func paymentManager(_ paymentManager: PaymentManager, didCancel payment: Payment) {
        // Handle this the same way as `didFinish`
    }
}

Before taking a payment with the SDK in a production environment, ensure that you've done the following:

• Requested the permissions necessary for the type of reader you're using.
• Installed and initialized the Mobile Payments SDK in your iOS application.
• Authorized the Mobile Payments SDK with a Square account.

Additionally, before taking a payment in production, you can use the Square Sandbox and mock readers to test your application.

To begin a payment with the Mobile Payments SDK, you create a PaymentParameters object, which includes the payment amount, idempotency key, optional tip, application fee (if applicable), and the processing mode, which is used if you want to take payments while offline. For the complete list of payment parameter values and details, see the iOS technical reference.

The PaymentParameters must also include an idempotency key. An idempotency key is a unique string identifier that accompanies an API operation, allowing Square to recognize potential duplicate calls. Using an idempotency key in each payment request ensures that a payment can only occur once, protecting against unintended outcomes like charging a customer twice for the same transaction. In Swift, you can use UUID().uuidString to generate a unique string value and store it as the idempotency key.

Along with an idempotency key, include a unique referenceID value in your PaymentParameters for each payment. This persists with the payment and can be used to identify the payment if you need to reference it later. Typically, the referenceID is used to match Square payments to an external system, but it can also be used to identify payments within Square.The idempotency key and reference ID are not interchangeable, and you shouldn't use the same value for both parameters, because sometimes a payment requires multiple attempts (for example, if the first is declined for expiration or insufficient funds, you can retry the payment with the same referenceID and a new idempotency key).

If you don't receive a server response from a payment made with the Mobile Payments SDK, you should check the status of the payment to determine whether to retry. Use the ListPayments endpoint and query by location ID, total payment amount, or other fields to filter the list. You can use your referenceID to identify the correct payment and check its status. You might need to retry the payment with a new idempotency key if the prior payment exists, but cannot be completed. In this situation, you can cancel the original incomplete payment by using CancelPaymentByIdempotencyKey.

The following is an example of payment parameters created for a payment of $1 USD. The parameters include a unique idempotencyKey, referenceID for that key, and an optional orderID that allows you to associate a payment with an itemized Square order.

import UIKit
import SquareMobilePaymentsSDK

extension <#YourViewController#> {
    func makePaymentParameters() -> PaymentParameters {
        let amount = Money(amount: 100, currency: .USD)

        // This is your own, reference ID that will be referenced by Square
        let referenceID = "MY_REFERENCE_ID_1234"

        // `idempotencyKeyStorage` is a simple key-value store that is persisted to
        // disk and associates a unique idempotency key with your reference ID.
        let idempotencyKey = idempotencyKeyStorage.get(id: referenceID) ?? {
            let newKey = UUID().uuidString
            idempotencyKeyStorage.store(id: referenceID, idempotencyKey: newKey)
            return newKey
        }()

        // Create the payment parameters
        let paymentParams = PaymentParameters(
            idempotencyKey: idempotencyKey,
            amountMoney: amount
        )

        // An optional note to add to the payment.
        paymentParams.note = "My Note"

        // Optional parameter to associate this payment with a previously created Square order. 
        paymentParams.orderID = "CAISENgvlJ6jLWAzERDzjyHVybY"

        // A user-defined ID to associate with the payment. You can use this field 
        // to associate the payment to an entity in an external system.
        paymentParams.referenceID = referenceID

        return paymentParams
    }
}

Each payment must also have PromptParameters configured. The PromptParameters contain a PromptMode and AdditionalPaymentMethods. PromptParameters.mode determines whether your application uses the default Square-provided payment prompt UI or a custom payment prompt where you build your own UI. The default UI provided by Square is presented on top of the viewController passed to PaymentManager.startPayment and presents buyers with the payment options available from PaymentManager.availableCardInputMethods.

PromptParameters.additionalMethods specifies a list of additional payment methods available to use for this payment. The current option is keyed (a manually entered credit card payment). If you're using the default PromptMode, you can make AdditionalPaymentMethods an empty list to remove the manual card entry button from the payment prompt.

If you create your own custom PromptMode, you should use the additionalPaymentMethods available from the PaymentHandle to render these options for buyers in your own payment prompt UI.

To begin processing a payment, call PaymentManager.startPayment() with the PaymentParameters and PromptParameters you previously created. During the payment, Square takes control of the screen display to ensure that buyer information is handled securely and that the final confirmation of the payment is correctly shown to the buyer. You must conform to the PaymentManagerDelegate to handle results and errors from the payment flow.

When the payment begins, the Payment Manager provides a PaymentHandle as a way for you to interact with the ongoing payment (for example, if you need to cancel the payment). When the payment completes, regardless of whether it completed successfully, control is returned to the user interface and your application is notified of the result using PaymentManagerDelegate. Your application can display receipt or error information to the user and continue other processing.

import SquareMobilePaymentsSDK

class <#YourViewController#>: UIViewController {

    private var paymentHandle: PaymentHandle?

    func startPayment() {
        paymentHandle = MobilePaymentsSDK.shared.paymentManager.startPayment(
            makePaymentParameters(),
            promptParameters: PromptParameters(
                mode: .default,
                additionalMethods: .all
            ),
            from: self,
            delegate: self
        )
    }

    func makePaymentParameters() -> PaymentParameters {
        PaymentParameters(
            idempotencyKey: UUID().uuidString,
            amountMoney: Money(amount: 100, currency: .USD)
        )
    }
}

As part of your application's workflow, you might want to authorize a customer's transaction but delay the capture of the payment (the transfer of funds from customer to seller) for a period of time. For example, in a restaurant with a kiosk ordering system, you might want to authorize a customer's credit card when they order, but not complete the payment until they receive their food.

While creating PaymentParameters, set the autocomplete value to false if you want to delay the capture of the payment. While the payment is delayed, use the Payments API to complete or cancel the payment. You can also update the payment to add a tip using the tip_money parameter.

When autocomplete is false, there are two more PaymentParameter values which handle the delayed payment:

• delayDuration: The number of seconds to wait before canceling the payment or taking another delayAction, specified as a TimeInterval, with a minimum value of 60 seconds. Use a value of “0” to fallback to the default 36 hours for card present payments, or 7 days for manually entered payments.
• delayAction: The action to take once the delayDuration has passed with no other action on the payment. The possible options are cancel (default) or complete.

For more information about delayed payment capture using the Payments API, see Delayed Capture of a Card Payment.

The PaymentHandle provided by the Mobile Payments SDK during a payment can be used to cancel that payment. Note that only a payment where autocomplete is set to false can be canceled during processing. You can first check the value of PaymentHandle.isPaymentCancelable. If it's set to true, the payment can be canceled by calling paymentHandle.cancelPayment(). For example, if your code dismisses the view controller passed to PaymentManager.startPayment and the payment was not successful, you must call cancelPayment.

During any payment flow, if the application is backgrounded or the application activity is interrupted, the payment in progress is canceled.

Your application must provide buyers with the option to receive a digital or printed receipt. These receipts aren't sent directly by Square. Therefore, to remain compliant with EMV-certification requirements, you must generate receipts including the following fields from the payment response cardDetails object, when available:

• cardholderName (example: James Smith)
• cardBrand and last4 (example: Visa 0094)
• applicationName (example: AMERICAN EXPRESS)
• applicationIdentifier (example: AID: A0 00 00 00 25 01 09 01)
• entryMethod (example: Contactless)