MerchantKit

A modern In-App Purchases management framework for iOS developers.

MerchantKit dramatically simplifies the work indie developers have to do in order to add premium monetizable components to their applications. Track purchased products, offer auto-renewing subscriptions, restore transactions, and much more.

Designed for apps that have a finite set of purchasable products, MerchantKit is a great way to add an unlockable 'pro tier' to an application, as a one-time purchase or ongoing subscription.

Example Snippets

Find out if a product has been purchased:

let product = merchant.product(withIdentifier: "iap.productidentifier")
print("isPurchased: \(merchant.state(for: product).isPurchased))"

Buy a product:

let task = merchant.commitPurchaseTask(for: purchase)
task.onCompletion = { result in 
    switch result {
        case .succeeded(_):
            print("purchase completed")
        case .failed(let error):
            print("\(error)")
    }
}

task.start()

Get notified when a subscription expires:

public func merchant(_ merchant: Merchant, didChangeStatesFor products: Set<Product>) {
    if let subscriptionProduct = products.first(where: { $0.identifier == "subscription.protier" }) {
        let state = merchant.state(for: subscriptionProduct)
        
        switch state {
            case .isPurchased(let info):
                print("subscribed, expires \(info.expiryDate)")
            default:
                print("does not have active subscription")
        }
    }
}

Project Goals

Straightforward, concise, API to support non-consumable, consumable and subscription In-App Purchases.
Simplify the development of In-App Purchase interfaces in apps, including localized formatters to dynamically create strings like "£2.99 per month" or "Seven Day Free Trial".
No external dependencies beyond what Apple ships with iOS. The project links Foundation, StoreKit, SystemConfiguration and os for logging purposes.
Prioritise developer convenience and accessibility over security. MerchantKit users accept that some level of piracy is inevitable and not worth chasing.
Permissive open source license.
Compatibility with latest Swift version using idiomatic language constructs.

The codebase is in flux right now and the project does not guarantee API stability. MerchantKit is useful, it works, and will probably save you time. That being said, MerchantKit is by no means finished and there are major components that are in the project's scope but completely unimplemented (predominantly, consumable products are not supported). The test suite is currently bare.

Getting Started

Compile the MerchantKit framework and embed it in your application. The easiest way to get up and running right now is to use Cocoapods; add MerchantKit to your app's Podfile and build. You can also just download the source code from Github and embed the Xcode project into your app, although you'll have to upgrade to the latest releases manually.
In your app delegate, import MerchantKit create a Merchant instance in application(_:, didFinishLaunchingWithOptions:). Supply a storage object (recommended: KeychainPurchaseStorage) and a delegate.

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    ...
    
    self.merchant = Merchant(storage: KeychainPurchaseStorage(serviceName: "AppName"), delegate: self)
    
    ...
}

Implement the two required methods in MerchantDelegate to validate receipt data and receive notifications when the PurchasedState changes for registered products.

func merchant(_ merchant: Merchant, didChangeStatesFor products: Set<Product>) {
    for product in products {
        print("updated \(product)")
    }
 }
    
func merchant(_ merchant: Merchant, validate request: ReceiptValidationRequest, completion: @escaping (Result<Receipt>) -> Void) {
    let validator = LocalReceiptValidator(request: request)
    validator.onCompletion = { result in
        completion(result)
    }
        
    validator.start()
}

Register products as soon as possible (typically within application(_:, didFinishLaunchingWithOptions:)). You may want to load these products from a resource file. The included LocalConfiguration object provides a mechanism for this.

let config = try! MerchantKit.LocalConfiguration(fromResourceNamed: "MerchantConfig", extension: "plist")
self.merchant.register(config.products)

Call setup() on the merchant instance before escaping the application(_:, didFinishLaunchingWithOptions:) method. This tells the merchant to start observing the payment queue.

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    ...
    
    self.merchant = Merchant(storage: KeychainPurchaseStorage(serviceName: "AppName"), delegate: self)    
    self.merchant.register(...)
    self.merchant.setup()
    
    ...
}

Profit! Or something.

Product Interface Controller

The tasks vended by a Merchant give developers access to the core operations to fetch and purchase products with interfaces that reflect Swift idioms better than the current StoreKit offerings. Many apps will not need to directly instantiate tasks. ProductInterfaceController is the higher-level API offered by MerchantKit that covers the use case of many projects. In an iOS app, a view controller displaying an upgrade screen would be backed by a single ProductInterfaceController which encapsulated all necessary product and purchasing logic.

The ProductInterfaceController class encompasses common behaviours needed to present In-App Purchase for sale. However, it remains abstract enough not be tied down to one specific user interface appearance or layout.

Developers simply provide the list of products to display and tells the controller to fetch data. The delegate notifies the app when to update its custom UI. It handles loading data, intermittent network connectivity and in-flight changes to the availability and state of products.

Formatters

MerchantKit includes several formatters to help developers display the cost of In-App Purchases to users.

PriceFormatter is the simplest. Just give it a Price and it returns formatted strings like '£3.99' or '$5.99' in accordance with the store's regional locale. You can specify a custom string if the price is free. SubscriptionPriceFormatter takes a Price and a SubscriptionDuration. Relevant Purchase objects exposes these values so you can simply pass them along the formatter. It generates strings like '$0.99 per month', '£9.99 every 2 weeks' and '$4.99 for 1 month' depending on the period and whether the subscription will automatically renew.

In addition to the renewal duration, subscriptions can include free trials and other introductory offers. You can use a SubscriptionPeriodFormatter to format a text label in your application. If you change the free trial offer in iTunes Connect, the label will dynamically update to reflect the changed terms without requiring a new App Store binary. For example:

func subscriptionDetailsForDisplay() -> String? {
    guard let terms = purchase.subscriptionTerms, let introductoryOffer = terms.introductoryOffer else { return nil }
    
    let formatter = SubscriptionPeriodFormatter()
    
    switch introductoryOffer {
        case .freeTrial(let period): return "\(formatter.string(from: period)) Free Trial" // something like '7 Day Free Trial'
        default: ...
    }
}

PriceFormatter works in every locale supported by the App Store. SubscriptionPriceFormatter and SubscriptionPeriodFormatter are currently offered in a small subset of languages. Voluntary translations are welcomed.

Consumable Products

Merchant tracks the purchased state of non-consumable and subscription products. Consumable products are considered transitory purchases and not recorded beyond the initial time of purchase. Because of their special nature, there is a special method in the MerchantDelegate for consumable products. This method is required to be implemented by applications that manage consumable products.

func merchant(_ merchant: Merchant, didConsume product: Product) {
    self.addCreditsToUserAccount(for: product) // application-specific handling 
}

The Merchant will always report a consumable product's state as PurchasedState.notPurchased. Forgetting to implement the delegate method will result in a runtime error.

To Be Completed (in no particular order)

Increase the number of localizations for SubscriptionPriceFormatter and SubscriptionPeriodFormatter.
Add extensive documentation with example usage projects, including usage of ProductInterfaceController.
Improve handling of failure cases with richer error messages and feedback for when things go wrong.
Support downloadable content In-App Purchases.
Probably a lot more stuff I haven't thought of yet.

Credits

Developed and managed by Benjamin Mayo, @bzamayo on Twitter.

joaopsilveira / merchantkit