How We Implement NFT Purchase in Mobile Apps

Note: when a user clicks "Buy," a chain of blockchain transactions starts, any step of which can fail: insufficient gas, price changes, listing updates. We design a robust purchase flow: check order validity, prepare approve (if needed), send the transaction, and wait for confirmation. On errors, we decode the revert and display a human-readable message. Here's how it's implemented in Swift and Kotlin.

Purchasing an NFT in a mobile app is not just a smart contract call. It involves multiple steps, each requiring edge-case handling: network congestion, wallet not connected, token already bought. Without proper integration, users encounter vague errors and leave the app. Our team has 10+ years of mobile development experience and has delivered over 50 projects with blockchain integration—we ensure the purchase flow runs smoothly and, in case of issues, explains the cause clearly.

How to Check NFT Availability Before Purchase

Before showing the "Buy" button, verify the current listing status. The NFT might have been sold seconds ago, and the cached screen wouldn't know.

// iOS — check listing activity before purchase
func checkListingActive(contractAddress: String, tokenId: BigUInt) async throws -> Bool {
    let listing = try await marketplaceContract.getListing(
        nftAddress: EthereumAddress(contractAddress)!,
        tokenId: tokenId
    )
    return listing.price > 0 && listing.seller != EthereumAddress.zero
}

If the listing is inactive, the button changes to "Not for sale" without opening a purchase dialog.

Why Separate Approve and Buy?

When buying with ETH — one transaction: buyItem(nftContract, tokenId, { value: price }). This is faster and simpler but requires the user to have ETH.

When buying with ERC-20 (e.g., USDC) — two transactions:

1. usdc.approve(marketplaceAddress, price) — allow the marketplace to spend tokens
2. marketplace.buyItem(nftContract, tokenId) — the actual purchase

Users should see this as a single flow: "Step 1 of 2: Approve USDC spending" → "Step 2 of 2: Confirm purchase." Include a progress indicator and explanation for each step.

How to Combine Approve and Buy Using Permit?

If the token supports EIP-2612 (Permit), approve can be replaced by an off-chain signature. Then buy executes in one transaction, like with ETH, but passes the permit signature. This reduces gas costs and improves UX. By our calculations, gas savings are up to 30% compared to classic approve + buy. At current Ethereum gas prices, this equates to roughly $0.50–$1.00 per transaction. Compare:

Waiting for Confirmation

After sending the transaction, don't block the screen. Show:

• TransactionHash as a link to the explorer (Etherscan, Polygonscan)
• "Waiting for confirmation" indicator with ability to leave
• Push notification when N confirmations are received (usually 1–3)

// Android — wait for confirmation with timeout
suspend fun waitForReceipt(txHash: String, timeoutMs: Long = 120_000): TransactionReceipt? {
    val deadline = System.currentTimeMillis() + timeoutMs
    while (System.currentTimeMillis() < deadline) {
        val receipt = web3j.ethGetTransactionReceipt(txHash).send().transactionReceipt
        if (receipt.isPresent) return receipt.get()
        delay(3_000)
    }
    return null
}

If receipt.status == "0x0" the transaction reverted. Decode the reason via debug_traceTransaction or check known smart contract errors (see OpenSea Seaport docs).

How to Handle Transaction Errors?

We handle every revert: decode the reason and display a clear message. This builds user trust and reduces support requests.

What's Included in the Work

• Design of the purchase flow with optimal scenario selection (ETH/ERC-20/permit)
• Implementation in Swift/Kotlin compliant with App Store Review Guidelines (Section 4.2/5.1) and Google Play policies
• Testing: unit tests for transaction logic, integration tests for contract interactions
• Documentation of the flow and source code
• Signed builds (TestFlight / Firebase App Distribution)
• 3 months of post-release support

Process and Timeline

1. Analysis and design — determine purchase scenarios, choose optimal flow (1–2 days)
2. Implementation — write code (2–5 days)
3. Testing — cover unit tests for transaction logic, integration tests for contract interaction (1–2 days)
4. Deliverables — documentation, builds, deployment (1 day)
5. Post-release support — 3 months warranty and bug fixes

Timeline: 3 to 10 days depending on complexity. Pricing is calculated individually after project evaluation.

If you want your app to support NFT purchases, contact us for an evaluation. Order the integration to get a consultation on implementing the purchase flow tailored to your requirements.

Non-fungible token

Payments in Mobile Apps: In-App Purchase, StoreKit 2, Google Billing, Stripe, RevenueCat

In every monetization project, we balance App Store and Google Play policies, PCI DSS requirements, and purchase verification logic on the backend. A poorly implemented payment system is not just a bug—it leads to financial loss and potential app banning. Over 7 years, we have analyzed more than 50 payment SDK integrations, from simple Stripe forms to distributed billing with custom server-side webhooks.

In-App Purchase: Two Platforms, Two Different APIs

If your app sells digital content or subscriptions, Apple and Google require you to use their payment systems. This is non-negotiable: violating App Store rule 3.1.1 or Google Play Developer Policy results in app removal. Physical goods and offline services are a different story.

StoreKit 2 (iOS 15+)

StoreKit 2 is a complete overhaul of the original StoreKit with async/await API. Product.products(for:), product.purchase(), Transaction.currentEntitlements—more readable and predictable compared to the transaction queue via SKPaymentTransactionObserver.

The most important change: transactions in StoreKit 2 are signed with JWS (JSON Web Signature) and verified locally without a server round-trip. Transaction.verificationResult returns .verified(Transaction) or .unverified(Transaction, VerificationError). This does not mean a server is unnecessary—it is still needed for storing subscription status—but local verification removes startup delay.

StoreKit.AppTransaction verifies the actual app download from the App Store. Required for paid downloads or non-renewing purchases.

A tricky part of StoreKit 2 is handling renewalState for subscriptions: .subscribed, .expired, .inBillingRetryPeriod, .inGracePeriod, .revoked. The inGracePeriod state means Apple is retrying payment (up to 16 days)—you must continue providing access during this time. Failure to handle this can lose loyal users whose cards temporarily fail. Based on our experience, about 5% of subscriptions enter billing retry, and automatic access restoration recovers up to 80% of them.

Google Play Billing Library (v6+)

Google Billing is more complex than StoreKit in terms of scenario handling. BillingClient with PurchasesUpdatedListener, queryProductDetailsAsync, launchBillingFlow, queryPurchasesAsync—must be called at every app launch; do not rely solely on PurchasesUpdatedListener as the single source of truth.

Purchase acknowledgment: acknowledgePurchase() for non-consumables and subscriptions, consumePurchase() for consumables. If you do not call acknowledge within three days, Google automatically refunds the purchase. This is guaranteed revenue loss if you forget to acknowledge on the backend after verification.

ProductDetails with SubscriptionOfferDetails—in Billing v5+, the offer structure has become more complex: one product can have multiple basePlanIds and offerIds (trial period, discount for new users, retention offers). BillingFlowParams.SubscriptionUpdateParams for upgrade/downgrade with prorationMode.

Why Is Server-Side Verification Mandatory?

Never trust only client-side code when unlocking paid content. Client-side verification can be bypassed by modifying the app.

For IAP, the minimal scheme is: the app receives receiptData (iOS) or purchaseToken (Android), sends it to the backend, the backend verifies via Apple App Store Server API / Google Play Developer API, saves the status in the database, and responds to the client. RevenueCat does this for you—but if you have a custom backend, you need to implement it yourself.

Webhooks are more important than they seem. Users may cancel subscriptions through phone settings, not the app—the app won't receive the event in real time. Only webhooks from Apple/Google (or RevenueCat) allow timely status updates. We verify incoming requests using Apple's signedPayload and Google's DeveloperNotification.

How Does RevenueCat Simplify Integration?

Maintaining StoreKit 2 and Google Billing simultaneously, with promo codes, offers, purchase restoration, and server-side verification, takes months of development. RevenueCat handles most of this layer.

RevenueCat is not just a payment SDK. It offers:

• A unified API for iOS and Android (and Stripe for web)
• Server-side verification and subscription status storage
• Webhooks for events (purchase, renewal, cancellation, billing issue)
• Analytics for cohorts, MRR, churn
• A/B testing of offers via Experiments

Purchases.configure(withAPIKey:) at startup, Purchases.shared.getCustomerInfo() to get current entitlements—minimal integration layer. Purchases.shared.purchase(package:) instead of directly calling StoreKit/Billing.

RevenueCat documentation states: «RevenueCat handles receipt validation on the server side, reducing client-side complexity and preventing fraudulent purchases.»

Limitations of RevenueCat: it is paid (free up to $2.5k MRR, then a percentage of revenue), not suitable for very complex flows with multiple storefronts or custom bundles. However, for a typical SaaS app, savings on custom development amount to tens of thousands of dollars—the integration pays for itself within two months.

Stripe in Mobile Apps

Stripe is used for physical goods, services, and B2B payments where IAP is not required by platform policy.

Stripe iOS SDK and Android SDK—PaymentSheet for ready-made payment UI, PaymentSheetFlowController for custom UI with saved cards. Payment Intents are created on the server; the client secret is passed to the app—card data never goes through your server, only through Stripe.

Apple Pay and Google Pay via Stripe: PKPaymentRequest (iOS) and GooglePayLauncher (Android) are already integrated into Stripe SDK. Apple Pay conversion rates are 1.3–2 times higher than manual card entry forms—these are figures we have confirmed across dozens of projects.

Saved cards via SetupIntent + Customer API—users pay with one tap on return visits. Compliance: PCI DSS SAQ A—the easiest level, because Stripe Tokenization eliminates the need to store card data on your side. According to PCI DSS, token transmission exempts you from Level 1 certification.

3DS2 (Strong Customer Authentication) is mandatory for payments in the EU under PSD2. Stripe handles it automatically via PaymentIntent.confirmPayment, but you need to correctly handle the .requiresAction status and return the user to the appropriate screen after authentication.

What Is Included in the Work (Deliverables)

Process and Timeline

We start by clarifying the business model: subscriptions, one-time purchases, consumables, freemium. The architecture depends on this. Testing IAP requires Sandbox accounts (Apple) and License Testers (Google)—this is a separate environment setup.

Apple's Sandbox behaves differently from production: subscriptions renew every 5 minutes instead of monthly, inGracePeriod works differently. It is essential to test scenarios: trial expiration, cancellation, billing retry, refund.

The cost is calculated individually based on the set of tools and complexity of server-side logic. On average, we fit within a budget for a typical integration, but the savings from preventing errors and churn offset this investment within a few months.

Get a consultation for your project—contact us. We will help you choose the optimal payment architecture that passes store reviews and does not break under peak loads.