Cross-Chain Bridge Integration in a Mobile Crypto Wallet

Cross-Chain Bridge Integration in a Mobile Crypto Wallet

The most common scenario in a mobile crypto wallet: a user sends USDC from Ethereum to Polygon, and the funds get stuck during finalization. Without correct tracking, this causes panic and negative support tickets. We solve this by integrating cross-chain bridge aggregators with full status monitoring. Over 5 years, we have implemented bridge functionality for 15+ crypto wallets and handled hundreds of thousands of cross-chain transfers. Our experience allows us to anticipate typical problems: insufficient bridge liquidity, destination network configuration errors, delays due to forks. We have solved and documented each such case so that users never lose funds.

How do bridge aggregators work?

Aggregators are middleware that combine multiple bridge protocols into a single API. The app sends a transfer request, and the aggregator calculates the best route by speed, fee, and final amount. We use proven solutions: Li.Fi, Squid Router, Stargate Finance. Each has its strengths. Compare them:

Li.Fi is the most flexible, supporting recoverTx for stuck transfers. Squid is better for Cosmos. Stargate is simple and fast for USDC/USDT.

How to choose the right bridge for your wallet?

Consider: which networks do your users need? Which tokens? If only EVM and stablecoins, Stargate is sufficient. If Cosmos or Polkadot are needed, we integrate Squid via Li.Fi. In any case, integration through an aggregator gives one API for dozens of bridges.

// iOS — routes via Li.Fi SDK
import LiFi
let lifi = LIFI()
let routesRequest = RoutesRequest(
    fromChainId: 1,          // Ethereum
    toChainId: 137,          // Polygon
    fromTokenAddress: "0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48", // USDC
    toTokenAddress: "0x2791Bca1f2de4661ED88A30C99A7a9449Aa84174",
    fromAmount: "100000000"  // 100 USDC (6 decimals)
)
let routes = try await lifi.getRoutes(request: routesRequest)

Detailed API description can be found in the Li.Fi documentation.

Routing and bridge selection

Li.Fi returns multiple routes with different parameters. Show the user:

Default sorting by maximum output. Provide a toggle "faster" / "cheaper".

How to implement background transfer tracking?

After sending the transaction in network A, the user gets a txHash in the source chain. Funds will appear in the destination chain after some time, depending on the finalization mechanism.

Li.Fi provides a status API: GET /v1/status?txHash={hash}&fromChain={chainId}&toChain={chainId}&bridge={bridgeName}. Statuses: PENDING → DONE / FAILED.

// Android — polling bridge status
suspend fun pollBridgeStatus(txHash: String, fromChain: Int, toChain: Int): BridgeStatus {
    repeat(180) { // 30 minutes * 10 sec
        delay(10_000)
        val status = lifiBridgeApi.getStatus(txHash, fromChain, toChain)
        if (status.status == "DONE" || status.status == "FAILED") return status
    }
    return BridgeStatus(status = "TIMEOUT")
}

Background polling via WorkManager (Android) or BackgroundTasks (iOS) so the user gets a push notification on completion, even if the app was closed.

Handling stuck transfers

Some bridges do not automatically refund on error. Li.Fi provides a recoverTx API for some cases. You need to store the entire bridge transaction history locally with the ability to recheck and a link to bridge support. We guarantee that no transaction goes unattended — all statuses are logged and accessible in the interface. If funds haven't arrived within 30 minutes, the user can initiate a recheck. In rare cases, manual recovery via the aggregator's API is needed — our engineers assist with this as part of support.

Our integration process

1. Requirements analysis: supported networks, tokens, use cases.
2. Aggregator selection and architecture design.
3. API call implementation and response handling.
4. Background tracking with push notifications.
5. Testing on testnets (Rinkeby, Goerli, Mumbai) with real transactions.
6. Production deployment and monitoring.

What is included in the work

When ordering integration, we provide:

• Integration documentation for the aggregator and architecture.
• Access to API and SDK with code examples for iOS and Android.
• Training of the client's team on bridge functionality.
• Support during testing and deployment.
• Warranty support after launch.

Timeline

Integration through an aggregator with one supported route: 1-2 weeks. Multi-aggregator system with route selection, background tracking, and history: 3-6 weeks. Contact us — we will estimate your project in one business day. We are ready to implement cross-chain functionality in your wallet — get a consultation from an engineer with 10 years of experience in mobile development turnkey.

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.