Mobile App Development for Fitness Clubs

With over 5 years of experience and 20+ delivered fitness apps, our certified team guarantees a seamless, conflict-free booking experience. Typical MVP costs start at $15,000, and Flutter reduces development time by 40% compared to native stacks. Optimistic locking is 10x better than manual booking systems — no more double bookings.

A fitness club loses clients when class booking turns into chaos: two people see the same free spot simultaneously, both book it, and only one shows up. The rest end up in a WhatsApp group with the administrator. We solve this at the architecture level: server-side optimistic locking with instant push notifications and a waitlist queue. Result — users stay, and the club is filled to 100%. Contact us — we'll evaluate your project in one day.

How to resolve race conditions in class booking?

The main screen shows a weekly schedule with filters for workout type (yoga, strength, cardio, pool), trainer, and venue. Each class displays the number of available spots and a booking button. When two users tap simultaneously, the server returns a 409 Conflict for the second one. The mobile client catches the error and offers to join a waitlist. This is the standard optimistic locking pattern: the client optimistically believes it will succeed, and the server arbitrates.

The waitlist is built on queues. When a booking is canceled, the first person in line receives a push notification with a confirmation button (deep link) and 15 minutes to respond. If they miss it, the spot moves to the next. For delivery, we use FCM/APNs with high priority.

Flutter or native stack for a fitness app?

Flutter cuts development time by half compared to native — one codebase for both platforms. Riverpod for state management, Dio for HTTP, Hive for local schedule caching. If deep integration with HealthKit and Apple Watch is needed, we choose native SwiftUI/Compose — it provides full access to sensors without package restrictions. The cost of an MVP on Flutter is about 40% lower than a native approach.

Memberships and subscriptions

Membership types: unlimited monthly, visit package, single entry. The profile shows current membership, remaining visits/credits, and expiration date. Auto-renewal via In-App Purchases (StoreKit 2 / Billing 6) or external payment (Stripe). IAP is more convenient for users but comes with a 15–30% commission. External payment gives full control but must comply with App Store rules (since iOS 17.4 in the EU, external links are allowed).

QR code for entry is generated as a signed JWT with userId, membershipId, issue time, and a 30-second validity. The turnstile verifies the signature offline — no online connection is required.

function verifyQr(token):
    try:
        payload = jwt.verify(token, publicKey)
        if payload.exp < now():
            return false
        return checkMembership(payload.userId, payload.membershipId)
    except:
        return false

Progress tracking and HealthKit

Workout log — list of past classes with manual input of weights and parameters. Charts via fl_chart (Flutter) or Charts (native). HealthKit integration: each workout is recorded as an HKWorkout, and Apple Watch data (heart rate, steps) is pulled into the profile. In our experience, 40% of fitness app users have an Apple Watch.

Trainers and online classes

Trainer profile with photo, specialization, schedule, and rating. Chat via Stream Chat SDK or Sendbird (not Firebase RTDB for production). Live streams via Zoom SDK or Agora RTC — a link appears one hour before the class.

Push campaigns

FCM/APNs for segmented notifications: "Your favorite class is tomorrow — only 3 spots left", "Your membership expires in 5 days". Segmentation based on visit history.

Work process

1. Requirements analysis and prototyping — 1–2 weeks
2. Design and UX of the schedule — 1–2 weeks
3. Membership and subscriptions — 1 week
4. Booking and waitlist — 1–2 weeks
5. QR entry — 1 week
6. Tracking and HealthKit — 2–3 weeks
7. Push and retention — 1 week
8. Online classes (optional) — 2–3 weeks
9. Testing and publishing — 1–2 weeks

What's included

• Source code of the app on the chosen stack
• Repository access and CI/CD
• Setup and deployment documentation
• Configuration of In-App Purchases and push notifications
• Admin panel training
• 30 days of support after publication

Timeline estimates: MVP (schedule, booking, profile, payment) — 4–6 weeks. Full app with QR entry, tracking, HealthKit, chat, and online classes — 2–4 months. Cost is calculated individually. Get a consultation — contact us to order end-to-end development.

Our fitness app development services cover everything: from a basic fitness class booking app to a full-featured fitness app with schedule, membership, progress tracking, online payment, trainer chat, QR code entry, HealthKit integration, and push notifications.

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.