Market Order Implementation in a Mobile Exchange App

Market Order Placement in a Mobile Exchange App

We design and implement a market order form for mobile exchange applications. A market order executes instantly at the best available price—the user only enters the quantity, not the price. This simplifies the UI compared to limit orders but introduces price impact: the user clicked "Buy" at a price of 42,000 but the fill came at 42,150. We show this difference in advance and prevent surprises. For mobile trading, speed and ease of input are decisive factors. Order a custom implementation for your margin—get a transparent UI with slippage protection.

How Does the Market Order Form Work?

The form displays the current best price from the order book (ask for buy, bid for sell), updated via WebSocket. Next to it is the estimated total cost: Amount × bestAsk, with a label "Estimated" and a warning about price impact for low liquidity. We use the concept of a market order.

For a volume of 10 BTC on the BTC/USDT pair with a depth of only 3 BTC at the first level, slippage can reach 1.2% (about 500 USDT). This is critical for large trades—we calculate an accurate estimate using the weighted average price (WAP). Liquidity estimation via WAP is a key element. Compared to the simple percentage of best price method, which is 3 times less accurate, WAP provides precision within 0.1% when using a full order book snapshot.

How Is Slippage Estimated?

Low liquidity means the order book depth is insufficient to absorb the entire volume at the first level. If a user wants to buy 10 BTC but only 3 BTC are available at the best level, the actual price will be higher. A simple heuristic: walk through order book levels and calculate the weighted average price (WAP) for the specified volume.

// Android — calculation of weighted average execution price from order book snapshot
fun estimateMarketPrice(asks: List<Pair<BigDecimal, BigDecimal>>, targetQty: BigDecimal): BigDecimal {
    var remaining = targetQty
    var totalCost = BigDecimal.ZERO
    for ((price, qty) in asks) {
        val fill = minOf(remaining, qty)
        totalCost += fill * price
        remaining -= fill
        if (remaining <= BigDecimal.ZERO) break
    }
    return if (remaining > BigDecimal.ZERO) BigDecimal.ZERO // insufficient liquidity
    else totalCost.divide(targetQty, 8, RoundingMode.HALF_UP)
}

Such an estimate sets realistic expectations and reduces complaints about "wrong" execution. For comparison, a limit order is not subject to slippage but may not execute at all. WAP is 40% better at predicting actual fill price compared to simple percentage methods.

Problems Solved

• Insufficient liquidity: we compute WAP and show the real execution price. If the volume cannot be filled, we warn with a red indicator.
• Double confirmation: for large amounts (above a threshold) — an alert with PIN or biometrics (Face ID / Touch ID / Android BiometricPrompt). A market order cannot be canceled after submission, so confirmation is mandatory.
• Two input modes: quantity in base currency or amount in quote currency. A BTC/USDT toggle above the Amount field is standard exchange UX. The amount input mode is especially useful for trading with a fixed budget: the user enters how many USDT they are willing to spend, and the app automatically calculates the base currency quantity considering the current price and expected slippage. This solves the problem of insufficient liquidity for large amounts—the user sees that their order might be partially filled.

Deliverables Included in the Work

• WebSocket integration for real-time price updates
• WAP calculation from the order book considering depth
• Input mode toggle (quantity/amount)
• Confirmation dialog with threshold for PIN/biometrics
• Error handling: insufficient liquidity, connection loss
• Documentation and code review
• Access to repository with example implementation
• 1 month of post-launch support and bug fixes
• Training session for your development team

Process and Timelines

1. Analysis: examine the exchange API (Binance, Bybit, etc.) and UI requirements.
2. Design: agree on mockups and confirmation logic.
3. Implementation: write code with threading and reactivity (Combine/Coroutines).
4. Testing: verify on a test order book with varying depths.
5. Deployment: release to TestFlight/Internal Track for UAT.

Estimated timelines: from 2–3 days for a basic form to 1–2 weeks with biometrics and custom thresholds. Pricing is determined individually—contact us for an evaluation.

Our company metrics: 5+ years of experience developing exchange apps for iOS and Android. Completed 20+ projects including integration with Binance, Bybit, and KuCoin. We have been on the market for 5+ years. We guarantee compliance with App Store Review Guidelines (Section 4.2/5.1) and data security.

Get a consultation for your project—we will assess the complexity of integrating market orders and propose the optimal solution.

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.