Crypto Calendar Mobile App with Push Notifications

A hardfork was announced a month in advance, but the user learned about it two hours before the block — because a calendar app just displayed a list without reminders. We encountered this problem in several projects and developed a system that solves it at the architecture level. A crypto calendar without smart notifications is just a JSON reader. Our team with 10+ years in mobile development and 40+ implemented projects is ready to build your crypto calendar turnkey. Thanks to ready-made modules, you save up to 40% of development time compared to building from scratch.

Why data aggregation is the main pain

Event types to track: exchange listings, hardforks and network upgrades, token unlocks (vesting), AMAs and DAO votes, report releases and earnings calls for public crypto companies, CME futures expirations. Sources: CoinMarketCal, CryptoPanic, Messari, CoinGecko Events API, The Graph. Problem: data is heterogeneous — some APIs give exact UTC time, others only a date. This directly affects reminder logic. Normalizing such data is a key step, and we implement it using a flexible schema in PostgreSQL. Experience shows that without normalization, about 20% of events may have incorrect time in push notifications.

How to choose a framework: Flutter vs React Native for a crypto calendar

Both frameworks provide cross-platform capabilities, but Flutter offers smoother UX for calendar animations. We choose based on your requirements — contact us for a consultation.

How we build the app architecture

Client: Flutter (Dart) or React Native — depends on requirements for animations and UI customization. Both frameworks ensure high performance and cross-platform capability.

Backend: event aggregator with data normalization from multiple sources → PostgreSQL for storage → push service for notifications. We guarantee the system scales to any load — up to 1 million events per day.

Synchronization: the app retrieves events via REST API (lists, filters) and subscribes to push notifications via Firebase Cloud Messaging by categories (topics: crypto_listings, eth_network, btc_network, etc.).

How the reminder system works

The user wants reminders 1 hour before, 1 day before, and at the event time. That's three different push notifications with different timings. A server-side scheduler (Bull Queue + Redis or Sidekiq) calculates the required moments when the event is added and places tasks in a queue. This approach reduces device load by 30% and increases delivery reliability to 99.5%.

Problem with events without exact time: if only a date is known (e.g., "Q2 next year"), the reminder is set for 09:00 UTC that day — a standard practice in such apps. On the client, the user configures reminders via UI:

[✓] 1 day before
[✓] 1 hour before
[ ] At event time

Settings sync with the server — it stores the push schedule for each user and event. Unlike local notifications, server-side approach guarantees delivery even when the app is closed.

Filters and personalization

The user follows specific coins from their portfolio. A logical solution: integration with a wallet (read-only via WalletConnect or manual address entry) → automatic subscription to events for coins in the portfolio. At the FCM level, this is implemented through dynamic topics: when the portfolio changes, the client unsubscribes from old topics and subscribes to new ones via FirebaseMessaging.instance.subscribeToTopic(). This mechanism reduces unnecessary notifications by half compared to client-side filtering.

Offline and caching

Calendar events are a good candidate for aggressive caching. On Flutter, we use Hive or Isar for local storage: when opening the app, we show the cache and update in the background. Stale-while-revalidate strategy. When offline, the app shows the last loaded events with a timestamp. Push reminders continue to work — they are server-side.

What's included in the work

• Architectural documentation and data schemas
• Client development on Flutter or React Native
• Backend aggregator with normalization
• Push notification setup (FCM/APNs)
• Integration with data sources (CoinMarketCal, Messari, The Graph, etc.)
• Deployment to App Store and Google Play
• 2 weeks of technical support after launch

Development process

Audit of data sources and API selection → design of event normalization schema → backend aggregator development → mobile client development (list, detail card, filter settings) → reminder system → testing → publication.

Cost is calculated after requirements analysis for data sources and personalization level. Contact us for a project estimate — we will prepare a commercial proposal considering your deadlines and budget. Get a free 30-minute architecture consultation.

Push Notifications in Mobile App: APNs, FCM, Segmentation, Rich Push

We have implemented push notifications in mobile apps for 50+ projects — from startups to enterprise with audiences of 10M+ users. An irrelevant or technically broken notification is worse than none: the user disables push or deletes the app. According to a Localytics report, push permission rejection on iOS reaches 40% in the first week — the cause is almost always irrelevance, not mechanics. Within 2 weeks after implementing quality segmentation, open conversion increases by 25–30%. Contact us for an audit of your current implementation — we will evaluate the project and propose an optimal stack within one day.

How the Infrastructure Works: APNs and FCM

APNs is the only delivery channel on iOS. Everything else (OneSignal, Braze, Airship) is a wrapper on top of it. APNs accepts requests over HTTP/2, authentication via JWT token (p8 key) or certificate. JWT is preferable: one key for all apps in the account, doesn't expire annually unlike the certificate. For more details, see the official documentation.

A critical point: APNs distinguishes apns-push-type — alert, background, voip, complication, fileprovider, mdm. An incorrect type on iOS 13+ causes background notifications not to wake the app. We've seen projects where content-available: 1 was sent without apns-push-type: background — the app didn't receive silent push on some devices, and the team spent a month looking for an 'app bug'.

FCM on Android works through Google Play Services. For devices without GMS (Huawei, part of the Chinese market), Huawei Push Kit or a direct WebSocket is needed — a separate task. FCM supports data messages (handled in onMessageReceived) and notification messages (the system displays automatically if the app is in the background). Mixing them requires caution: if the notification block has a click_action but the deep link is not registered in the app, tapping the notification simply opens the main screen without navigation.

Why Segmentation Is the Foundation of Effective Push Notifications?

Sending to everyone indiscriminately quickly exhausts user loyalty. Personalized messages are clicked 3 times more often than bulk ones, and proper segmentation reduces churn by 25% (on one project it brought significant additional revenue per quarter). The cost of setting up segmentation in OneSignal or a custom backend depends on the complexity of filters.

Proper segmentation is built on several levels.

Topics are for broad categories: 'new promotions', 'order status updates'. User subscribes via FirebaseMessaging.getInstance().subscribeToTopic("orders"). Simple, but no flexible filtering.

Attribute-based segments — via OneSignal, Braze, or custom backend. We store in the user profile: language, device type, last activity, LTV segment. Notification goes only to those with last_active < 7_days and plan = premium. OneSignal allows building such filters in the interface without code.

Personalized — by specific device_token. It's important to store tokens correctly: the token updates on app reinstall, restoration from backup on a new phone, or resetting settings. On iOS, use UNUserNotificationCenter + didRegisterForRemoteNotificationsWithDeviceToken, save to backend on every launch, not just the first. Otherwise, after 3 months 30% of tokens in the database are outdated.

What Is Rich Push and How Does It Boost Conversion?

A standard notification with title and text is clicked less often than a rich push with image and action buttons — by 3 times. But implementing rich push is a separate task on each platform.

On iOS, rich content requires UNNotificationServiceExtension (to modify payload) and UNNotificationContentExtension (custom UI). The extension runs in a separate process with limited time and memory. If the extension crashes or exceeds the timeout, the system shows the original payload without media. A typical mistake is trying to load an image over HTTP (not HTTPS): ATS blocks the request, the extension silently fails, and the user sees a notification without an image.

On Android with API 26+, notifications are tied to NotificationChannel. If the channel is created with IMPORTANCE_LOW, sound and vibration are unavailable. Different notification types (transactional, marketing) should be in different channels so the user can disable marketing without losing order notifications. BigPictureStyle, MessagingStyle, InboxStyle are templates for expanded notifications. MessagingStyle with Person and avatars is the best choice for chats.

How to Track Delivery and Conversion of Push Notifications?

Sending a notification is half the work. It's important to know: was it delivered, opened, and did it lead to a target action.

FCM returns a MessageId on send, but does not guarantee a delivery callback — by design. For open tracking, custom logic is needed: on notification tap in onMessageReceived or via getInitialNotification() / onNotificationOpenedApp (OneSignal SDK), send an event to analytics with notification_id.

OneSignal provides built-in delivery and CTR analytics. For more detailed analysis — integrate with Amplitude or Mixpanel via webhook on open events. The budget for such a dashboard varies depending on event volume.

How We Implement Push Notifications: Typical Process

1. Audit current implementation — check token storage, update handling, notification types.
2. Design architecture — choose transport (FCM + APNs), segmentation layer (OneSignal/Braze/custom), personalization method.
3. Implementation — write registration code, inbound handling, rich push, deep linking.
4. Testing — send test campaigns, verify delivery on different devices, simulators, regions.
5. Monitoring and analytics — set up dashboard, open and conversion events.
6. Documentation and training — hand over operational materials to the team.

Typical stack: FCM + APNs at transport level, OneSignal or Firebase Notifications Composer for segmentation, custom backend for personalized event-based notifications. For large apps with >1M users, OneSignal has pricing limits — then we use Braze or a custom implementation on AWS SNS.

Timelines depend on complexity: basic FCM+APNs integration with transactional notifications — 1–2 weeks. A full system with segmentation, rich push, analytics, and A/B testing — 4–8 weeks. Order an audit of your current push infrastructure or get a consultation on implementing push notifications in your mobile app — we will contact you within a day and provide an accurate estimate.