GePP — Modernizing the GST E-Invoice Preparation & Printing System

GePP - Modernizing the GST E-Invoice Preparation & Printing System A Case Study on Migrating a Government Legacy App to Flutter 3.24 + Dart 3.9 (Null Safety). This article explains the modernization of the GePP (GST E-Invoice Preparation & Printing System), a critical tool for generating and printing e-Invoices aligned with India’s GSTN standards, using Flutter 3.24 and Dart 3.9.

Originally built with older technologies and desktop dependencies, the goal of this modernization was to create a cross-platform, secure, and responsive application that runs seamlessly on:

• Web
• Android / iOS

Objective

Re-engineer the legacy GePP desktop system into a unified, maintainable Flutter application—secure, offline-ready, and optimized for multi-platform deployment. Key outcomes:

• Modern Tech Stack: Migrate to Flutter 3.24 (Dart 3.9 Null Safety)
• Data Security: Encrypted local caching using Flutter Secure Storage
• Offline Support: Integrated Hive DB for caching & quick sync
• State Management: Provider → BLoC pattern migration for scalable logic
• Reports & Printing: Custom PDF templates using pdf + printing package
• Modular Code Base: Repository pattern + Dependency injection with GetIt
• User Experience: Animated transitions using flutter_animate

Tech Stack

• Layer: Technology, Framework
• Flutter: Flutter 3.24 (Dart 3.9 — Null Safety)
• State Management: Provider + BLoC
• Local DB: Hive
• Storage: Flutter Secure Storage
• Dependency Injection: GetIt
• Networking: HTTP Client + Interceptor
• Printing: PDF / Printing Plugin
• UI/UX: Material 3 + Custom Theme (Adaptive Light/Dark)
• Platform Targets: Android • iOS • Web

Architecture

A clean-architecture approach ensures scalability and testability.

lib/

├── core/

│ ├── theme/

│ ├── utils/

│ └── services/

├── data/

│ ├── datasources/

│ │ ├── remote/

│ │ └── local/

│ ├── repositories/

│ └── models/

├── presentation/

│ ├── blocs/

│ ├── screens/

│ └── widgets/

└── main.dart

Each layer communicates through abstract repositories, separating logic from UI.

Implementation Highlights

• Authentication Flow (BLoC): Implemented AuthBloc for managing login, OTP verify, resendOTP, and logout. Used event-state pattern for predictable transitions. Integrated secure storage to persist tokens offline.
• Dependency Injection with GetIt: This pattern eliminates tight coupling and improves testability.
• Dynamic PDF Generation: Invoices and student verification certificates are generated using the pdf + printing package.
• Offline Sync with Hive: Data such as user profiles, reports, and cached invoices are persisted locally. Offline-first design ensures the app remains usable during poor connectivity.
• UI & Theming: Implemented adaptive theming through a centralized AppTheme class. Each module (Login, Dashboard, Verification, Reports) adheres to Flutter’s responsive design for tablet and mobile layouts.
• Dashboard & Reporting: Visual summary of invoices generated, verified, and printed CSV export of queued logs for audit Backend link launcher to open trace IDs directly in Sentry or ELK dashboard Real-time performance snapshots (FPS + memory stats)
• Security & Audit: FeatureImplementationSecure StorageFlutter Secure Storage + AES EncryptionSensitive Data ProtectionHTTPs only + JWT Token AuthError LoggingCustom ErrorLogger wrapper with Trace IDsAccess ControlRole-based Auth + Offline Session Expiry

Each network call carries a Trace ID, allowing backend-to-frontend log correlation during debugging.

Performance Optimization

• Lazy loading for reports and student lists
• Deferred widgets for large screens
• Image compression for face matching module
• Reduced build repaints using ValueListenableBuilder

Key Challenges & Solutions

• Challenge: Migrating legacy code
• Solution: Complete rewrite in Dart 3.9 (Null Safety)
• Challenge: Offline storage consistency
• Solution: Hive DB + secure sync service
• Challenge: Theme and routing conflicts
• Solution: Centralized ThemeBloc + RouteGenerator
• Challenge: Dynamic PDF layouts
• Solution: Modular template system using pdf widgets
• Challenge: Integration with external GST APIs
• Solution: Async HTTP wrapper + error retries

Outcome

• ✅ Unified Flutter codebase
• ✅ 40 % faster load times (compared to legacy app)
• ✅ Simplified deployment via single CI/CD pipeline
• ✅ Consistent look & feel across web and mobile
• ✅ Secure offline operation for field officers

Next Enhancements

• Integration with real GST IRP APIs for direct IRN fetching
• Push notifications for IRN status updates
• Multi-tenant support for district-level offices
• Automated analytics dashboards in Firebase/ELK