FREELANCER PROFILE Senior Flutter Developer Kantar – Data Science Center (THX) — Groningen, Netherlands

About THX

THX is a division of Kantar and operates as a passive measurement platform across European and APAC markets. We combine ACR (audio content recognition), location tracking, and survey research to measure media exposure and advertising effectiveness for enterprise clients. Our consumer-facing platform — THX. — recently launched in the APAC region (Japan) and we are actively expanding into additional markets beyond our current footprint in the Netherlands, Belgium, France, Germany, UK, Spain, and Italy.
Our Data Science Center in Groningen is a young, professional, and talented team of 15+ specialists who work fast and deliver on a global scale. We move quickly, we solve problems at their root, and we build technology that works across markets and cultures.

The Role

We are looking for a Senior Flutter Developer with multi-year experience in handling large-scale refactoring projects within Flutter codebases. This is not a “attach frontend to endpoints” role. We need someone who deeply understands how mobile applications connect to backend services, how API integrations work end-to-end, and how to optimize that entire chain for performance and reliability at scale.
The THX. consumer platform runs on both iOS and Android across multiple European and Asian markets. You will be working on a codebase that serves real users in production, where decisions have immediate impact. You will be expected to identify architectural bottlenecks, propose solutions, and execute them — not wait for instructions.

What We’re Looking For

Technical Expertise

• Deep proficiency in Dart and the Flutter framework, with demonstrable experience on production applications used by a substantial user base across multiple markets
• Proven track record of leading or significantly contributing to large-scale codebase refactoring — migrating legacy architectures, implementing null safety, restructuring state management (BLoC, Riverpod, or equivalent), and reducing technical debt
• Strong understanding of RESTful API integration, GraphQL, and how to optimize network layer performance including caching strategies, request batching, error handling, and retry logic
• Experience with CI/CD pipelines for mobile (Fastlane, GitHub Actions, Bitrise) including automated testing, build distribution, and app store submission workflows
• Ability to diagnose and resolve performance bottlenecks: memory leaks, excessive widget rebuilds, rendering jank, and battery consumption issues using Flutter DevTools and platform-specific profilers
• Understanding of platform-specific considerations: push notifications, background processing, local storage, secure authentication flows, and offline-first capabilities

Beyond Frontend

• Clear understanding of backend architectures and how API contracts affect mobile performance and UX — ability to constructively challenge and improve API designs
• Experience with data synchronization patterns, conflict resolution for offline scenarios, and efficient data serialization
• Familiarity with cloud infrastructure concepts (AWS, GCP, or Azure) and how mobile clients interact with backend services at scale
• Knowledge of monitoring and observability from the mobile side: crash reporting (Firebase Crashlytics, Sentry), analytics integration, and real-user performance metrics

Mindset & Working Style

• Decisive: you assess situations quickly, form a clear opinion, and act on it
• Consulting: you communicate your reasoning, bring the team along, and help non-technical stakeholders understand trade-offs
• Solution engineer and fixer: when something is broken, you don’t just flag it — you fix it, and you make sure it stays fixed
• Comfortable working in a fast-moving team that ships across multiple international markets simultaneously
• Self-starter who can pick up context quickly from an existing codebase and team, without needing extensive onboarding

Practical Details

Engagement Freelance (via Octagon), with potential to convert to a permanent position Location Based in or around Groningen, Netherlands. Present in the Groningen office at least 3 days per week Duration Initial engagement 3–6 months, with extension likely based on performance and roadmap Availability Full-time preferred (32–40 hours/week) Start As soon as possible Rate Competitive, commensurate with experience — to be discussed
Note to Octagon: This role combines short-term freelance support with a long-term hiring opportunity. We are simultaneously opening a permanent position for this Flutter Developer role. If the freelance candidate proves to be a strong fit, conversion to a permanent contract is explicitly on the table.