Krystian is available for hire
Hire KrystianKrystian Wsul
Verified Expert in Engineering
Mobile App Development Developer
Location
Málaga, Spain
Toptal Member Since
March 3, 2020
Krystian is a seasoned Android developer with over a decade of experience across various software roles. He manages the full app lifecycle—from initial design and deployment, to ongoing growth and enhancement. He has helped launch startups, modernized legacy systems, and worked across entire tech stacks. Adept in both dynamic startups and structured corporate settings, Krystian excels both in roles that demand a wide-ranging skill set, and deep technical knowledge.
Portfolio
Goldman Sachs - Main
Android, Kotlin, Mobile App Development, Technical Leadership, Android API...
Mobile Motion
SQL, Mobile App Development, Firebase, Scrum, Continuous Integration (CI)...
Freelance Work
Video Streaming, Mobile App Development, Video & Audio Processing, GraphQL, SQL...
Experience
Availability
Full-time
Preferred Environment
IntelliJ IDEA, Android Studio
The most amazing...
...project I've worked on is a social video app that reached 100+ million downloads, caught the attention of major platforms, and was acquired by Reddit.
Work Experience
Senior Android Engineer
2021 - 2024
Goldman Sachs - Main
- Participated in the comprehensive rewrite and rebranding of the legacy Marcus Android app, updating it to use Compose, unidirectional data flow, coroutines, and Clean Architecture.
- Led the rewrite of UK savings accounts, and contributed to the development of other major app features, such as credit cards.
- Supported app production by monitoring releases, triaging and delegating runtime issues, and writing hotfixes where needed.
- Conducted technical interviews for Android engineering candidates, providing detailed evaluations to hiring managers to support recruitment decisions, and actively participated in the mentorship of junior team members.
Technologies: Android, Kotlin, Mobile App Development, Technical Leadership, Android API, Android SDK, Android Studio, Dagger 2, Git, JUnit, Reactive Programming, Functional Reactive Programming, Retrofit 2, RxJava 2, Agile, Continuous Integration (CI), Model View Intent (MVI), Scrum, Charles Proxy, Coroutines, GraphQL, Jetpack Compose, Room, GitLab, Unidirectional Data Flow, Mobile
Senior Android Engineer
2020 - 2021
Mobile Motion
- Introduced innovative features in a social video app, culminating in over 100 million downloads and the app's subsequent acquisition by Reddit.
- Engineered advanced video editing tools, including filters, clip composition, live streaming, and interactive stickers/effects, significantly improving the app's creative capabilities.
- Transitioned the app from Java MVP to Kotlin MVI architecture, improving app maintainability.
- Contributed extensively to the development of additional functionalities such as real-time chats and in-app payments.
- Participated in a mature, well-organized Scrum process, with cross-functional teams that rapidly delivered and iterated on feature development.
Technologies: SQL, Mobile App Development, Firebase, Scrum, Continuous Integration (CI), Video Streaming, Video & Audio Processing, Reactive Programming, Model View Intent (MVI), Model View Presenter (MVP), Google APIs, Room, Dagger 2, GraphQL, RxJava 2, Kotlin, Java, Android, Android Studio, Android SDK, JUnit, Mockito, Android API, Git, ExoPlayer, Functional Reactive Programming, Agile, Mobile
Freelance Full-Stack Android Engineer
2017 - 2020
Freelance Work
- Served as the lead Android engineer for a white-label carsharing app, achieving hundreds of thousands of installations across Europe and Asia.
- Contributed as an Android engineer to a platform that enhances home delivery services from local businesses.
- Developed an MVP for an app that engages users with marketing content to earn points, which can be redeemed for ad-free experiences within partner applications.
- Created a social, video-based Android app for sharing smartphone reviews on websites.
- Developed an Android app designed to help users not familiar with social media to exchange greetings for birthdays, anniversaries, and other significant occasions.
- Engineered an Android application for a security contractor that supports the collaborative creation of security assessment reports, featuring editable multimedia annotations and geospatial data.
- Built both an MVP Android app and web-based administrative tools for a marketplace platform focused on the exchange of clothing items.
- Designed an Android app that assists users in planning and coordinating menus for dinner parties.
Technologies: Video Streaming, Mobile App Development, Video & Audio Processing, GraphQL, SQL, Continuous Integration (CI), Full-stack, Model View ViewModel (MVVM), Model View Intent (MVI), Model View Presenter (MVP), Dagger 2, Accelerometers, Cloud Firestore, JWPlayer, OpenStreetMap API, WhatsApp API, Branch.io, Mapbox SDK, Near-field Communication (NFC), Reactive Programming, GPS, Realm, Google Location API, Google APIs, Material Design, Firebase Realtime Database, RxJava 2, Android Studio, Bluetooth, Yelp API, Facebook API, Test-driven Development (TDD), Google Drive API, Google Maps SDK, Google App Engine, SQLite, AdMob, Google Cloud Platform (GCP), Firebase, Node.js, Java, Kotlin, Android, Android SDK, JUnit, Mockito, Retrofit 2, Android API, Git, ExoPlayer, Functional Reactive Programming, Technical Leadership, Relational Database Design, Firebase Cloud Functions, Data Science, Mobile, Architecture
Full-Stack Engineer
2014 - 2017
MaxiPizza
- Developed an Android application for customer order placement and delivery tracking.
- Created a Android and iOS apps to aid delivery drivers with order details and route navigation.
- Overhauled the company's website and online ordering system.
- Developed desktop software for generating and printing customer invoices, and managing employee payroll and work schedules.
- Implemented automation for integrating orders with third-party order placement and delivery services.
- Redesigned intranet web tools for restaurant and franchise management.
- Automated email and SMS promotional campaigns to boost customer engagement and sales.
Technologies: Mobile App Development, SQL, Swift, Full-stack, Flutter, Model View ViewModel (MVVM), Model View Intent (MVI), Model View Presenter (MVP), Dagger 2, IntelliJ IDEA, Reactive Programming, GPS, Google Location API, Google APIs, Material Design, RxJava 2, iOS, Visual Studio, Android Studio, Facebook API, Mobile Payments, Test-driven Development (TDD), Google Maps SDK, Firebase, MySQL, JavaScript, CSS, HTML, PHP, Ruby, MaxDB, ASP.NET, C#, Java, Kotlin, Android, Android SDK, JUnit, Mockito, Android API, Git, Functional Reactive Programming, Relational Database Design, Technical Leadership, Data Science, .NET, Mobile, Architecture
Network Technician
2006 - 2011
University of Wisconsin-La Crosse
- Automated the deployment of Cisco routers and computer network configurations.
- Installed and maintained fiber-optic and copper cabling.
- Performed remote and on-site diagnostics of networked devices.
- Developed and maintained the networking department's web page.
Technologies: Ruby, Cisco
Release Engineer
2007 - 2010
SAP BusinessObjects
- Built a centralized build server to replace individual build servers used by seven teams on site.
- Designed an intranet web page to display build results for all teams.
- Automated nightly builds along with sending build reports to team managers.
Technologies: SQL, Apache Ant, Perforce, Ruby
Experience
Marcus by Goldman Sachs
https://marcus.comGoldman Sachs' personal finance app.
As a senior engineer, I participated in the comprehensive rewrite and rebranding of the legacy Marcus Android app, modernizing both the UI and architecture with Jetpack Compose and Kotlin Coroutines. I significantly contributed to the development and integration of major features, including credit card support, and led the redesign of UK savings accounts.
My role also included maintaining app stability by monitoring releases, managing runtime issues through triage and delegation, and writing necessary hotfixes. Additionally, I supported the team expansion by conducting technical interviews and mentoring junior team members.
As a senior engineer, I participated in the comprehensive rewrite and rebranding of the legacy Marcus Android app, modernizing both the UI and architecture with Jetpack Compose and Kotlin Coroutines. I significantly contributed to the development and integration of major features, including credit card support, and led the redesign of UK savings accounts.
My role also included maintaining app stability by monitoring releases, managing runtime issues through triage and delegation, and writing necessary hotfixes. Additionally, I supported the team expansion by conducting technical interviews and mentoring junior team members.
Dubsmash
http://www.dubsmash.comA social video app designed for video creation and interaction with multimedia content. The app achieved over 100 million downloads and was ultimately acquired by Reddit.
As a senior Android engineer, I introduced advanced video editing tools including filters, clip composition, live streaming, and interactive stickers/effects, which significantly enhanced the app's creative capabilities and user engagement. I migrated the app's architecture from Java MVP to Kotlin MVI, improving both performance and maintainability. My contributions also extended to developing key functionalities such as real-time chats, in-app payments, and streamlined phone registration processes. I participated actively in a mature, well-organized Scrum process, working closely with cross-functional teams to ensure rapid and iterative feature development.
As a senior Android engineer, I introduced advanced video editing tools including filters, clip composition, live streaming, and interactive stickers/effects, which significantly enhanced the app's creative capabilities and user engagement. I migrated the app's architecture from Java MVP to Kotlin MVI, improving both performance and maintainability. My contributions also extended to developing key functionalities such as real-time chats, in-app payments, and streamlined phone registration processes. I participated actively in a mature, well-organized Scrum process, working closely with cross-functional teams to ensure rapid and iterative feature development.
CarTrek
https://cartrek.online/A white-label car-sharing app serving companies with clients across Europe and Asia, enabling the rental of both private and company-owned cars.
As the lead Android developer, I was responsible for implementing significant new features, including support for multiple map providers and advanced payment processing. Additionally, I managed the app's customization for rebranding to various client specifications, and performed ongoing maintenance and modernization of the app and infrastructure.
As the lead Android developer, I was responsible for implementing significant new features, including support for multiple map providers and advanced payment processing. Additionally, I managed the app's customization for rebranding to various client specifications, and performed ongoing maintenance and modernization of the app and infrastructure.
Shadda
A platform designed to bridge the gap between clients and local businesses capable of home deliveries but lacking proprietary ordering infrastructure, with separate apps for both clients and businesses.
As the Android developer for the business app, I integrated key functionalities including Google Maps and Mapbox navigation. My responsibilities also involved communicating with the Firebase backend and executing a comprehensive visual redesign of the app.
As the Android developer for the business app, I integrated key functionalities including Google Maps and Mapbox navigation. My responsibilities also involved communicating with the Firebase backend and executing a comprehensive visual redesign of the app.
Airon
Airon was a platform that enabled users to post and browse video reviews of smartphones, enriched with social media features.
As the sole Android developer and lead architect, I designed and developed the Android app, ensuring seamless integration with the other platforms and video hosting service. I played a major role in both the UX and UI design processes and led architectural decisions for the iOS and Web clients, as well as the backend infrastructure. My responsibilities also included integrating social functionalities to actively engage the user community.
As the sole Android developer and lead architect, I designed and developed the Android app, ensuring seamless integration with the other platforms and video hosting service. I played a major role in both the UX and UI design processes and led architectural decisions for the iOS and Web clients, as well as the backend infrastructure. My responsibilities also included integrating social functionalities to actively engage the user community.
FireWatch Solutions (Internal-use App)
https://www.firewatchsolutions.com/FireWatch Solutions specializes in providing tailored security services to clients traveling to high-risk areas around the world.
I developed an internal-use Android app designed specifically for employees conducting collaborative security evaluations at future client destinations. As the sole developer, my responsibilities included enabling the capture and integration of multimedia and geospatial data, the construction of detailed area maps, and the creation of custom graphical elements such as floor plans.
I developed an internal-use Android app designed specifically for employees conducting collaborative security evaluations at future client destinations. As the sole developer, my responsibilities included enabling the capture and integration of multimedia and geospatial data, the construction of detailed area maps, and the creation of custom graphical elements such as floor plans.
MaxiPizza
https://www.maxipizza.pl/MaxiPizza is a pizza restaurant chain with numerous locations across Poland.
As the second member of a growing in-house development team, I was responsible for architecting and developing the comprehensive technological framework spanning multiple platforms, including Windows and Unix servers, various databases, and communication protocols. My role as the chief developer involved building the backend infrastructure, a robust online ordering system, and Android applications for customer order placement and delivery tracking. Additionally, I engineered desktop software for generating invoices and managing employee tasks, integrated third-party ordering services, and redesigned intranet tools for restaurant management, and implemented automated email and SMS marketing campaigns.
As the second member of a growing in-house development team, I was responsible for architecting and developing the comprehensive technological framework spanning multiple platforms, including Windows and Unix servers, various databases, and communication protocols. My role as the chief developer involved building the backend infrastructure, a robust online ordering system, and Android applications for customer order placement and delivery tracking. Additionally, I engineered desktop software for generating invoices and managing employee tasks, integrated third-party ordering services, and redesigned intranet tools for restaurant management, and implemented automated email and SMS marketing campaigns.
TimeGuru
https://avibortnick.com/time-guru/An Android app designed as a sophisticated metronome for musical practice, equipped with advanced features including the ability to handle combinations of time signatures.
Over the years, my role has encompassed implementing new functionalities, executing comprehensive app redesigns, and maintaining overall app performance to ensure reliability and user satisfaction.
Over the years, my role has encompassed implementing new functionalities, executing comprehensive app redesigns, and maintaining overall app performance to ensure reliability and user satisfaction.
Skills
Languages
Kotlin, Java, SQL, C#, HTML, CSS, Swift, GraphQL, PHP, JavaScript, Ruby
Frameworks
Android SDK, ASP.NET, Dagger 2, JUnit, Mockito, .NET, Flutter
Libraries/APIs
RxJava 2, Android API, Google Location API, Google APIs, Google Maps SDK, Google Drive API, Retrofit 2, Node.js, Facebook API, OpenStreetMap API, Yelp API, WhatsApp API, Accelerometers
Tools
Android Studio, Visual Studio, Git, IntelliJ IDEA, Perforce, Apache Ant, JWPlayer, Branch.io, GitLab, ExoPlayer, Charles Proxy
Paradigms
Model View Presenter (MVP), Model View Intent (MVI), Model View ViewModel (MVVM), Reactive Programming, Functional Reactive Programming, Test-driven Development (TDD), Agile, Continuous Integration (CI), Scrum, Data Science
Platforms
Android, Mobile, Firebase, Google Cloud Platform (GCP), Google App Engine, iOS
Storage
Firebase Realtime Database, MySQL, SQLite, Realm, MaxDB, Cloud Firestore
Other
Full-stack, Mobile App Development, GPS, Video & Audio Processing, Video Streaming, Material Design, AdMob, Jetpack Compose, Coroutines, Relational Database Design, Firebase Cloud Functions, Technical Leadership, Architecture, Cisco, Room, Near-field Communication (NFC), Mapbox SDK, Mobile Payments, Bluetooth, Unidirectional Data Flow
Education
2006 - 2010
Bachelor of Science Degree in Computer Science, Mathematics, Physics
University of Wisconsin-La Crosse - La Crosse, WI, USA
Collaboration That Works
How to Work with Toptal
Toptal matches you directly with global industry experts from our network in hours—not weeks or months.
1
Share your needs
Discuss your requirements and refine your scope in a call with a Toptal domain expert.
2
Choose your talent
Get a short list of expertly matched talent within 24 hours to review, interview, and choose from.
3
Start your risk-free talent trial
Work with your chosen talent on a trial basis for up to two weeks. Pay only if you decide to hire them.
Top talent is in high demand.
Start hiring