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Denis Hennessy
Verified Expert in Engineering
Software Developer
Greystones, Ireland
Toptal member since November 4, 2020
Bio
Denis is a passionate engineer who loves building products that people actually use. In particular, he's been building iOS apps since the first iPhone launched. He has a high level of expertise in Swift and Objective-C. Since a modern iOS app is rarely a standalone entity, he's also worked with many external frameworks, including building REST endpoints where SDKs didn't already exist. He's also got experience building large server products, as well as some embedded-systems work.
Portfolio
Advance Local
Swift, SwiftUI, Firebase, iOS, Android, RevenueCat, CI/CD Pipelines, Analytics...
Peer Assembly
Objective-C, Swift, GraphQL, GraphQL API, UIKit, CI/CD Pipelines...
Major US Media Company
REST, Core Data, Swift, Objective-C, UIKit, CI/CD Pipelines, Automated Testing...
Experience
- Linux - 20 years
- iOS - 12 years
- Objective-C - 10 years
- Swift - 6 years
- Python - 5 years
- REST - 5 years
- Cloud Firestore - 3 years
- SwiftUI - 2 years
Work Experience
Mobile Systems Architect
2018 - PRESENT
Advance Local
- Managed the evolution of a suite of mobile consumer apps from a dated, Objective-C codebase to a modern Swift/SwiftUI platform (and Kotlin on Android).
- Acted both as a manager and architect for the mobile solution and as an individual contributor on the iOS codebase.
- Established a test suite and CI/CD pipeline, which enabled us to ship an average of over 500 releases per year.
Technologies: Swift, SwiftUI, Firebase, iOS, Android, RevenueCat, CI/CD Pipelines, Analytics, REST, Publishing, Media, Mobile Development, iOS SDK, Cloud Firestore, Software Development, iOS Native Libraries, App Store, Deep Linking, Model View ViewModel (MVVM), WidgetKit, TestFlight, App Store Acceptance Process, App Intents, iOS App Development, Native Mobile Apps, Mobile UI Design, User Experience (UX), User Interface (UI), Bug Fixes, Visual Studio Code (VS Code), Software, Video Streaming, Mobile App Development, App Store Connect, APIs, SDKs, Software Development Toolkits (SDK), Kotlin, IT Security, Scalability, Claude, Claude Code, Payment APIs, Mobile, API Integration, eCommerce, Mobile Apps, Amazon Web Services (AWS)
Principal Developer
2007 - PRESENT
Peer Assembly
- Developed a medicine dose tracking app for a pharmaceutical company that provided consumers of the drug an easy-to-use way to track their usage while providing a significant market advantage for the company.
- Built a language-learning app for teaching Hebrew to children. I managed all phases of the project, from requirements capture and development to testing and successful launch.
- Developed a mobile game for a US fast-food chain. Besides being fun to play, the game was also an important part of their customer loyalty program.
Technologies: Objective-C, Swift, GraphQL, GraphQL API, UIKit, CI/CD Pipelines, Automated Testing, ESP32, Embedded Systems, Firmware, FreeRTOS, Serial Peripheral Interface (SPI), Architecture, Software Architecture, Technical Leadership, Back-end Development, Python, PostgreSQL, Mobile Development, Back-end Architecture, MQTT, Internet of Things (IoT), Web Development, iOS SDK, iOS, Cloud Firestore, RevenueCat, Software Development, iOS Native Libraries, App Store, TestFlight, App Store Acceptance Process, iOS App Development, Native Mobile Apps, Mobile UI Design, User Experience (UX), User Interface (UI), Django, Bug Fixes, React, Embedded C, Visual Studio Code (VS Code), Fractional CTO, Large Language Models (LLMs), Agentic AI, Artificial Intelligence (AI), Software, SDK Development, AVFoundation, Full-stack, Video Processing, Video Streaming, Mobile App Development, App Store Connect, APIs, React Native, TypeScript, Full-stack Development, Embedded Software, RS-232, SDKs, Software Development Toolkits (SDK), Claude, Claude Code, Mobile, API Integration, eCommerce, Mobile Apps, Tailwind CSS, Supabase, Amazon Web Services (AWS)
Principal iOS Developer
2018 - 2023
Major US Media Company
- Converted a large portfolio of apps from an ad-supported revenue strategy to a subscription-based offering.
- Managed a major project evolution from a primarily Objective-C codebase to a primarily Swift codebase, while significantly enhancing the app feature set.
- Established a continuous-integration system to handle the quality assurance and automated release process.
Technologies: REST, Core Data, Swift, Objective-C, UIKit, CI/CD Pipelines, Automated Testing, Architecture, Payment Gateways, Software Architecture, Technical Leadership, Mobile Development, iOS SDK, Software Development, iOS App Development, Native Mobile Apps, Mobile UI Design, User Experience (UX), User Interface (UI), Bug Fixes, Software, Mobile App Development, App Store Connect, APIs, Software Development Toolkits (SDK), Mobile, API Integration, Mobile Apps
Principal iOS Developer
2016 - 2018
Makalu Inc
- Took over development of their iOS location bookmarking app, updating it with the latest iOS platform features and adding new photo capabilities.
- Developed the "event app" for a major annual rock concert under demanding schedule constraints.
- Developed an educational app which encouraged children to write letters to elected officials.
Technologies: Core Data, Swift, Objective-C, UIKit, Automated Testing, Software Architecture, iOS SDK, iOS, Software Development, iOS Native Libraries, App Store, TestFlight, App Store Acceptance Process, iOS App Development, Native Mobile Apps, Mobile UI Design, User Experience (UX), User Interface (UI), Bug Fixes, Software, Full-stack, Mobile App Development, App Store Connect, APIs, React Native, SDKs, Software Development Toolkits (SDK), Mobile, Mobile Apps
Principal iOS Developer
2014 - 2016
Time Marketing
- Developed an innovative wine tasting and cataloging app for serious wine enthusiasts.
- Worked with the project owner and external design freelancer to come up with a unique look and feature set.
- Managed the evolution of the app from a paid-up-front model to a free-with-IAP model.
Technologies: Core Data, Swift, Objective-C, UIKit, CI/CD Pipelines, Automated Testing, Architecture, Payment Gateways, iOS SDK, iOS, RevenueCat, Software Development, iOS Native Libraries, App Store, TestFlight, App Store Acceptance Process, iOS App Development, Native Mobile Apps, Mobile UI Design, User Experience (UX), User Interface (UI), Bug Fixes, Software, Mobile App Development, App Store Connect, APIs, Software Development Toolkits (SDK), Mobile, Mobile Apps
CTO and Co-founder
1999 - 2007
Valista Inc
- Handled the architecture of Valista's range of telco-grade payment and messaging products.
- Managed a distributed development team with centers in Ireland, Sri Lanka, and California.
- Set up a continuous integration system to improve the quality of software releases and increase the pace of updates.
Technologies: Ruby, Java, C++, C, CI/CD Pipelines, Automated Testing, Architecture, Payment Gateways, Software Architecture, Technical Leadership, Back-end Development, Python, PostgreSQL, Back-end Architecture, Web Development, Software Development, User Experience (UX), User Interface (UI), Embedded C, Visual Studio Code (VS Code), CTO, Software, SDK Development, Full-stack, APIs, Full-stack Development, SDKs, Software Development Toolkits (SDK), IT Security, Scalability, Payment APIs, eCommerce
Project History
Lines of Zen
https://linesofzen.comLines of Zen is a new kind of meditation app. Instead of trying to empty your mind, you gently focus it by following a series of guided drawing exercises. It's built using Swift and SwiftUI, employing Firebase as a back-end and RevenueCat for subscription management. I was responsible for all development of the project.
Rego - Bookmark Your Places
https://regoapp.comRego is an app that lets you easily save "bookmarks" for places you've visited, and attach photos and other data. People use it to save favorite tourist spots, restaurants, and even parking places. The app was originally an upfront payment app and is now free-with-IAP.
Goal Streaks
http://www.goalstreaks.comGoal Streaks helps you achieve your goals by tracking your daily progress and encouraging you to stick with it for longer and longer streaks. The idea of maintaining a continuous streak is a great motivator to help users stick to a project when they might otherwise quit.
Focus Time
https://focustimeapp.comFocus Time is an app that helps you practice the Pomodoro Technique—a way of achieving distraction-free work by alternating periods of intense focus, and short breaks to clear your head. I originally wrote the app in 2012.
Skills
Libraries/APIs
iOS Native Libraries, GraphQL API, React
Tools
Xcode, MQTT, TestFlight, Claude, Claude Code
Languages
Swift, Objective-C, C, Embedded C, Python, Ruby, GraphQL, Embedded C++, TypeScript, Kotlin, C++, Java
Frameworks
iOS SDK, SwiftUI, UIKit, WidgetKit, App Intents, Django, React Native, Tailwind CSS, Core Data
Paradigms
REST, Automated Testing, Mobile Development, Back-end Architecture, Mobile UI Design, Model View ViewModel (MVVM)
Platforms
MacOS, Linux, iOS, Firebase, RevenueCat, Visual Studio Code (VS Code), Raspberry Pi, Mobile, Amazon Web Services (AWS), Android
Storage
SQLite, PostgreSQL, Cloud Firestore
Other
Software Development, CI/CD Pipelines, Architecture, Payment Gateways, Software Architecture, Technical Leadership, Internet of Things (IoT), Web Development, App Store, Deep Linking, App Store Acceptance Process, iOS App Development, Native Mobile Apps, User Experience (UX), User Interface (UI), Bug Fixes, CTO, Fractional CTO, Large Language Models (LLMs), Agentic AI, Artificial Intelligence (AI), Software, SDK Development, AVFoundation, Full-stack, Video Processing, Video Streaming, Mobile App Development, App Store Connect, APIs, Full-stack Development, Embedded Software, RS-232, SDKs, Software Development Toolkits (SDK), IT Security, Scalability, Payment APIs, API Integration, Mobile Apps, Supabase, Embedded Systems, Firmware, Serial Peripheral Interface (SPI), Back-end Development, eCommerce, ESP32, FreeRTOS, Analytics, Publishing, Media
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