Creating a Privacy-First Fitness App

From concept to creation

May 11, 2024 - Willem L. Middelkoop

As a little side-project, squeezed between my normal work, I have been working on something of personal interest: a native workout tracking app for iOS. I wanted to make my smartwatch obsolete, instead using my phone to track workouts. How hard could it be to gather detailed sensor data using native Swift APIs?

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I got some magic in my pocket: The app tracking workouts from the background - during recordings it just shows a widget on the homescreen

Early prototype of the app working with Bluetooth heartrate sensor and its early dataset shown in another app that makes some pretty graphs (HealthFit)

Different configurations of realtime data

Apple Watch workout app shows large buttons to start a workout - no need for additional configuration

Screenshots of the app's onboarding process: asking for permissions and guiding new users

"Goin' the extra mile": 8 hours on my bike to test battery usage (note: there is still 23% power left!)

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Articles (156)

2025-01-18 Master of Change
2024-12-19 Data versus Feeling
2024-12-09 The Problem With Bitcoin
2024-11-11 Write your own words
2024-11-10 Go Viral on X
2024-10-20 Marathon
2024-09-10 I see dead people
2024-05-11 Developing a native iOS app
2024-03-24 Half Marathon
2024-03-05 No, I Won't Build That!
2024-02-16 Vision Pro
2024-01-16 Health and fitness data
2023-12-15 Backup Rotation Scheme
2023-11-10 Apple Watch as Phone
2023-11-01 And... there is light!
2023-10-14 Giving AI Training
2023-10-13 Hello, stranger
2023-08-23 The Same, Differently
2023-05-31 Impact of training
2023-05-08 Tablet as Tool
2023-05-03 Using AI to generate code
2023-05-01 Working Offline First
2023-04-20 Tips for Long-Distance Cycling
2023-04-05 Cutting Cloud Costs
2023-02-08 Back to the University
2022-12-22 Rolex DateJust 36
2022-11-01 Moving to the countryside
2022-10-23 MUSE in Carré
2022-09-15 The Ultimate Freedom Bike
2022-09-11 Computer says no
2022-07-30 CAPTCHA Alternative
2022-07-17 Offline Music
2022-04-18 Snake '97 High Scores
2022-01-14 Wearing WHOOP 4.0
2021-12-09 For the love of mini
2021-11-19 Escaping the flat tires
2021-10-25 Using Google Free Android
2021-09-30 Inspiration from a child
2021-08-31 Protecting VMware ESXi
2021-07-13 The birth of a child
2021-06-30 Migrating to the Cloud
2021-06-05 Wearing Grand Seiko
2021-05-31 Using the reMarkable paper tablet
2021-05-04 Hospitalised with MCADD
2021-04-15 Enter text without keys
2021-04-10 Fixed Gear Gran Fondo
2021-03-26 Art of visualisation
2021-03-25 Three tips to make a content strategy work
2021-02-28 Wearing two watches
2021-02-22 The value of notifications
2021-02-02 Realtime service uptime monitoring
2021-01-31 Building a product platform
2021-01-14 Abandoning my own tablet OS
2021-01-10 Commuter bike checkup
2020-12-31 Working with special hardware from China
2020-12-15 Why Apple Silicon is a big deal
2020-12-02 Designing a multi dimensional capacity queue
2020-11-30 Designing my own watch
2020-11-22 Replacing a Shimano shift cable
2020-11-03 The joy of a simple laptop
2020-10-31 Increase traffic to your blog with a RSS feed
2020-10-30 How to use rsync to make backups
2020-10-15 Rescuing photos from a crashed iMac
2020-09-29 Riding with Omata One
2020-09-28 Refining my tablet OS experience
2020-09-16 Syncing files seamlessly between smartphone and tablet
2020-08-31 Free from the iCloud: Escaping Apple Photos
2020-08-23 Cleaning a vintage watch
2020-08-11 The best keyboard for iPad
2020-07-31 Influencing purchase behaviour
2020-07-30 Upgrading a 27-inch iMac 5K to 14 terabytes
2020-07-09 Repairing a Giro AEON bicycle helmet
2020-06-24 Some thoughts on touch screen user interface design
2020-06-17 Tudor Black Bay 36 long-term review
2020-06-12 Linking Lemmid Store with kitchens
2020-05-25 Different ways to visualise health
2020-05-11 Scalable application design without magic
2020-05-01 Designing an interface for a food ordering page
2020-04-30 Minimalistic road bike with Gates carbon drive
2020-04-14 Talking tablets: what makes a great tablet?
2020-04-01 Traffic shaping using iptables and tc
2020-03-25 Designing and implementing a (micro) payment system
2020-03-09 Making my own tablet OS
2020-03-03 Monolithic vs Microservices software architecture
2020-02-28 Your own addressbook and calendar cloud
2020-02-13 The day I sold my car
2020-01-28 Measuring cholesterol levels in blood
2020-01-24 Improve your sleep by blocking light
2019-12-31 Whoop strap review: 24/7 wearable sensor
2019-12-01 Realtime human pose recognition through computer vision
2019-11-26 Supernova E3 Pro 2 dynamo powered headlight
2019-11-22 Building a professional 72TB NAS
2019-10-19 Flying in a C47-A (DC-3) classic plane
2019-10-07 Multi-Window support in your iPad app
2019-09-27 Visiting an international hackers conference
2019-09-10 Fighting backscatter spam at server level
2019-08-28 Riding with a power meter on your bike
2019-08-18 Creating a minimal road bike
2019-07-16 Limitations of sleep tracking using a wearable
2019-07-15 Optical vs chest strap heart rate monitors
2019-06-25 Assembling a cargo bike
2019-06-24 Six months with MCADD
2019-05-29 Solo around the Markermeer (211KM)
2019-05-24 Dig for dummies
2019-04-30 The best bike computer app: Cyclemeter
2019-04-25 Search like a pro: Google search operators
2019-03-31 WordPress: 10 tips to secure your website
2019-03-20 Servicing a robot vacuum cleaner
2019-02-28 Understanding the security concerns in shared hosting
2019-02-27 One year on the ultimate commuter bike
2019-01-29 Born with MCADD
2019-01-24 My first weeks as dad
2018-12-22 Business in a bag
2018-12-06 Do amazing things with a focused mind
2018-11-16 Around town in an electric car
2018-11-02 Is the Apple Watch the modern tool watch?
2018-10-16 Rescuing files from a broken harddisk
2018-10-09 Using a budget Android as main smartphone
2018-09-27 Swimming and cycling with Apple Watch
2018-09-26 Optimising images for the web and performance
2018-08-21 Space travelling from El Teide
2018-08-20 Google Analytics vs AWStats
2018-07-20 Helping people with free software
2018-07-10 Enjoy a festival without alcohol
2018-06-09 Removing paint from a bike frame
2018-06-02 Creating a minimal iPhone stand
2018-05-17 Listen to your body
2018-05-09 Lunch by plane
2018-04-30 Off the grid: Working on 100% renewable energy
2018-04-18 Outside the comfort zone: amateur acting
2018-03-10 Cyber security: 5 easy tips to protect your server against hackers
2018-03-07 Tablet as main computer
2018-02-24 Creating the ultimate commuter bike
2018-02-21 Updating Snake '97
2018-01-21 Something the smartwatch will never have: patina
2018-01-08 Lessons from a takeaway plastic bag
2017-12-02 Commuting by bike
2017-11-15 Collecting health data with Biostrap
2017-10-23 From tree to table
2017-09-14 Saving a MacBook Air with exploded battery
2017-09-02 Climbing Mount Etna
2017-08-31 Clouds below my floor
2017-07-31 The day I killed my LAN
2017-06-30 Fietselfstedentocht 2017
2017-05-31 Beast of the Green Hell
2017-04-30 Smartwatches vs Mechanical watches
2017-03-26 Repairing a Nintendo Gameboy Advance SP
2017-02-16 Programming on Apple Watch
2017-01-30 iPad Pro next to my primary machine
2016-12-31 One year of cycling
2016-11-21 Being a volunteer in Amsterdam-Zuidoost
2016-10-01 Harvesting boredom to let creativity flourish
2016-09-03 Batavus Champion bike from 1978
2016-08-15 iPad Pro as primary computer
2016-07-15 Cash is king, or is it?
2016-06-23 Building the blog... in style!