The Fitbit Ionic has a pretty open JavaScript SDK that we’re still working hard on to improve. You have access to a lot of the device public API, and can share your apps via links without getting it approved for the gallery.

https://dev.fitbit.com/

There once was Pebble.

You could write dev apps in a straightforward easy to program no lock-in no review workflow.

And then guess who buys them out and flushes all that dev goodness in the toilet?

Yes, fitbit.

There still is pebble. I recently found my pebble time at the back of a desk after not using it for over a year. I was sure it'd be a crippled mess without cloud services and I'd have to jump through hoops to make it do anything useful. I was pleasantly surprised to find that the community is still strong and the watch works even better now without any cloud requirements. Features like voice to text which I recall being clunky and error prone work like a charm. I was worried I'd have to write my own watchface but there are literally hundreds available to choose from. The benefit of fostering an open ecosystem from day one is paying off even a year after the company's "demise." I suggest picking one up on eBay.

The Fitbit SDK is a direct continuation of a lot of the awesome work Pebble did, much of it by the same people. The Ionic may be aimed at a slightly different market, but we’re still working hard at trying to keep that dev goodness alive.

There was once Pebble.

They went out of business and their assets went up for sale.

Then guess who buys some of their assets and keeps some of that dev goodness alive?

Yes, Fitbit.

"Principal Software Engineer, Fitbit, Inc."

Common practice here is to disclose that when making comments...

You can find many examples in my comment history where I mention that I work for but don't speak for Fitbit. I don't feel it's necessary to mention it for extremely basic fact correction as in this case. The details of what was and wasn't acquired are public and widely known.

Garmin. https://developer.garmin.com/connect-iq/

I have experince coding on all three platforms (Pebble, FitBit Ionic, and Garmin). If you’re serious about developing fitness apps, then Garmin is a great option. The language is easy to pick up, and the SDK and ecosystem make it simple to access, stream, and record sensor data.

@mettamage, maybe Aidlab will fit in your needs (https://www.aidlab.com -> check Aidlab for Developers part) - it's an open health tracker (open to communicate with and create apps full of body information for anything that supports Bluetooth 4). SDK supports Python, Unity, iOS and Android bindings.