I’m working on a project where I need to get temperature from a wireless sensor into my phone app. The exact sensor doesn’t matter – it just needs to be inexpensive. Decent battery life would also be nice. And so began yet another typical “searching for a sensor” trip for me.
After a little research, I ordered a SensePeanut. Looked great. Decent prototype cost. Available off-the-shelf and in a package. Just what I need! Except I got it in hand only to find out that you can get data from it only using their app, which they don’t give out the source for. Soon they’ll have a “cloud SDK” that will let me get the data, but for a whole host of reasons, that just isn’t an option.
So I did a bit more research. I needed a sensor that provided a “direct to the sensor” SDK. I ended up ordering some proximity beacons from Estimote. Proximity isn’t what I’m after, but they also can transmit temperature so it seemed promising.
Well last weekend I finally got time to start writing code. Turns out that the SDKs they provide are, at least as far as I can tell, native binary SDKs for iOS and Android. I’m a C# dev this week and I really want this to be cross platform, but they don’t have a Xamarin SDK. Someone at Xamarin did create an SDK for the devices a few years back, but that project doesn’t actually work with the latest Android SDK – discovery returns nothing. Evidently that’s not uncommon when using a library built against an older Android SDK.
Well it’s turns out that the existing Xamarin library is really just a bindings library that wraps the Java SDK. So let me get the latest Estimote SDK and recompile. Well, evidently the Estimote SDK is now deployed through JCenter (think NuGet for Java). That appears to require that I install Java tools and I really have no desire to pollute my working machine with a toolchain I don’t intend to use. No idea what can of worms that might open.
They don’t appear to have an AAR download, nor do they have the source for the SDK available – only source for some sample apps.
A little reverse-engineering with JCenter, however, let me to find the URL to direct-download the AAR. I pulled it down and created a Xamarin Bindings Library project targeting it. This is my first attempt at a Binding Library, but how hard could it be, right? Turns out that the project generates a load of error, and this is not at all uncommon for a Bindings Library. Auto-generating wrapper code is hard, I realize that. But I really don’t have the desire, or especially the time at this point, to climb the learning curve for Bindings Libraries, especially since I just want to read a simple temperature.
Maybe the existing Xamarin project can be used against the new Estimote AAR? Surely the original dev had to solve these errors, right? I loaded up the existing project and stuffed in the newer AAR. Build and…still a bunch of errors. Admittedly only about half the errors than the new project had, so it’s progress, but still isn’t getting me data.
I then spent a few hours reading and working on the project. It’s a steep learning curve with not a whole lot of “Bindings-Libraries-for-Dummies” tutorials out there. I got rid of maybe half the errors (the easy ones) and then got stuck.
Time to pivot again. There’s got to be a way to do this. Back to researching. I found a Windows 10 C# library that can read data from iBeacons and Eddystone. The Estimote can be configured to transmit the temperature in an Eddystone telemetry packet so that seems at least somewhat promising. At least I have full source for the library.
I ran the sample app on my laptop and, lo-and-behold, I see temperature data which is hugely promising. Again, it’s a Windows 10 library but I’ve been making a career out of making desktop code mobile for a long time so it’s certainly in my wheelhouse.
First task was to port the library to .NET Standard so I’d have some expectation I’d be able to use it over on Android and iOS. That required extracting the actual Bluetooth packet discovery out to an interface that was platform-specific. The starting code was pretty well thought out and constructed so the task was not too hard and I actually had a UWP app receiving and displaying the data in just a couple hours.
The next job was to create the Android implementation of the Bluetooth discovery bits. The UWP platform provided the Bluetooth data in a really nice format. I’m sure it looks that way since the starting code I was working with was built specifically for it. Fitting the reality of the packets that we get in Android into that structure wasn’t straightforward, but since Bluetooth and Eddystone are well documented it also wasn’t overly difficult. It took me about a day of working with it to understand the spec well enough to get the incoming packets integrated into library.
I got a hold of Andreas Jakl, the original library author, let him know about my fork, and requested collaborator status. He was gracious and added me to the project so my changes have all be back-integrated into the original tree, as well as fixes to the original sample non-Xamarin UWP app to use the new libraries. We’re in teh process of getting the new stuff out as NuGet packages.
Getting BLE temperature data into my app certainly wasn’t as straightforward as I had hoped when I first ordered a sensor. In fact, the journey to get there was nothing like what I’d hoped, or even expected once I started trying to write code for it. In the end, however, I got it working (well in UWP and Android anyway, iOS will have to wait until I start porting the app that started this whole thing) and the work is public and open source. I still have to manage the beacons with vendor-specific apps, which still irritates me, but at least we all now have a generic way to receive data from BLE beacons in our .NET apps.