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1 Feb 2016 : Pebble SDK Review #
Although Pebble smartwatches have been around for some time, I only recently became one of the converted after buying a second-hand Pebble Classic last October. Over Christmas I was lucky enough to be upgraded to a Pebble Time Round. This version was only released recently, and the new form factor requires a new approach to app development. Not wildly different from the existing Classic and Time variants, but enough to necessitate recompilation and some UI redesign of existing apps.

As a consequence many of the apps I'd got used to on my Classic no longer appear in the app store for the Round. This, I thought, offered a perfect opportunity for me to get to grips with the SDK by upgrading some of those that are open source.

Although I'm a total newb when it comes to Pebble and smartwatch development generally, I have plenty more experience with other toolchains, SDKs and development environments, from Visual Studio and QT Creator through to GCC and Arduino IDE, as well as the libraries and platforms that go with them. I was interested to know how the Pebble dev experience would compare to these.

It turns out there are essentially two ways of developing Pebble apps. You can use a local devchain, built around Waf, QEMU and a custom C compiler. This offers a fully command line approach without an IDE, leaving you to choose your own environment to work in. Alternatively there's the much slicker CloudPebble Web IDE. This works entirely online, including the source editor, compiler and pebble emulator.

CloudPebble IDE
I worked through some of the tutorials on CloudPebble and was very impressed by it. The emulator works astonishingly well and I didn't feel restricted by being forced to use a browser-based editor. What I found particularly impressive was the ability to clone projects from GitHub straight into CloudPebble. This makes it ideal for testing out the example projects (all of which are up on GitHub). Without having to clutter up your local machine. Having checked the behaviour on the CloudPebble emulator, if it suits your needs you can then easily find the code to make it work and replicate it in your own projects.

Although there's much to recommend it, I'm always a bit suspicious of Web-based approaches. Experience suggests they can be less flexible than their command line equivalents, imposing a barrier on more complex projects. In the case of CloudPebble there's some truth to this. If you want to customise your build scripts (e.g. to pre-generate some files) or combine your watch app with an Android app, you'll end up having to move your build locally. In practice these may be the fringe cases, but it's worth being aware.

So it can be important to understand the local toolchain too. There's no particular IDE to use, but Pebble have created a Python wrapper around the various tools so they can all be accessed through the parameters of the pebble command.
Pebble Tool command:

    build               Builds the current project.
    new-project         Creates a new pebble project with the given name in a
                        new directory.
    install             Installs the given app on the watch.
    logs                Displays running logs from the watch.
    screenshot          Takes a screenshot from the watch.
    insert-pin          Inserts a pin into the timeline.
    delete-pin          Deletes a pin from the timeline.
    emu-accel           Emulates accelerometer events.
    emu-app-config      Shows the app configuration page, if one exists.
    emu-battery         Sets the emulated battery level and charging state.
    emu-bt-connection   Sets the emulated Bluetooth connectivity state.
    emu-compass         Sets the emulated compass heading and calibration
    emu-control         Control emulator interactively
    emu-tap             Emulates a tap.
    emu-time-format     Sets the emulated time format (12h or 24h).
    ping                Pings the watch.
    login               Logs you in to your Pebble account. Required to use
                        the timeline and CloudPebble connections.
    logout              Logs you out of your Pebble account.
    repl                Launches a python prompt with a 'pebble' object
                        already connected.
    transcribe          Starts a voice server listening for voice
                        transcription requests from the app
    data-logging        Get info on or download data logging data
    sdk                 Manages available SDKs
    analyze-size        Analyze the size of your pebble app.
    convert-project     Structurally converts an SDK 2 project to an SDK 3
                        project. Code changes may still be required.
    kill                Kills running emulators, if any.
    wipe                Wipes data for running emulators. By default, only
                        clears data for the current SDK version.

Although it does many things, the most important are build, install and logs. The first compiles a .pbw file (a Pebble app, essentially a zip archive containing binary and resource files); the second uploads and runs the application; and the last offers runtime debugging. These will work on both the QEMU emulator, which can mimic any of the current three watch variants (Original, Time, Time Round; or aplite, basalt and chalk for those on first name terms), or a physical watch connected via a phone on the network.

CloudPebble IDE
It's all very well thought out and works well in practice. You quickly get used to the build-install-log cycle during day-to-day coding.

So, that's the dev tools in a nutshell, but what about the structure, coding and libraries of an actual app? The core of each app is written in C, so my first impression was that everything felt a bit OldSkool. It didn't take long for the picture to become more nuanced. Pebble have very carefully constructed a simple (from the developer's perspective) but effective event-based library. For communication between the watch and phone (and via that route to the wider Internet) the C hands over to fragments of Javascript that run on the phone. This felt bizarre and overcomplicated at first, but actually serves to bridge the otherwise rough boundary between embedded (watch) and abstract (phone) development. It also avoids having to deal with threading in the C portion of the code. All communication is performed using JSON, which gets converted to iterable key-value dictionaries when handled on the C side.

This seems to work well: the UI written in C remains fluid and lightweight with Javascript handling the more infrequent networking requirements.

The C is quite restrictive. For example, I quickly discovered there's no square root function, arguable one of the more useful maths functions on a round display (some trig is provided by cos and sin lookup functions). The libraries are split into various categories such as graphics, UI, hardware functions and so on. They're built as objects with their own hierarchy and virtual functions implemented as callbacks. It all works very well and with notable attention to detail. For example, in spite of it being C, the developers have included enough hooks for subclasses to be derived from the existing classes.

The downside to all of this is that you have to be comfortably multilingual: C for the main code and interface, Javascript for communication with a phone, Java and Objective-C to build companion Apps and Python for the build scripts. Whew.

Different people will want different things in a development environment: is it well structured? Does it support a developer's particular preference of language? Is it simple at the start but flexible enough to deal with more complex projects? Does it support different development and coding styles? How much boilerplate overhead is there before you can get going? How familiar does it all feel?

It just so happens that I really like C, but dislike Javascript, although I'm certain there are many more developers who feel the exact opposite. The Pebble approach is a nice compromise. I was happy dealing with the C and the Javascript part was logical (e.g. no need to deal with browser incompatibilities). If you're a died-in-the-wool Web developer, there's even a pre-build JS shim for creating watch faces.

So it also seems to work well together and I've come away impressed. Many developers will find the CloudPebble interface slicker and easier to use. But after wading through the underlying complexities – and opacity – of IDEs like Visual Studio and Eclipse, the thoughtful clarity of the Pebble SDK makes for a refreshing change. I wouldn't recommend it for a complete newcomer to C or JS, but if you have any experience with these languages, you'll find yourself up and running with the Pebble toolchain in no time.


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