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  • We can assume it’s obvious now that Apple Watch is works asynchronously, taking advantage of its bigger siblings faster processor, networking abilities, and battery. Interesting.. :)

    React-native is also asynchronous by nature! React-native let’s you write react code which then runs as native android and iOS applications, it does this in a similar way as us- generating a shadow tree which is then used to tell the native system what to draw, similar to us generating graphics commands. But unlike Espruino, it’s not like you can just load up some JavaScript in a native app and talk to the native view system. You need a bridge to communicate from a JavaScript context or VM to native and vice versa. In react-natives case, it has a queue of commands that build up on the JS side that describe things like layout (similar to our drawString!) native methods to call, etc. which occasionally gets “flushed” to the native side, where native code processes the commands and actually draws things on the screen. This is asynchronous, platform agnostic, and sounds familiar. Oh, and react-native also Yoga to implement flexbox.

    Now we know have some color on these two systems and we shall bring it together.

    From Apple Watch, we draw inspiration of its host / extension model. We will take advantage of phone for running our react code.

    We have Bluetooth on both our phone and the bangle, so we can easily communicate back and forth via Bluetooth.println and sending raw commands.

    Using our react-native inspired bridge, we can execute our react code on the phone, the react code which creates commands for drawing on the Espruino. Flush that to the native side, then use the BLE connection to transmit the drawing commands to the Bangle. And voila, we now have a way to do exactly the same thing we did earlier. Statically render content to the watch. Or did we?

    Now that the react code is constantly running on our phone, any changes to state will cause it (we are going to call it react-bangle) now to flush drawing commands for transmitting to the watch. This means things like timers will work! Which brings us to another super power we may have overlooked:

    We have access to real internet; at high fidelity (signal including)! Since the context our app is actually running in is on the phone in a full blown JS engine we have access to things like fetch! We can use our fancy timers and fetch and flexbox in react to now create a complex data view on our watch which updates automatically! Neat!

    You’ll probably have noticed a couple things by now but I’m going to point out the less obvious one first: in our case we use a phone as a host, but more specially it’s an app which uses JavaScriptCore as it’s JS engine for executing our watch app code. I drew parallels to react-native earlier because the phone app is architecturally similar, as is the react-reconciler. The biggest difference is we have a longer bridge: ours traverse JS -> Native -> BLE -> Watch. It’s basically the same thing, just a longer journey. That’s the less obvious and interesting fact!

    Now the more obvious thing: we like interacting. The bangle has setWatch commands for listening to button presses. We need to link that back across our bridge. This is done as follows: before sending the react generated command over Bluetooth, we wrap it in a few boilerplate commands which standardize 2 things:
    Capture all button presses and use Bluetooth.println to emit a chunk of JSON identifying the button and redrawing the originally sent command (so we don’t accidentally lose focus). On our host’s native side we then link these events directly to an event emitter on the hosts JavaScript engine. You can then subscribe to this event emitter as you normally would in your react watch app code and update state which... causes the entire process to be repeated and your watch will display the newly minted program with its fresh data!

    Wow, we’ve done it. We’ve figured out a way to run a react app which feels as if it were a native bangle app (given some lag, but you’d be surprised). So what’s next?

    Well, a couple things. For one- all we’ve managed to do is run a react app in a clobbered slow manner. Getting the JS files to execute is a pain, you must start the process from your phone, etc etc. so how do we make it easier? Well, we extend our native apps ability and do a little bootstrapping.


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