How Do I Wrap An Existing API?

One question that often comes up is how can I wrap an existing API into an Observable sequence? The answer is fairly simple and not a lot of lines of code to make that happen.

To make this a bit more concrete, let's take a familiar HTML5 API like Geolocation API, in particular, the navigator.geolocation.watchPosition method.

The typical use of this method might be the following where we would hook up an event handler to listen for success and errors on watching the geolocation by using the navigator.geolocation.watchPosition method. When one wishes to terminate listening for geolocation updates, you simply call the navigator.geolocation.clearWatch method passing in the watch ID returned from the watchPosition method.

function watchPositionChanged(e) {
    // Do something with the coordinates
}

function watchPositionError(e) {
    // Handle position error
}

var watchId = navigator.geolocation.watchPosition(
    watchPositionChanged,
    watchPositionError);

var stopWatching = document.querySelector('#stopWatching');
stopWatching.addEventListener('click', stopWatchingClicked, false);

// Clear watching upon click
function stopWatchingClicked(e) {
    navigator.geolocation.clearWatch(watchId)
}

In order to wrap this, we'll need to use the Rx.Observable.create method. From this, we can yield values to the observer or handle the errors. Let's see how the code might look, creating a watchPosition method which takes geolocation options.

function watchPosition(geolocationOptions) {
    return Rx.Observable.create(observer => {
        var watchId = window.navigator.geolocation.watchPosition(
            function successHandler (loc) {
                observer.onNext(loc);
            }, 
            function errorHandler (err) {
                observer.onError(err);
            }, 
            geolocationOptions);

        return () => {
            window.navigator.geolocation.clearWatch(watchId);
        };
    });
 }

We need to also be aware of ensuring we're not adding too many watchPosition calls as we compose it together with other observable sequences. To do that, we'll need to utilize the publish and refCount methods from rx.binding.js.

Our final result should look like the following:

function watchPosition(geolocationOptions) {
    return Rx.Observable.create(observer => {
        var watchId = window.navigator.geolocation.watchPosition(
            function successHandler (loc) {
                observer.onNext(loc);
            }, 
            function errorHandler (err) {
                observer.onError(err);
            }, 
            geolocationOptions);

        return () => {
            window.navigator.geolocation.clearWatch(watchId);
        };
    }).publish().refCount();
 }

And now we can consume the geolocation such as:

var source = watchPosition();

var subscription = source.subscribe(
    position => console.log(`Next: ${position.coords.latitude}, ${position.coords.longitude}`),
    err => {
        var message = '';
        switch (err.code) {
            case err.PERMISSION_DENIED:
                message = 'Permission denied';
                break;
            case err.POSITION_UNAVAILABLE:
                message = 'Position unavailable';
                break;
            case err.PERMISSION_DENIED_TIMEOUT:
                message = 'Position timeout';
                break;
        }
        console.log('Error: ' + message);
    },
    () => console.log('Completed')
);