Add promise- or callback-based notifications when motor actions are complete

[WasabiFan]

Although there is no kernel interface for completion events, we can still poll the state with an interval timer. It would be relatively easy to register a setInterval callback that polls the necessary properties and resolves promises as needed. The three big considerations are:

• It should use one universal timer which handles any and all pending promises
• The timer should only be running when needed
• Any promise library that is used must work in the version of Node that can run on the EV3

[WasabiFan]

I have decided to put this on hold until we have a real way of receiving event notifications from the kernel. To make a generic version of this API work, every motor start call would need to initiate a polling apparatus just in case the user decided to .then() on the promise. That's just too much overhead for the embedded processor to handle.

For now, I have implemented a polling system that lets the user register a predicate and callback on any generic Device instance. This API can be used if events are required.

[paulolc]

@WasabiFan, how can I use the polling system you refer? I can't seem to find the corresponding commit. How can I register the predicate and specify the callback? Thanks.

[WasabiFan]

These changes are on the develop branch and have been released in a beta release. If you haven't already installed it, you can do so by running (command untested but should work):

npm install ev3dev-lang@beta

I published this release as a beta because the documentation generator I use was having problems; this has since been resolved, so I plan to publish a full release and then update it for the most recent kernel soon. This version of the library supports the -10-ev3dev kernel. The newest kernel, -11-ev3dev, is not yet supported (sorry about that!) but it was only released a few days ago.

The diff of the addition of the polling feature can be found here. Primarily, the functions that you are looking for are registerEventCallback and registerEventPromise; you can look at their signatures in the source code to see what you need to pass them. I also have an example of the callback-based version of the function here. Note that, to use the promise-based version, you need to install bluebird from npm.