Create example for AsyncProgressWorker & AsyncProgressQueueWorker

[mastergberry]

There is currently on example for these progress workers. The documentation is kind of a bit confusing here: https://github.com/nodejs/node-addon-api/blob/master/doc/async_worker_variants.md

It doesn't really explain how to use the progress callback in the OnProgress function...just randomly makes a Callback().Call({Env().Null(), Env().Null(), Number::New(Env(), *data)}); call with two Env().Null()'s with no explanation as to what this is doing. I guess by passing two nulls it forces this to the Progress callback in javascript...but it's all a bit confusing.

Would be nice to get a proper example on here put together for these progressworkers. In the nan library you have to specify each the callback and the progresscallback to the worker, but it doesn't seem to be the case in the new node-addon-api, example/test can be found here: https://github.com/nodejs/nan/blob/e222068f9d99967897a8cefa6c505534b36116ad/test/cpp/asyncprogressworker.cpp

I am porting a project from nan -> node-addon-api currently, and feel an example on how the progress callbacks work would be beneficial to me and others also.

Thanks.

[mastergberry]

Upon a bit of further inspection of the source code, I found a similar test which shows something that makes more sense on how to use the async progress workers: nodejs/node-addon-api@ab01844#diff-3f536315f565c436ece66924a9d8073b7e0aa43366dedf4cda5ac9ac1a1e991eR18

Seems like the documentation is wrong and out of date possibly. Makes more sense now that I see two Function's being passed. Still think an example in this repo would not hurt.

[mhdawson]

@mastergberry any chance you would want to submit a PR to fix the documentation?

[mastergberry]

Hey @mhdawson

I have gone ahead and created a PR to clean up the documentation. Maybe later if I get some extra time I will convert this into a proper example here as well (or someone else can feel free to if they wish as well).

nodejs/node-addon-api#831

[mhdawson]

@mastergberry thanks for doing that. PR is landed :)