Support query.cancel()

[dplewis]

Closes: #997

Taken from Android API cancel

Can be improved in the future.

[codecov]

Codecov Report

Merging #1003 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1003      +/-   ##
==========================================
+ Coverage   91.91%   91.94%   +0.02%     
==========================================
  Files          53       53              
  Lines        5098     5114      +16     
  Branches     1145     1146       +1     
==========================================
+ Hits         4686     4702      +16     
  Misses        412      412

Impacted Files	Coverage Δ
src/ParseQuery.js	96.14% <100%> (+0.1%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 566f29a...e2fd677. Read the comment docs.

[dplewis]

@acinader @davimacedo This is hard to test in a node environment as the node-xmlhttprequest package (hasn't been published in 4 years) is lacking proper support for abort. i.e. every open connection calls abort when opened. This isn’t a problem for Parse.File cancel since it doesn’t use that package.

Canceling queries is tricky. The RESTController does a few promises like installationId and currentUser before the request. The requestTask might not be set when cancel is called. I added an onchange event to know when the request started. We could move this to a query.onStart or something like that.

It should work in browser though.

[dplewis]

@acinader @davimacedo I updated my comment. Thoughts?

[acinader]

if you think we should release this, given its limitations, is ok with me.