Make handling of skipped features faster

[elemoine]

Aaron Solberg reported serious performance problems occuring when adding features to the map's skippedFeatures collection.

The issue can be reproduced on the synthetic-points example:

• open http://ol3js.org/en/master/examples/synthetic-points.html?mode=raw

• in the console do

  features = vectorSource.getFeatures();
  for (i = 0; i < 10000; ++i) { map.getSkippedFeatures().push(features[i]); }

  The code will take a long time to execute… (and the map to be refreshed)

The issue has nothing to do with the rendering code. It's related to the way we handle "add" (and "remove") events in ol.Map when features are added to the skipped features collection. Currently, each time a feature is added we clear the skippedFeatureUids_ object and re-populate with a loop over the features in the skipped features collection. So the complexity of adding N features to a skipped features collection of length M is O(M * N).

With this PR we no longer repopulate the skippedFeatureUids_ object from scratch. The complexity of adding N features to a skipped features collection is O(N). Can be tested here: http://erilem.net/ol3/skipped-features-performance/examples/synthetic-points.html?mode=raw.

[tonio]

LGTM. Very noticeable effect (-:

Apart from this PR, I wonder if using skipped features is the best practice for the mailing-list use case.

[elemoine]

Another (probably minor) optimization is to remove the "to string" conversion. I'll do that separately.

[elemoine]

Aaron's jsfiddle changed to using my branch and skippedFeatures: http://jsfiddle.net/NC4vh/29/. Now the browser manages to complete the operation.

[tschaub]

I wonder if using skipped features is the best practice for the mailing-list use case

@tonio - I wonder the same. Before exporting getSkippedFeatures let's review the use cases and make sure we're handling them in the most sensible way.

[elemoine]

@tonio - I wonder the same. Before exporting getSkippedFeatures let's review the use cases and make sure we're handling them in the most sensible way.

Yep, same feeling here.