Option for queryRenderedFeatures: execute only when map.loaded()

[peterqliu]

Offshoot of #2471 discussion:

When I call queryRenderedFeatures(), I assume the results come from a fully loaded map, with zoom/center defined as the map's state at the time I invoked it. Currently, I can achieve this with:

map.on('render', function() {
  if (map.loaded()){
    map.queryRenderedFeatures(...);
    map.off('render');
  }
})

I think it's a fair assumption that no one wants to run .qRT on a partially loaded map, so it would be nice for the method to check for map.loaded() internally before doing the query. Either by default, or via some parameter like map.queryRenderedFeatures({onDataend:true});

[jfirebaugh]

What should it return if map.loaded() is false?

[peterqliu]

What should it return if map.loaded() is false?

I should say that this option would turn .qRT into an async method. It would not return anything until that evaluates to true.

[jfirebaugh]

We had multiple design discussions regarding whether .qRT should be synchronous or asynchronous, and the conclusion was that a sync API was preferable. I don't think we want to re-litigate that, and I don't think it would be good API design to make it synchronous by default but asynchronous if you pass a specific option. Please continue to use the on('render') pattern where needed.