HexagonLayer: Aggregate data using valid viewport.

[1chandu]

For #2111

Background

Re-aggregate and update hexbins when viewport is changed.

Change List

• HexagonLayer: Re-aggregate data when viewport is changed.

[ibgreen]

I suspect that this bug is a result of all the refactorings we have done to support multiple viewports.

I have a feeling that this PR mainly works around the real problem (viewport initially being undefined).

If that was fixed, would we still want to reaggregate when the viewport is changed? Wouldn't that turn this into a ScreenHexagonLayer?

Are the results of the aggregation actually viewport dependent (lng/lat/zoom) or do we just need a mercator projection? If the latter, can we just create a "working viewport" during aggregration (instead of depending on the passed in viewport)?

[ibgreen]

To get viewportChanged notifications, I think you need to define the shouldUpdateState method.

[1chandu]

Are the results of the aggregation actually viewport dependent (lng/lat/zoom) or do we just need a mercator projection? If the latter, can we just create a "working viewport" during aggregration (instead of depending on the passed in viewport)?

It uses viewport for 1. calls projectFlat and unprojectFlat 2. uses viewport's distanceScales.

So if any change in above factors, we should re-aggregate.

[1chandu]

Digged more into this

1. During the layer initialization, the viewport available in layer.context is the default viewport (non geospatial)
2. Actual webmercator viewport is created after layer is initialized and after GLContext is created, at this point it uses canvas width and height.
3. During render time, each viewport is activated using _activateViewport, where layer's context.viewport is updated, viewportChanged flag is set.
4. Layer gets chance to update its state (provided shouldUpdateState is overwritten)

For hexagon layers, we do need to re-project points at #4 , so we have to check for viewportChanged flag. But this might trigger aggregation un-necessarily for each pan/zoom events. So we need a new flag that is only set when mercator projection needs to be updated.

[1chandu]

Update the PR with possible solution (couple of tests failing, will fix before landing them)

We need to project points the first time viewport changed to a geospatial (WebMercatorViewport) viewport, since initial viewport is not a WebMercatorViewport. Added changes to aggregate only when layer has geospatial viewport and shouldUpdate returns only when viewportChanged and data is not aggregated yet.

[1chandu]

@ibgreen created #2215