Optional fixed draw order

[twpayne]

The fact that the draw order of features with the same z index is undefined has caused some confusion on the mailing list.

This PR adds an optional fixedOrder property to ol.layer.Vector that specifies that features should be drawn in a fixed order. At the moment this fixedOrder is taken from the feature's goog.getUid, which is a proxy for the order in which the features were created. Note that this is not the same order as the features might have been added to the source, but will often correspond to the order in which they were deserialised.

You can see the effect of the fixed order in the KML example by trying the following commands in the console:

map.getLayers().getAt(1).setFixedOrder(true);
map.getLayers().getAt(1).setFixedOrder(false);

The disadvantage of using a fixed order this that extra memory and time is required to sort the features before rendering.

There's probably a better name for the property than fixedOrder.

[twpayne]

Any comments on this? Travis does pass, it errored because GitHub was being DDOSed at the time.

[twpayne]

I've now updated this according to the discussion in the hangout. This makes ol3 render in a consistent order by default, while still adding the flexibility to use an undefined render order (to avoid the sort if you have many features) and even a custom render order (if you want to use some other property).

You can see the effect of changing draw order by running the kml example and typing the following into the console:

vector.setRenderOrder(null); // set undefined rendering order
vector.setRenderOrder(undefined); // set default rendering order
vector.setRenderOrder(function(f1, f2) { return Math.random() - 0.5; }) // random!

[tschaub]

Looks good to me.

At some point, we might want to get consistent with our use of goog.array.sort v. Array.prototype.sort (we rely on the latter being what we expect when finding points in rings, while modifying features, and in using libtess.js.

[elemoine]

Any reason for doing that in ObjectEvent instead of Object?

[twpayne]

Good catch :) Total error on my part! PR coming up.

[fredj]

I think we will never end up here because vectorLayerRenderOrder is set if undefined (line 186)

[twpayne]

I think you're right. I think the check at line 237 should be !goog.isNull(...).

[fredj]

PR to come