wip: proj transform

[michaelkirk]

This is a demo of an API I'd like to be able to use with geozero to do more kinds of processing with the library.

Currently, it seems like processing is usually limited to "input -> output", which works great for converting formats and for some kinds of processing:

let wkt = WktStr("POINT (4760096.421921 3744293.729449)");
wkt
  .to_json()
  .unwrap()

But often I want to do some kind of pipeline of operations. One such example here:

1. read geometry from some input format
2. project the geometries
3. write to some output format (maybe the same as input)

let from = "EPSG:2230";
let to = "EPSG:26946";
let proj = Proj::new_known_crs(from, to, None).unwrap();

let wkt = WktStr("POINT (4760096.421921 3744293.729449)");
wkt
  .to_projected(proj)
  .to_json()
  .unwrap());

I personally think the user facing API is pretty good - but implementing it has some problems that I'll highlight below.

[michaelkirk]

This is the crux - we transform the points before passing them in to the "next" processor in the pipeline.

We only "need" these two methods coordinate / xy, but we have to explicitly delegate our other impls to the "output" processor, otherwise we'd inherit the "no-op" implementations for the GeomProcessor trait. (which is really painful)

Effectively, I'm going for some kind of interceptor pattern here.

As well as being very verbose, it also seems like it'd be easy to break if we added a new method to GeomProcessor/FeatureProcessor/PropertyProcessor and failed to update these long list of delegations below.

[pka]

Sorry, I'm currently short on time and leaving for Easter holidays soon. My only input would be that maybe a delegation pattern like https://github.com/flatgeobuf/flatgeobuf/blob/845111d2ba6765c8f43379877eb1f3440b78d4b6/src/rust/src/file_writer.rs#L302 could work here as well.

[michaelkirk]

maybe a delegation pattern

If I understand correctly, that's exactly what I'm doing - I did use the delegate crate macro to make it a little more concise, but it seems like a lot of boilerplate.

I'm searching for, but have so far failed to find, a better way to do this.

[pka]

I'm currently experimenting with a new event based API, which should support chaining of processors. Other advantages of an event stream would be serializing and buffering.

[michaelkirk]

Excited to see what you come up with!