[Qgis-psc] Grant report: Rework handling of multi-layer, mixed-format datasets

[Nyall Dawson]

Hi lists,

Just reporting in the conclusion of the grant work: "Rework handling
of multi-layer, mixed-format datasets". You can read more about the
motivation behind this work at
https://github.com/qgis/QGIS-Enhancement-Proposals/issues/216

While the work was partly motivated as an opportunity to clean up some
older parts of the QGIS codebase which were fragile and had low test
coverage, it has also resulted in many improvements and polish in the
QGIS user interface. Specifically:

- There's a much improved dialog shown when a user has to pick from
multiple layers from a dataset dropped onto QGIS. The dialog will
populate in a background thread, avoiding the UI hangs older QGIS
versions experience when scanning for layers in large files. The
dialog shows a unified view of all layers in the dataset, regardless
of whether they are vector, raster or mesh layers. There's also a new
"filter" widget allowing users to quickly find matching layers in
larger datasets.

- The browser view of files and file-based datasources has been
reworked. You'll no longer see duplicate entries for the same files
when these files can be read as either raster/vector/mesh layers (e.g.
for netcdf files, geopdf files and other mixed-format types). A lot of
consistency has been added to the right click actions you see when
interacting with file based items, with many bugs fixed along the way.

Under the hood, we've now got a flexible API for querying sublayers of
a dataset which can be easily extended to new providers so that they
automatically participate in the unified sublayers dialog and browser
items. It's all protected by a huge number of new unit tests, the
majority of which cover critical functionality which was previously
not being tested.

Thanks for funding this work!
Nyall