Need methods to store objects

[courtiol]

Problem

It is no longer possible to directly save objects created by IsoriX onto the hard drive with terra.

Context

When relying on raster it was possible to save objects created by IsoriX onto the hard drive as *.rda or *.rds.

Objects created by IsoriX are composite in the sense that they may contain rasters, spatial vectors, and non-spatial info all bundled together as a list.

Those list cannot be saved with terra since this package has functions designed to save rasters or vectors specifically and no function to store any kinds of composite.

This matters for users because spatial objects created by IsoriX can be very slow to create.
This also matters to me for editing the bookdown.

Solution

Turning files into binaries ("serialisation") does not seem to be a recommended practice for spatial objects, and even with raster it led to issues if the values were not stored in memory.

So the solution seems to implement methods to save specific classes of IsoriX-created objects.
These methods would create a folder with a particular structure within which the different parts of the objects would be saved separately.

We would also have methods to read these folders so as to recreate the IsoriX objects.

Objects created in IsoriX containing spatial data

• objects of class ISOFIT: a list storing various info and no spatial data.
• objects of class MULTIISOFIT: a list storing various info and no spatial data.
• objects of class CALIBFIT: a list storing various info as well as 0 or 1 SpatVector and no SpatRaster.
• objects of class ISOSCAPE: a list storing 1 SpatRaster and 1 SpatVector.
• objects of class ISOFIND: a list of 3 lists, one containing 4 SpatRaster, one containing 1 SpatRaster, one containing (up to?) 3 SpatVector.

Open questions

• Could there be a simpler way for us to avoid having to create IsoriX-specific methods?
• Revise the internal structure of ISOFIND to make it simpler (e.g. single brick)? (I need to check if various sets of points can be combined within a single SpatVector)

[courtiol]

@rhijmans, if you have thoughts, please share them with us :-)

[rhijmans]

I think you can define a saveRDS method for these objects. These can then call terra::wrap for SpatRaster/SpatVector objects before calling base::saveRDS. That is what terra does for saveRDS<SpatVector> and saveRDS<SpatRaster>. And do the inverse for readRDS.