Build a reader from a catalogue and a slice function

The reader contract is one function that does two jobs, switched by returnfiles. That shape exists so that functions written before this package, raadtools' read* among them, satisfy it without being touched. It is not a nice thing to write by hand: the return type depends on the value of an argument, which is awkward to document and easy to get subtly wrong.

Usage

xyt_source(slice, catalogue)

Arguments

slice

a function of (date, files, ...) returning a single-layer SpatRaster for date. files is the catalogue.

catalogue

the catalogue: a data.frame with a date column, or a function returning one. A function is called once, the first time it is needed, and the result is reused for the life of the source. Build a separate source if you need a different catalogue.

Value

a function of class xyt_reader satisfying the reader contract.

Details

xyt_source() writes it for you. You supply the two jobs separately, each as an ordinary function with an ordinary signature, and get back something extract_xyt() accepts.

See also

extract_xyt() takes a files argument too, which is the other way to avoid writing the flag: hand it the catalogue directly and the returnfiles call is never made.

Examples

## a series of two rasters held in a list
r <- lapply(1:2, function(i) {
  x <- terra::rast(terra::ext(0, 4, 0, 4), resolution = 1, crs = "EPSG:4326")
  terra::values(x) <- i
  x
})
src <- xyt_source(
  slice = function(date, files, ...) r[[which(files$date == date)]],
  catalogue = data.frame(date = as.POSIXct(c("2000-01-01", "2000-01-02"),
                                           tz = "UTC"))
)
extract_xyt(src, data.frame(x = 1, y = 1,
                            t = as.POSIXct("2000-01-02", tz = "UTC")))
#> [1] 2