Using elevatr in webr — httr/curl not available

[psychemedia]

I'm trying to use elevatr in ShinyLive / webR, a WASM build of R, but curl, as used in httr, is not available (example issue).

However, download.file() is available and can be used as follows (I haven't checked if the CORS proxy is required, but include it as an option just in case:

# CORS proxy function for webR environment
make_cors_url <- function(url, params = NULL) {
  if (!is.null(params)) {
    param_string <- paste(
      mapply(
        function(k, v) paste0(URLencode(k), "=", URLencode(v)),
        names(params),
        params
      ),
      collapse = "&"
    )
    url <- paste0(url, "?", param_string)
  }
  paste0("https://corsproxy.io/", URLencode(url))
}

# Apply CORS proxy if needed
 use_cors=T

dem_list <- lapply(urls, function(x) {
    
    tmpfile <- tempfile(tmpdir = tmp_dir, fileext = ".tif")
    
    # Apply CORS proxy if needed
    download_url <- if(use_cors) make_cors_url(x) else x
    
    # Use download.file with method="internal" for webR compatibility
    tryCatch({
      download.file(
        download_url, tmpfile
      )
      
      # Basic check if file exists and has content
      if (!file.exists(tmpfile) || file.size(tmpfile) == 0) {
        stop(paste("Failed to download from:", x))
      }
      
      # Note: We can't get headers like x-amz-meta-x-imagery-sources with base R
      # So we'll just return the tmpfile without the source attribute
      tmpfile
      
    }, error = function(e) {
      stop(paste("Error downloading from:", x, "\nError:", e$message))
    })
  })

Would it be possible to have fallback support for download.files() rather than httr so that elevatr can be used in a WebR/WASM context?

[jhollist]

@psychemedia thanks for bringing this up!

I am amenable to including this; however, I don't have the time at the moment to make the addition. If you can submit a PR, with a test for this, I would be happy to merge it in.

Thanks!