Reads all Mapbox Vector Tiles that fall within the supplied viewport, merging into a single sf object. This function is optimized for running with Leaflet under Shiny with multiple users, avoiding re-fetching and redrawing tiles.
Arguments
- info
a GeoServer MVT info object for target layer, from layer.info
- nw
northwestern corner tile in viewport, from get.tile
- se
southeastern corner tile (default: just read a single tile at nw)
- data.zoom
zoom level read data at
- drawn
from session$userData, bit matrix of tiles we've already drawn or NULL for the first call. Omit this argument if you don't need to track whether tiles have been drawn
Value
A two-element list:
tiles an sf object with vector data from tiles, or NULL if no new data are available in viewport
drawn updated drawn matrix
Details
Reads all tiles within viewport, merging them into a single sf object.
This function is intended to be used with Leaflet under Shiny with multiple users, though it
is expected to work fine in other applications that are probably less demanding. As multiple
users under Shiny may share an R session, data fetched by any user may be used by other users
without re-fetching. This is accomplished by caching with memoise::memoise(), which will be
initialized in this function if needed.
Additionally, Leaflet internally caches data that have already been drawn, so it is
undesirable for this function to return tiles that have already been drawn when using with
Leaflet. Tiles are only read and returned if they haven't been drawn yet for this user.
This behavior is tracked with the drawn argument. After calling read.viewport.tiles(), you should
save the drawn result in the Shiny user data like this:
x <- read.viewport.tiles(..., drawn = session$userData[[layername$layer]])
session$userData[[layername$layer]] <- x$drawn
Note that saving drawn as a local variable will not preserve it across repeated observe calls in Shiny, and saving it as a global variable will interfere with other users.
If there are no new data available in the viewport, NULL will be returned. You'll need to check for NULL before calling a leaflet drawing function.
Author
Bradley W. Compton bcompton@umass.edu