Build dotted group-drag boundaries for expanded groups

Computes one padded rectangular boundary per group. The boundary is returned as real geometry with the group's id so it can be drawn inside the same container as the children and therefore moves with the group when dragged. In Spatial Studio this boundary is a drag handle, not a collapse control.

Usage

make_group_boundaries(sf_obj, group_col, padding = NULL)

Arguments

sf_obj

An sf object containing the (expanded) child regions.

group_col

Column in sf_obj identifying the expanded group, usually the parent id column. Missing or empty values are ignored; an error is raised when no valid group values remain.

padding

Single positive number in map units, or NULL (default) to use 2.5 percent of the bounding-box diagonal of sf_obj.

Value

An sf object with one row per group and columns group_id, xmin, xmax, ymin, ymax (padded bounds), plus the rectangle geometry.

See also

build_elastic_transition(), which calls this automatically when group_drag_frame = TRUE.

Examples

rect <- function(x0, x1, y0, y1) {
  sf::st_polygon(list(cbind(c(x0, x1, x1, x0, x0), c(y0, y0, y1, y1, y0))))
}
children <- sf::st_sf(
  county = c("A", "A", "B"),
  geometry = sf::st_sfc(
    rect(0, 1, 0, 1), rect(2, 3, 0, 1), rect(5, 6, 0, 1),
    crs = 3857
  )
)
make_group_boundaries(children, group_col = "county")
#> Simple feature collection with 2 features and 5 fields
#> Geometry type: POLYGON
#> Dimension:     XY
#> Bounding box:  xmin: -0.1520691 ymin: -0.1520691 xmax: 6.152069 ymax: 1.152069
#> Projected CRS: WGS 84 / Pseudo-Mercator
#>   group_id       xmin     xmax       ymin     ymax
#> 1        A -0.1520691 3.152069 -0.1520691 1.152069
#> 2        B  4.8479309 6.152069 -0.1520691 1.152069
#>                         geometry
#> 1 POLYGON ((-0.1520691 -0.152...
#> 2 POLYGON ((4.847931 -0.15206...