if (!isGeneric("+")) {
  setGeneric("+", function(x, y, ...)
    standardGeneric("+"))
}

#' mapview + mapview; adds data from the second map to the first
#'
#' @author Adapted from
#' \href{https://github.com/r-spatial/mapview/blob/develop/R/plus.R}{
#' tim-salabim code}.
#' @param e1 a mapview map to which e2 should be added.
#' @param e2 a mapview map from which the objects should be added to e1.
#' @examples
#' \dontrun{
#' eeobject <- ee$FeatureCollection("users/csaybar/DLdemos/train_set")
#' center <- eeobject$geometry()$centroid()$getInfo()$coordinates
#' vizparams <- list(color = "FF0000", strokeWidth = 5)
#' m1 <- ee_map(eeobject, vizparams, center, objname = "Arequipa-landuse")
#'
#' collection <- ee$ImageCollection("LANDSAT/LC08/C01/T1_TOA")$
#'   filter(ee$Filter()$eq("WRS_PATH", 44))$
#'   filter(ee$Filter()$eq("WRS_ROW", 34))$
#'   filterDate("2014-01-01", "2015-01-01")$
#'   sort("CLOUD_COVER")
#' eeobject <- collection$median()
#' vizparams <- list(bands = c("B4", "B3", "B2"), max = 0.3)
#' center <- c(-122.3578, 37.7726)
#' m2 <- ee_map(eeobject, vizparams, center, objname = "SF")
#' m1 + m2
#' }
#'
setMethod(
  "+",
  signature(
    e1 = "mapview",
    e2 = "mapview"
  ),
  function(e1, e2) {
    e2_token <- e2@object$tokens
    e2_name <- e2@object$names
    e2_opacity <- e2@object$opacity
    e2_shown <- e2@object$shown

    for (x in seq_len(length(e2_name))) {
      e1@map <- e1@map %>%
        addTiles(urlTemplate = e2_token[x],
                 group = e2_name[x],
                 options = tileOptions(opacity = e2_opacity[x])) %>%
        ee_mapViewLayersControl(names = e2_name[x]) %>%
        hideGroup(if (!e2_shown[x]) e2_name[x] else NULL)
    }
    return(e1)
  }
)