A quick hack to have the legends associated with radio-button "Base Groups" in R leaflet maps toggle along with layers

toggle-radio-layer-legends.R

library(leaflet)
library(tidyverse)

outline <- quakes[chull(quakes$long, quakes$lat),]

map <- leaflet(quakes) %>%
  addTiles(group = "OSM (default)") %>%
  # Overlay groups
  addCircles(~long, ~lat, ~10^mag/5, stroke = F, group = "Quakes") %>%
  addPolygons(data = outline, lng = ~long, lat = ~lat,
              fill = F, weight = 2, color = "#FFFFCC", group = "Outline") %>%
  addLayersControl(
    baseGroups = c("Quakes", "Outline"),
    options = layersControlOptions(collapsed = FALSE)
  ) %>%
  addLegend("bottomright", colors = "green", labels = "1",
            title = "Quakes Legend",
            layerId="Quakes" # Give a LayerId the same as the layer group name
  ) %>%
  addLegend("bottomright", colors = "yellow", labels = "1",
            title = "Outline Legend",
            layerId = "Outline" # Give a LayerId the same as the layer group name
  ) %>%
  htmlwidgets::onRender("
    function(el, x) {
      var initialLegend = 'Outline' // Set the initial legend to be displayed by layerId
      var myMap = this;
      for (var legend in myMap.controls._controlsById) {
        var el = myMap.controls.get(legend.toString())._container;
        if(legend.toString() === initialLegend) {
          el.style.display = 'block';
        } else {
          el.style.display = 'none';
        };
      };
    myMap.on('baselayerchange',
      function (layer) {
        for (var legend in myMap.controls._controlsById) {
          var el = myMap.controls.get(legend.toString())._container;
          if(legend.toString() === layer.name) {
            el.style.display = 'block';
          } else {
            el.style.display = 'none';
          };
        };
      });
    }")

map