Modifying Existing Maps with leafletProxy

This works, but reactive inputs and expressions that affect the renderLeaflet expression will cause the entire map to be redrawn from scratch and reset the map position and zoom level.

For some situations that may be acceptable or desirable behavior. But in other situations, you may want finer-grained control over the map, such as changing the color of a single polygon or adding a marker at the point of a click – without redrawing the entire map.

To modify a map that’s already running in the page, you use the leafletProxy() function in place of the leaflet() call, but otherwise use Leaflet function calls as normal.

Normally you use leaflet to create the static aspects of the map, and leafletProxy to manage the dynamic elements, like so:

library(shiny)
library(leaflet)
library(RColorBrewer)

ui <- bootstrapPage(
  tags$style(type = "text/css", "html, body {width:100%;height:100%}"),
  leafletOutput("map", width = "100%", height = "100%"),
  absolutePanel(top = 10, right = 10,
    sliderInput("range", "Magnitudes", min(quakes$mag), max(quakes$mag),
      value = range(quakes$mag), step = 0.1
    ),
    selectInput("colors", "Color Scheme",
      rownames(subset(brewer.pal.info, category %in% c("seq", "div")))
    ),
    checkboxInput("legend", "Show legend", TRUE)
  )
)

server <- function(input, output, session) {

  # Reactive expression for the data subsetted to what the user selected
  filteredData <- reactive({
    quakes[quakes$mag >= input$range[1] & quakes$mag <= input$range[2],]
  })

  # This reactive expression represents the palette function,
  # which changes as the user makes selections in UI.
  colorpal <- reactive({
    colorNumeric(input$colors, quakes$mag)
  })

  output$map <- renderLeaflet({
    # Use leaflet() here, and only include aspects of the map that
    # won't need to change dynamically (at least, not unless the
    # entire map is being torn down and recreated).
    leaflet(quakes) %>% addTiles() %>%
      fitBounds(~min(long), ~min(lat), ~max(long), ~max(lat))
  })

  # Incremental changes to the map (in this case, replacing the
  # circles when a new color is chosen) should be performed in
  # an observer. Each independent set of things that can change
  # should be managed in its own observer.
  observe({
    pal <- colorpal()

    leafletProxy("map", data = filteredData()) %>%
      clearShapes() %>%
      addCircles(radius = ~10^mag/10, weight = 1, color = "#777777",
        fillColor = ~pal(mag), fillOpacity = 0.7, popup = ~paste(mag)
      )
  })

  # Use a separate observer to recreate the legend as needed.
  observe({
    proxy <- leafletProxy("map", data = quakes)

    # Remove any existing legend, and only if the legend is
    # enabled, create a new one.
    proxy %>% clearControls()
    if (input$legend) {
      pal <- colorpal()
      proxy %>% addLegend(position = "bottomright",
        pal = pal, values = ~mag
      )
    }
  })
}

shinyApp(ui, server)

Screenshot of a Shiny app

Besides adding layers, you can set the map bounds, remove specific objects by ID, or clear categories of layers.

proxy <- leafletProxy("mymap")

# Fit the view to within these bounds (can also use setView)
proxy %>% fitBounds(0, 0, 11, 11)

# Create circles with layerIds of "A", "B", "C"...
proxy %>% addCircles(1:10, 1:10, layerId = LETTERS[1:10])

# Remove some of the circles
proxy %>% removeShape(c("B", "F"))

# Clear all circles (and other shapes)
proxy %>% clearShapes()

Understanding Layer IDs

Layer IDs can be used to replace or remove specific map elements. (Note: For managing groups of map elements, see the Show/Hide Layers topic.)

When you give an object a layer ID, if a similar object exists with the same ID, it will be removed from the map when the new object is added.

All layer-adding functions take a layerId argument. Generally this is a vectorized argument; if adding 50 polygons, you’ll need to pass either NULL or a 50-element character vector as your layerId value. (If you use a single-element character vector as your layerId, then all of the 50 polygons will have the same ID, and all but the last polygon will be removed; so don’t do that!)

Layer IDs are namespaced by category. Layer IDs only need be unique within their category; a call to addCircles will not cause any tile layers to be removed, regardless of layer ID, because circles and tiles are in different categories.

Inputs/Events

Leaflet maps and objects send input values (which we’ll refer to as “events” in this document) to Shiny as the user interacts with them.

Putting it all together

For an extensive example of Shiny and Leaflet working together, take a look at the SuperZip Explorer example app (note the “Get Code” button at the top of the page). (Hint: Try clicking on the bubbles, and also notice that the plots in the sidebar change as you pan and zoom around the map.)