Leaflet and Shiny R: Why Your Circles Disappear When You Change Tile Layers
Have you ever encountered a frustrating scenario where your Leaflet circles mysteriously vanish when you switch tile layers in your Shiny R app? This common issue arises due to the way Leaflet handles map updates and how Shiny interacts with JavaScript. Let's delve into the problem and explore solutions to keep your circles visible across all tile layers.
The Problem: Circles Vanishing Act
Imagine you have a Shiny app displaying a Leaflet map with some circles representing points of interest. You allow users to switch between different basemaps (like OpenStreetMap or Stamen Toner) using a dropdown menu. However, as soon as you switch layers, your precious circles disappear, leaving you with a blank canvas.
Here's an example code snippet that showcases the issue:
library(shiny)
library(leaflet)
ui <- fluidPage(
leafletOutput("map"),
selectInput("basemap", "Basemap",
choices = c("OpenStreetMap" = "osm",
"Stamen Toner" = "stamen-toner"))
)
server <- function(input, output, session) {
output$map <- renderLeaflet({
leaflet() %>%
addTiles(group = "OpenStreetMap") %>%
addProviderTiles("Stamen.Toner", group = "Stamen Toner") %>%
addCircleMarkers(lng = -74.0060, lat = 40.7128, radius = 10, group = "circles") %>%
addLayersControl(baseGroups = c("OpenStreetMap", "Stamen Toner"),
overlayGroups = c("circles"),
options = layersControlOptions(collapsed = FALSE))
})
}
shinyApp(ui, server)
In this code, we create a Leaflet map with two tile layers: OpenStreetMap and Stamen Toner. We also add a circle marker. Switching between tile layers using the dropdown menu will result in the circle vanishing!
The Root of the Disappearance: Leaflet's Map Update Logic
The culprit behind the disappearing circles is the way Leaflet updates the map when you change tile layers. Leaflet treats these updates as complete map refreshes. It redraws the entire map, discarding any existing elements (like our circles) that were previously added. Since our circles weren't explicitly re-added during the map update, they become invisible.
The Solution: Re-adding the Circles
The key to fixing this issue is to re-add the circles after each tile layer change. We can achieve this using Shiny's reactivity and Leaflet's JavaScript API:
library(shiny)
library(leaflet)
ui <- fluidPage(
leafletOutput("map"),
selectInput("basemap", "Basemap",
choices = c("OpenStreetMap" = "osm",
"Stamen Toner" = "stamen-toner"))
)
server <- function(input, output, session) {
output$map <- renderLeaflet({
leaflet() %>%
addTiles(group = "OpenStreetMap") %>%
addProviderTiles("Stamen.Toner", group = "Stamen Toner") %>%
addCircleMarkers(lng = -74.0060, lat = 40.7128, radius = 10, group = "circles") %>%
addLayersControl(baseGroups = c("OpenStreetMap", "Stamen Toner"),
overlayGroups = c("circles"),
options = layersControlOptions(collapsed = FALSE))
})
observeEvent(input$basemap, {
leafletProxy("map") %>%
clearGroup("circles") %>%
addCircleMarkers(lng = -74.0060, lat = 40.7128, radius = 10, group = "circles")
})
}
shinyApp(ui, server)
In this updated code, we add an observeEvent function that listens for changes in the input$basemap value. When the user changes the basemap, the function uses leafletProxy to interact with the existing Leaflet map object. We first clear any existing circles from the "circles" group using clearGroup. Then, we re-add the circle markers using addCircleMarkers.
Beyond Circles: Applying the Solution to Other Leaflet Features
This approach of re-adding elements after a tile layer change applies not only to circles but also to other Leaflet features like markers, polygons, popups, and more. You can simply adjust the Leaflet methods used to add or remove the specific elements according to your needs.
Additional Tips
- Consider Using the
leafletProxyFunction:leafletProxyallows you to interact with the existing Leaflet map object within your Shiny app, enabling more dynamic map manipulation. - Keep Your Code Organized: Separate your code into logical sections for better readability and maintainability, particularly when working with complex Shiny and Leaflet interactions.
- Explore Leaflet's JavaScript API: Understanding Leaflet's JavaScript API can empower you to create more sophisticated map interactions and solve complex issues.
By understanding the way Leaflet updates maps and using Shiny's reactivity and Leaflet's API, you can overcome the challenge of disappearing Leaflet elements when changing tile layers. This knowledge will help you create more interactive and user-friendly Shiny apps with dynamic and engaging Leaflet maps.
