R Shiny Leaflet prevent view from resetting when input is changed - r

Question

I have a map which has a series of lines with associated values, which is rendered based on a reactive dataframe, filtered by a sliderInput.
When a line is clicked, the map view is updated with new lng, lat and zoom values, based on that line. What I would like is that the view does not reset when the slider is updated. I understand that it's resetting to the values in the initial setView() because the map is rerendering when the slider is changed, but I'm unsure how I can prevent this behaviour.
In short, when a user clicks on the line, I want it to stay at that lng/lat/zoom even as the slider changes. How can I accomplish this?
Below is a simplified reproducible example, with only one line:
library(shiny)
library(leaflet)
library(tidyverse)
library(RColorBrewer)
# Example data frame
line1 <- data.frame(
lng = rep(c(13.35011, 13.21514), 4),
lat = rep(c(52.51449, 52.48042), 4),
id = rep("10351A", 8),
period = rep(c(1, 2, 3, 4), each = 2),
value = rep(c(1200, 2300, 3140, 1111), each = 2)
)
ui <- fluidPage(
sidebarPanel(
sliderInput(
inputId = "period_picker",
label = "Period",
min = 1,
max = 4,
value = 1
),
uiOutput("clicked_info")
),
mainPanel(
leafletOutput("map")
)
)
server <- function(input, output) {
# Reactive dataframe based on period_picker
dat <- reactive({
filtered <- line1 %>%
filter(period == input$period_picker)
return(filtered)
})
# Render map
output$map <- renderLeaflet({
# Create color palette based on reactive frame
pal <- colorNumeric(palette = "Purples", domain = c(0, max(line1$value)))
# Render leaflet map
leaflet(data = dat()) %>%
addTiles() %>%
setView(lng = 13.38049, lat = 52.51873, zoom = 13) %>%
addPolylines(
lng = ~lng,
lat = ~lat,
layerId = ~id,
color = ~pal(dat()$value),
opacity = 1
)
})
# Zoom in and readjust view if shape matching id is clicked - this is the
# lng/lat/zoom value I want to keep when the sliderInput is changed
observeEvent(input$map_shape_click, {
x <- input$map_shape_click
if(x$id == "10351A") {
leafletProxy(
mapId = "map",
) %>%
flyTo(
lng = 13.282625,
lat = 52.497455,
zoom = 12
)
}
# Render dataset in the UI
output$clicked_info <- renderUI({
div(
tags$span("Line ID:", dat()$id[1]),
br(),
tags$span("Period:", dat()$period[1]),
br(),
tags$span("Value:", dat()$value[1])
)
})
})
}
shinyApp(ui = ui, server = server)

Answer 1

You need to put the addPolylines not in the map rendering, but in another observeEvent with leafletProxy. Replace your output$map block of code by the following lines :
# Render map
output$map <- renderLeaflet({
leaflet() %>%
addTiles() %>%
setView(lng = 13.38049, lat = 52.51873, zoom = 13)
})
observeEvent(dat(), {
req(dat())
pal <- colorNumeric(palette = "Purples", domain = c(0, max(line1$value)))
leafletProxy("map") %>%
addPolylines(
data = dat(),
lng = ~lng,
lat = ~lat,
layerId = ~id,
color = ~pal(dat()$value),
opacity = 1
)
})