A (mostly) minimal example of using sidebar drop downs to 1) filter counties within states, and 2) `flyTo` the centroid of the selected state in shiny + leaflet

leaflet_flyTo_Shiny.r

library(shiny)
library(shinydashboard)
# devtools::install_github("nik01010/dashboardthemes")
library(dashboardthemes)
library(tidyverse)
library(sf)
library(leaflet)
library(usmap) # us counties and states as table
library(leaflet)

# Zoom to coordinates for each state centroid
state_coords <- data.frame(abbr = datasets::state.abb,
                           x = datasets::state.center["x"],
                           y = datasets::state.center["y"],
                           area = datasets::state.area,
                           stringsAsFactors = FALSE) %>% 
  rbind(., data.frame(abbr = "DC", # add D.C.
                      x = -77.0369,
                      y = 38.9072,
                      area = 68))
county_state <- countypop %>% 
  left_join(select(statepop, state = full, abbr), by = "abbr") 
state_pop_coords <- statepop %>% 
  left_join(state_coords, by = "abbr") %>% 
  rename(state = full) %>% 
  st_as_sf(coords = c("x","y"), crs = 4326)
state_pop_coords <- cbind(state_pop_coords, st_coordinates(state_pop_coords))

header <- dashboardHeader(
  title = 'Leaflet flyTo',
  # task list for status of data processing
  dropdownMenuOutput('task_menu'))

sidebar <- dashboardSidebar(
  sidebarMenu(
    htmlOutput("state_selector"),#add selectinput boxs
    htmlOutput("county_selector")# from objects created in server
    
  )
)
body <- dashboardBody(
  shinyDashboardThemes(
    # https://github.com/nik01010/dashboardthemes
    theme = "grey_dark"
  ),
  # the main leaflet map
  box(leafletOutput('map'), width = 12)
)

# Shiny UI
ui <- dashboardPage(
  header,
  sidebar,
  body
)

server <- function(input, output, session) {
  
  output$state_selector = renderUI({ 
    selectInput(inputId = "state", #name of input
                label   = "State:", #label displayed in ui
                choices = c("Choose one" = "", usmap::statepop$full),
    )
  })
  
  output$county_selector = renderUI({
    data_available = county_state[county_state$state == input$state, "county"]
    #creates a reactive list of available counties based on the State selection made
    selectInput(inputId  = "county", #name of input
                label    = "County:", #label displayed in ui
                choices  = unique(data_available), #calls list of available counties
                selected = unique(data_available)[1])
  })
  
  output$map <- renderLeaflet({
    leaflet() %>%
      addTiles(urlTemplate = "//{s}.tiles.mapbox.com/v3/jcheng.map-5ebohr46/{z}/{x}/{y}.png",
               attribution = 'Maps by <a href="http://www.mapbox.com/">Mapbox</a>'
      ) %>%
      setView(lng = -98, lat = 39, zoom = 4)
  })
  
  center <- reactive({
    subset(state_pop_coords, state == input$state)
    # or whatever operation is needed to transform the selection
    # to an object that contains lat and long
  })
  
  mymap_proxy <- leafletProxy("map")
  
  observe({
    #https://stackoverflow.com/questions/53090523/leaflet-map-zooms-based-on-a-dynamic-selectinput
    fdata <- center()
    mymap_proxy %>%
      flyTo(lng = fdata$X, lat = fdata$Y, zoom = 8)
  })
  
}

shinyApp(ui, server)