in R Shiny, update an object based on user input

I would like to have a plot of my graph (g) showing in my app at all times. I would like to have a 'default' value of g hard-coded into the app, and the user can update g via the UI. I am using the {{igraph}} library and its functions, such as make_empty_graph and add_vertices.

I believe my main issue at this point is that I hard-code g, but then the reactive version of g is really g(). I'm sure this problem has been solved many times before, but I can't see an example in the Shiny demos that does it.

Here's what I have inside server():

g <- make_empty_graph(directed = TRUE) %>%    add_vertices(nv = 1, depth = 0L, x = 0, y = 0, cur = TRUE)g <- eventReactive(    eventExpr = 'investigate',    valueExpr = {        add_children(g = g, vidx = input$vidx)    })output$the_plot <- renderPlot(    expr = {           plot_graph(g())    })

add_children() and plot_graph() are defined in the global namespace. I am happy to share more code (or the whole app) if that is helpful.

Based on input from SO, the working version is:

g <- reactiveVal(    make_empty_graph(directed = TRUE) %>%        add_vertices(nv = 1, depth = 0L, x = 0, y = 0, cur = TRUE))    observeEvent(    eventExpr = input$investigate,    handlerExpr = g(add_children(g = g(), vidx = input$vidx)),    ignoreInit = TRUE)output$the_plot <- renderPlot(plot_graph(g()))