在 R Shiny 中检测 selectInput 值更改为 NULL

Question

在下面的代码中，我无法检测selectInput到 的值更改为NULL

library(shiny)
ui <- fluidPage(
  selectInput(
    inputId = "var",
    label = "Select a variable:",
    choices = c("A", "B", "C"),
    selected = NULL,
    multiple = T),
  textOutput("selected_var")
)
server <- function(input, output) {
  observeEvent(input$var, {
    showNotification("var changed")
    output$selected_var <- renderPrint(paste0("selected var: ", input$var))
    if(is.null(input$var)) {                          # I want to be able to
      showNotification("var changed to null")         # detect this action
    }
  })
}
shinyApp(ui = ui, server = server)

如果用户选择 A 然后按退格键将其删除，我希望能够检测到该操作。

input$var您将如何检测更改为 的值NULL？

Answer 1

默认observeEvent设置为忽略NULL。添加ignoreNULL = FALSE到observeEvent将解决这个问题。您可能还希望添加ignoreInit = TRUE以阻止observeEvent在启动时触发。

这是完整的代码：

library(shiny)

ui <- fluidPage(

    selectInput(inputId = "var", label = "Select a variable:", choices = c("A", "B", "C"), selected = NULL, multiple = T),

    textOutput("selected_var")

)

server <- function(input, output) {

    observeEvent(input$var, {

        if(is.null(input$var)) {    

            showNotification("var changed to null")    

        }

    }, ignoreInit = TRUE, ignoreNULL = FALSE)

}
shinyApp(ui = ui, server = server)