Action button and observeEvent

问题

I want an button to triggers an action on a prespecified non-reactive variable x. Each time the button is hit x <- x + 1 shall be executed. To check if it was done correctly the result shall be displayed. To achieve this I tried observeEvent(). But it is doing what it should only once. How does it work properly?

It seems that rv is only available within the observeEvent() function. If output$text_2 <- renderText({ rv$a }) is placed outside of observeEvent() an error occurs. How can I use something done within observeEvent() outside of it?

library(shiny)
x  <- 0

ui <- fluidPage(

  actionButton(inputId = "action", 
               label = "action"),
  textOutput("text_1"),
  textOutput("text_2")

)

server <- function(input, output) {

  output$text_1<- renderText({ input$action })

  observeEvent(input$action, {
               x  <- x + 1 
               rv <- reactiveValues(a = x)
               output$text_2 <- renderText({ rv$a })
               })
}

shinyApp(ui = ui, server = server)

回答1:

Daattalis answer is spot on but I figured I could post some examples on how you can use Shinys reactive values to do this.

library(shiny)

ui <- fluidPage(

  actionButton(inputId = "action", 
               label = "action"),
  textOutput("txt_example1"),
  textOutput("txt_example2"),
  textOutput("txt_example3")

)

server <- function(input, output) {

  # ------------------ Example 1 ------------------
  #  Use a list of reactive values
  rv <- reactiveValues(x=0)

  # Will update text output when rv$x is updated
  observe({
    output$txt_example1 <- renderText({ rv$x })
  })


  # ------------------ Example 2 ------------------
  #   Make variable a reactive
  x <- 0
  makeReactiveBinding('x')

  # The only 'trick' here is that x is made a reactive so that the observer is
  # triggered every time x is updated.
  # If x is not made a reactive the value of x will still be updated but the observer
  # wont trigger (see example 3).

  observe({
    output$txt_example2 <- renderText({ x })
  })

  # ------------------ Example 3 ------------------
  #   Use ordinary R scoping

  x2 <- 0 

  printUpdate <- function(){
    output$txt_example3 <- renderText({ x2 })
  }

  printUpdate() # Print first value

  # onClick listener, same for all examples
  observeEvent(input$action, {
    rv$x  <- rv$x + 1 # Example 1, update reactive list
    x     <<- x + 1   # Example 2, Save x to parent enviroment

    # Example 3
    x2 <<- x2 + 1     # Again, save x2 to parent enviroment, see help('<<-') 
    printUpdate()     # Update textOutput
  })


}

shinyApp(ui = ui, server = server)

Hope this helps!

回答2:

As of 2017, there's an easier, better way of doing this using reactiveVal:

library(shiny)

ui <- fluidPage(
  actionButton("go", "Click me"),
  helpText("All values in the table below are incremented ",
    "each time you press the button"),
  verbatimTextOutput("tbl")
)

server <- function(input, output, session) {
  x <- reactiveVal(1)

  observeEvent(input$go, {
    x(x() + 1)
  })

  output$tbl <- renderPrint({
    x()
  })
}

shinyApp(ui, server)

来源：https://stackoverflow.com/questions/36671011/action-button-and-observeevent

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