shiny reactive, reactiveValue and eventReactive, observe and observeEvent in Shiny observe

Example

An observe expression is triggered every time one of its inputs changes. The major difference with regards to a reactive expression is that it yields no output, and it should only be used for its side effects (such as modifying a reactiveValues object, or triggering a pop-up).

Also, note that observe does not ignore NULL's, therefore it will fire even if its inputs are still NULL. observeEvent by default does ignore NULL, as is almost always desirable.

library(shiny)

ui <- fluidPage(
  headerPanel("Example reactive"),
  
  mainPanel(
    
    # action buttons
    actionButton("button1","Button 1"),
    actionButton("button2","Button 2")
  )
)

server <- function(input, output) {
  
  # observe button 1 press.
  observe({
    input$button1
    input$button2  
    showModal(modalDialog(
      title = "Button pressed",
      "You pressed one of the buttons!"
    ))
  })
}

shinyApp(ui = ui, server = server)