title: R Shiny observeEvent continues to trigger
body: <p>I'm struggling to get an observeEvent process to run only once after it's triggering event - a button click.  This illustrates:</p>
  
  <pre><code>require(shiny)
  
  ui = fluidPage(
    textInput("input_value", '1. input a value. 2. click button. 3. input another value', ''),
    actionButton("execute", 'execute'),
    textOutput('report')
  )
  
  server = function(input, output, session) {
    observeEvent(input$execute, {
      output$report = renderText(input$input_value)
    })
  }
  
  shinyApp(ui = ui, server = server, options = list(launch.browser = T))
  </code></pre>
  
  <p>You'll see that after the button has been clicked once, the textOutput becomes responsive to textInput changes rather than button clicks.</p>
  
  <p>I've tried this approach:</p>
  
  <pre><code>server = function(input, output, session) {
    o = observeEvent(input$execute, {
      output$report = renderText(input$input_value)
      o$destroy
    })
  }
  </code></pre>
  
  <p>No effect. I've also tried employing the <code>isolate</code> function with no luck. Grateful for suggestions.</p>