Change 07_widgets example to use an action button

007-widgets/Readme.md

@@ -1 +1 @@
-This example demonstrates some additional widgets included in Shiny, such as `helpText` and `submitButton`. The latter is used to delay rendering output until the user explicitly requests it.
+This example demonstrates some additional widgets included in Shiny, such as `helpText` and `actionButton`. The latter is used to delay rendering output until the user explicitly requests it (a construct which also introduces two important server functions, `eventReactive` and `isolate`).

007-widgets/server.R

@@ -1,26 +1,32 @@
 library(shiny)
 library(datasets)
 
-# Define server logic required to summarize and view the 
+# Define server logic required to summarize and view the
 # selected dataset
 function(input, output) {
-
-  # Return the requested dataset
-  datasetInput <- reactive({
+
+  # Return the requested dataset. Note that we use `eventReactive()`
+  # here, which takes a dependency on input$update (the action
+  # button), so that the output is only updated when the user
+  # clicks the button.
+  datasetInput <- eventReactive(input$update, {
     switch(input$dataset,
            "rock" = rock,
            "pressure" = pressure,
            "cars" = cars)
-  })
-  
+  }, ignoreNULL = FALSE)
+
   # Generate a summary of the dataset
   output$summary <- renderPrint({
     dataset <- datasetInput()
     summary(dataset)
   })
-
-  # Show the first "n" observations
+
+  # Show the first "n" observations. The use of `isolate()` here
+  # is necessary because we don't want the table to update
+  # whenever input$obs changes (only when the user clicks the
+  # action button).
   output$view <- renderTable({
-    head(datasetInput(), n = input$obs)
+    head(datasetInput(), n = isolate(input$obs))
   })
 }

007-widgets/ui.R

@@ -2,40 +2,40 @@ library(shiny)
 
 # Define UI for dataset viewer application
 fluidPage(
-  
+
   # Application title.
   titlePanel("More Widgets"),
-  
+
   # Sidebar with controls to select a dataset and specify the
   # number of observations to view. The helpText function is
   # also used to include clarifying text. Most notably, the
-  # inclusion of a submitButton defers the rendering of output
+  # inclusion of an actionButton defers the rendering of output
   # until the user explicitly clicks the button (rather than
   # doing it immediately when inputs change). This is useful if
   # the computations required to render output are inordinately
   # time-consuming.
   sidebarLayout(
     sidebarPanel(
-      selectInput("dataset", "Choose a dataset:", 
+      selectInput("dataset", "Choose a dataset:",
                   choices = c("rock", "pressure", "cars")),
-      
+
       numericInput("obs", "Number of observations to view:", 10),
-      
+
       helpText("Note: while the data view will show only the specified",
                "number of observations, the summary will still be based",
                "on the full dataset."),
-      
-      submitButton("Update View")
+
+      actionButton("update", "Update View")
     ),
-    
+
     # Show a summary of the dataset and an HTML table with the
     # requested number of observations. Note the use of the h4
     # function to provide an additional header above each output
     # section.
     mainPanel(
       h4("Summary"),
       verbatimTextOutput("summary"),
-      
+
       h4("Observations"),
       tableOutput("view")
     )