save reactiveValues

server.R

library(shiny)
library(RCurl)

shinyServer(function(input, output, session){  
  
  urls <- reactiveValues()  
  if(file.exists("data.Rdata")){
    load("data.Rdata")
    urls$url <- df$url
    urls$isbn <- df$isbn
    urls$link <- df$link
    urls$comment <- df$comment
  }else{    
    urls$url <- c()
    urls$isbn <- c()
    urls$link <- c()
    urls$comment <- c()
  }
  
  observe({
    input$submit
    isolate({
      url <- input$url
      isbn <- getISBN(url)
      comment <- input$comment
      link <- getImageLink(url, isbn)
      urls$url <- c(urls$url, url)
      urls$isbn <- c(urls$isbn, isbn)
      urls$link <- c(urls$link, link)
      urls$comment <- c(urls$comment, comment)
      df <- as.data.frame(cbind(url=as.character(urls$url), 
                       isbn=as.character(urls$isbn),
                       comment=as.character(urls$comment), 
                       link=as.character(urls$link)), stringsAsFactors=F)
      save(df, file="data.Rdata")
      return(urls)      
    })
  })  
  
  getISBN <- function(url){    
    tmp <- getURL(url)
    a <- regexpr("ISBN-13", tmp)
    isbn <- substring(tmp, a + 13, a + 26)
    isbn <- gsub("-", "", isbn)
    return(isbn)    
  }
  
  getImage <- function(isbn){
    url <- "http://content-3.powells.com/cgi-bin/imageDB.cgi?isbn="
    full_url <- paste0(url, isbn)
    download.file(full_url, destfile=paste0("images/", isbn, ".jpg"))
  }    
        
  getImageLink <- function(url, isbn){
    # input: amazon book url
    # output: HTML image link for book
    html <- paste0('<a href=', url, '><img src="http://content-3.powells.com/cgi-bin/imageDB.cgi?isbn=', isbn,'"></a>')
    return(html)
  }  
  
  output$books <- renderUI({
    HTML(paste(urls$link, collapse="</br>"))
  })
})

ui.R

library(shiny)

shinyUI(pageWithSidebar(
  titlePanel("Reading List"),
  sidebarPanel(
    tags$head(
      tags$style(type="text/css", "label.radio { display: inline-block; }", ".radio input[type=\"radio\"] { float: none; }"),
      tags$style(type="text/css", "select { max-width: 200px; }"),
      tags$style(type="text/css", "textarea { max-width: 185px; }"),
      tags$style(type="text/css", ".jslider { max-width: 200px; }"),
      tags$style(type='text/css', ".well { max-width: 250px; }"),
      tags$style(type='text/css', ".span4 { max-width: 250px; }")
    ),
    textInput("url", "Enter URL here", value=""),
    textInput("comment", "Enter comment here", value=""),
    actionButton("submit", "Submit")
  ),
  mainPanel(
    uiOutput("books")
  )
))