title: Passing reactive values to functions and accessing reactive attributes
body: <p>I want to take a reactive source through more than one reactive conductor before sending it to a reactive endpoint.</p>
  
  <p>Generalized example:</p>
  
  <pre><code>x &lt;- reactive({ function(input$var) })
  
  y &lt;-reactive({ function(x) })
  
  output$display&lt;-renderText({ y()$attr })
  </code></pre>
  
  <p>Below is a specific Example.</p>
  
  <p>Thanks in advance</p>
  
  <pre><code># Install packages if needed
  packageNeeds &lt;- c('shiny', 'httr', 'dataRetrieval')
  packageNeeds &lt;- packageNeeds[!packageNeeds %in% rownames(installed.packages())]
  if(length(packageNeeds)&gt;0){
    install.packages(packageNeeds, repos='http://cran.cnr.Berkeley.edu')
  }
  
  library(shiny)
  library(httr)
  library(dataRetrieval)
  
  shinyApp(
    ui = fluidPage(
      fluidRow(
              wellPanel(
                selectInput("site", label = p("Pick a site"), choices = list('01594440', 'sysys')),
                selectInput("start", label = p("pick a date"), choices = list('1985-01-01', "xyz")))),
      fluidRow(wellPanel(verbatimTextOutput("URL"))),
      fluidRow(wellPanel(verbatimTextOutput("REC")))),
    server = function(input, output, session){
      url&lt;-reactive({ 
        # Here the user inputs are used to constuct a url - 
        # but really this could be any function that I'd like to take the output of
        # and pass it on to other functions and/or subeset
        # before sending to a reactive output      
        constructWQPURL(paste("USGS",input$site,sep="-"),
                              c('01075','00029','00453'),
                              input$start, endDate = "")
        })   
      # This is the function I've passed the reactive value to
      # it returns an object with a headers attributes that has the 
      # 'total-result-count' attribute I want 
      header_call = reactive({HEAD(url)})
  
      output$URL&lt;-renderText({
        # The url displays fine
         url()
      })
      output$REC&lt;-renderText({
        # The record count from the header pull does not display
        # the value is the number of records stored as a string
        header_call()$headers$'total-result-count'
      })
    }
  )
  </code></pre>