title: Can S4 object be set as reactive variables?
body: <p>I'm developing a shiny app, where I want to set an S4 object as a global varibale. But I get an error message:</p>
  
  <pre><code>Error in .getReactiveEnvironment()$currentContext() : 
    Operation not allowed without an active reactive context. (You tried to do something that can only be done from inside a reactive expression or observer.)
  </code></pre>
  
  <p>I wonder if S4 objects can not be set as reactive values in shiny.</p>
  
  <p>My code is here</p>
  
  <pre><code>ensemblObj=reactiveValues()
  ensemblObj$ensembl=useMart(biomart='ensembl')
  ensemblObj$archieves=listEnsemblArchives()
  ensemblObj$specials=listDatasets(ensemblObj$ensembl)
  </code></pre>
  
  <p>The error occurs at the last line. useMart(biomart='ensembl') will create an object that link to a server, and listDatasets(ensemblObj$ensembl) wants to get all available datasets from the server.</p>