title: Google spreadsheet custom function: How to get a continuously updated valued?
body: <p>I wrote a custom google app script function in a script associated with my google doc spreadsheet.  The function calls a third party service to get data.  I can put the function in a cell:</p>
  
  <pre><code>=myfunction("something")
  </code></pre>
  
  <p>and it returns the correct value from the service.  However, how can I keep this value updated so that it's showing the latest data from the service?</p>
  
  <p><strong>Update</strong></p>
  
  <p>For example:</p>
  
  <pre><code>=temperature("90120")
  </code></pre>
  
  <p>For getting the current temperature in a given zip code.  Also my sheet may have dozens or hundreds of these so I'd prefer something that is performant and maintainable.  It doesn't truly need to be continuous, polling once a minute or ideally more frequently could work.  I'm wondering if there's some way from the script to set a timer to run to update a range of cells?</p>