title: stringByEvaluatingJavaScriptFromString return value is not reliable?
body: <p>I'm calling <strong>stringByEvaluatingJavaScriptFromString</strong> while doing an async HTTP get operation in the callback for new bytes arriving. </p>
  
  <p>But I know I don't know for sure, so I'm asking. What is the expected behavior? </p>
  
  <p>And is <strong>stringByEvaluatingJavaScriptFromString</strong> not executed synchronously, but actually run in a thread, which would be perilous for my purposes and would explain the empty response string... but not the valid response strings?</p>
  
  <p>Lastly, is there any way to ask UIWebView whether it's busy and therefore cannot run  <strong>stringByEvaluatingJavaScriptFromString</strong> reliably?</p>
  
  <p>Thanks.</p>