title: UIWebView in multithread ViewController
body: <p>I have a UIWebView in a viewcontroller, which has two methods as below. The question is if I pop out(tap back on navigation bar) this controller before the second thread is done, the app will crash after [super dealloc], because "Tried to obtain the web lock from a thread other than the main thread or the web thread. This may be a result of calling to UIKit from a secondary thread.". Any help would be really appreciated.</p>
  
  <pre><code>-(void)viewDidAppear:(BOOL)animated {
      [super viewWillAppear:animated];
      NSInvocationOperation *operation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(load) object:nil];
      [operationQueue addOperation:operation];
      [operation release];
  }
  
  -(void)load {
      [NSThread sleepForTimeInterval:5];
      [self performSelectorOnMainThread:@selector(done) withObject:nil waitUntilDone:NO];
  }
  </code></pre>