Activity indicator not animating=iphone

In my application i am 开发者_开发知识库uploading some images to the servelet, which takes some time to complete upload. I have implemented an uiactivityviewer using IB and connected it to an outlet named acticityIndicator. But when i call [activityIndicator startsAnimating]; nothing is happening!!! What might be the problem. i've debugged the code, the statement is executing, but nothing happens!!!

Are you performing your uploading on the main thread, or kicking off a background thread to do it?

If the former then you are blocking the UI thread so it doesn't have a chance to update the activity indicator.