synchronous rpc calls in gwt, it's possible?

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Java

GWT

have a use case where I need to return a value from a asynchronous call. (I'm using GWT-Platform, but the concepts are the same.)
I need to block somehow until the AsyncCallback completes. I need the returned value in another method, or I'd just do what I need to in onSuccess().

I've tried loops, Timers, and a few other methods with no luck. Can anyone help?

thank