How to block until promise completes?

I have a use case that needs to use Promises, but the old calling code is synchronous. In Swift, is there a way to block the synchronous caller to wait for the promise to finish? I don't think I can use a custom dispatch queue since it needs to execute on the main thread.

Callback methods unfortunately do not work since the promise needs to start and complete at the middle of a long chain of synchronous commands.