Cancellation of Brokerage Fees

What are the obligations to the agent where the contract was signed but subsequently revoked? Is the realtor entitled to the brokerage fee?
This is where the issue of when the obligation to pay the agent becomes paramount. If the date of payment was fixed as the completion of the Brokerage, then what needs to determined is at what point is the Brokerage completed. If the point of completion is determined as the time of the signature of the purchase contract, then that would be the date to pay the agent. Conversely, if the date of completion shall be defined as the time as of which his client received all of the benefits of the sale, the agent would not be entitled to receive a commission in the event that the contract was defaulted upon.
However, a contract that was annulled well after signature or by mutual consent of the buyer and seller, clearly the agent is entitled to his commission.