Get the sender/signer from the first argument of the transaction data field

getAddressFromData.md

/**
 * @dev Return the first param of the meta tx data field which must be an address.
 * E.g: in transferFrom(sender, to, amount) sender address is returned.
 * @param dataField The meta transactions data field.
 */
function getAddressFromData(bytes memory dataField)
  public
  pure
  returns (address)
{
  address sender;
  // solium-disable-next-line security/no-inline-assembly
  assembly {
      sender := mload(add(dataField, 36))
  }
  return sender;
}
```