def group_callback(self, iocb):
        """Callback when a child iocb completes."""
        if _debug: IOGroup._debug("group_callback %r", iocb)

        # check all the members
        for iocb in self.ioMembers:
            if not iocb.ioComplete.isSet():
                if _debug: IOGroup._debug("    - waiting for child: %r", iocb)
                break
        else:
            if _debug: IOGroup._debug("    - all children complete")
            # everything complete
            self.ioState = COMPLETED
            self.trigger()
Callback when a child iocb completes.