Called when a msg is received from the front-end

def _handle_msg(self, msg):
        """Called when a msg is received from the front-end"""
        data = msg['content']['data']
        method = data['method']

        if method == 'update':
            if 'state' in data:
                state = data['state']
                if 'buffer_paths' in data:
                    _put_buffers(state, data['buffer_paths'], msg['buffers'])
                self.set_state(state)

        # Handle a state request.
        elif method == 'request_state':
            self.send_state()

        # Handle a custom msg from the front-end.
        elif method == 'custom':
            if 'content' in data:
                self._handle_custom_msg(data['content'], msg['buffers'])

        # Catch remainder.
        else:
            self.log.error('Unknown front-end to back-end widget msg with method "%s"' % method)