ROS node listen for messages while executing instance function

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I have a ROS node that is listening for messages on a topic, and when it receives a 'START' message it makes a call to an instance function of another class JointRecorder.record_playback(). What I want to happen is for the listener to continue listening for a 'STOP' message and execute the instance function concurrently, so that when the 'STOP' message is received it can terminate execution of the instance function. But, right now it just makes a synchronous call to the instance function and waits for it to complete, so the listener never receives the 'STOP' message. How can I have it both listen and execute the instance function?

class JointTrajectoryRecorder:
    def __init__(self, limb, rate, filename):
        self.limb = limb
        self.rate = rate
        self.filename = filename
        self.pub = rospy.Publisher('playback_cmds', String, queue_size=10)
        rospy.init_node('joint_trajectory_recorder', anonymous=True)
        rospy.Subscriber('record_cmds', String, self.respond)
        self.recorder = JointRecorder(self.limb, self.rate)

    def respond(self, cmd):
        # not sure if it will take command while still recording
        if cmd.data == _START_REC:
            self.recorder.record_playback()
        elif cmd.data == _STOP_REC:
            self.recorder.stop()
        elif cmd.data == _DONE:
            self.recorder.persist_data(self.filename)
        else:
            print "Command unknown:", cmd.data

    def run(self):
        print("Getting robot state... ")
        rs = baxter_interface.RobotEnable(CHECK_VERSION)
        print("Enabling robot... ")
        rs.enable()
        print("Running. Ctrl-c to quit")


class JointRecorder:
    def __init__(self, limb, rate):
        self.limb = limb
        self.rate = rospy.Rate(rate)
        self.start_time = rospy.get_time()
        self.arm = baxter_interface.Limb(self.limb)
        self.nominal = Trajectory(self.arm.joint_names(), self.rate)
        self.recordings = []
        self.nav = baxter_interface.Navigator(self.limb)
        self.is_recording = False
        self.is_done = False

    def time_stamp(self):
        return rospy.get_time() - self.start_time

    def stop(self, button_value=True):
        self.is_done = True

    def done(self):
        if rospy.is_shutdown():
            self.stop()
        return self.is_done

    def set_recording(self, button_value):
        self.is_recording = True

    def record_playback(self):
        print "\nRecording playback trajectory..."
        self.is_done = False
        t = Trajectory(self.arm.joint_names(), self.rate)
        while not self.done():
            t.add_timestep(self.time_stamp())
            for name in self.arm.joint_names():
                t.add_position(name, self.arm.joint_angle(name)) 
            self.rate.sleep()
        self.recordings.append(t)
        print "Playback recording complete."