How to use subscriber in ROS node

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Hello ! I am working on ROS and morse ! I have written a ROS node which is subscribing to the sensors of the robot. Lets say sub1 is for position and sub2 is for camera; pub1 for motion and pub2 for height

Now when I subscribe to the position of the robot and stays inside that function. and wont come out of it ! I want to do something like this sub1 - get the position; pub1 - move a small distance; pub2- maintain a height; stop the motion and check if the object is in the camera view; if object found ; store the position and come out of it; pub3 - give the postion; if object NOT found; go back to sub1 and get the new position and impart motion again;

Here is what I am working on (the sample code):-

class test_vision_node_fly:
    def __init__(self):
        self.image_pub = rospy.Publisher("quadrotor/videocamera1/camera_info",Image)
        self.ROI = rospy.Publisher("roi", RegionOfInterest)
        rospy.sleep(1)
        self.cv_window_name = "Image window"
        cv2.namedWindow(self.cv_window_name, 0)
        cv2.namedWindow("trackbars" , 0) 

            self.bridge = CvBridge()

            self.image_sub = rospy.Subscriber("quadrotor/videocamera1/image",Image,self.callback)

        rospy.loginfo("Waiting for image topics...")

    def on_trackbar(self,x):
            global H_MAX
            global H_MIN
            global S_MAX
            global S_MIN
            global V_MAX
            global V_MIN

        H_MIN = cv2.getTrackbarPos('H_MIN', 'trackbars')
            H_MAX = cv2.getTrackbarPos('H_MAX', 'trackbars')  
            S_MAX = cv2.getTrackbarPos('S_MAX', 'trackbars')
            S_MIN = cv2.getTrackbarPos('S_MIN', 'trackbars')
            V_MIN = cv2.getTrackbarPos('V_MIN', 'trackbars')
            V_MAX = cv2.getTrackbarPos('V_MAX', 'trackbars')

            pass   
    def callback(self,data):

        """ Convert the raw image to OpenCV format using the convert_image() helper function """
            cv_image = self.convert_image(data)
        # Convert the image to a Numpy array since most cv2 functions
            # require Numpy arrays.
            cv_image = np.array(cv_image, dtype=np.uint8)
         "" Apply the CamShift algorithm using the do_camshift() helper function """
            cv_image = self.do_camshift(cv_image)

            """ Refresh the displayed image """
            cv2.imshow(self.cv_window_name, cv_image)

    def convert_image(self, ros_image):  
        try:
                cv_image = self.bridge.imgmsg_to_cv2(ros_image, "bgr8")
            print('q')
            return cv_image
            except CvBridgeError, e:
                print e
    def do_camshift(self, cv_image):            
            ''' converting bgr to hsv ''' 

            hsv=cv2.cvtColor(cv_image,cv2.COLOR_BGR2HSV) 

        '''creating trackbars'''
        cv2.createTrackbar('H_MIN', 'trackbars', H_MIN, H_MAX, self.on_trackbar) 
        cv2.createTrackbar('H_MAX', 'trackbars', H_MAX, H_MAX, self.on_trackbar)
            cv2.createTrackbar('S_MIN', 'trackbars', S_MIN, S_MAX, self.on_trackbar) 
        cv2.createTrackbar('S_MAX', 'trackbars', S_MAX, S_MAX, self.on_trackbar)    
        cv2.createTrackbar('V_MIN', 'trackbars', V_MIN, V_MAX, self.on_trackbar)    
            cv2.createTrackbar('V_MAX', 'trackbars', V_MAX, V_MAX, self.on_trackbar)

            ''' threshdolding '''
        COLOR_MIN = np.array([H_MIN, S_MIN, V_MIN],dtype=np.uint8)

            COLOR_MAX = np.array([H_MAX, S_MAX, V_MAX],dtype=np.uint8)

            mask=cv2.inRange(hsv, COLOR_MIN, COLOR_MAX)

            new_mask = mask.copy()

        # Bitwise-AND mask and original image
            res = cv2.bitwise_and(cv_image,cv_image, mask= mask)

        #removing noise 
        kernel = np.ones((12,12),np.uint8)

            new_mask = cv2.morphologyEx(new_mask, cv2.MORPH_CLOSE, kernel)

            new_mask = cv2.morphologyEx(new_mask, cv2.MORPH_OPEN, kernel)

        contours, hierarchy = cv2.findContours(new_mask,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)

        # Draw contours on the original image
            cv2.drawContours(cv_image, contours, -1, (0,0,0), 3)

        x,y,w,h = cv2.boundingRect(contours[-1])

        cv2.rectangle(cv_image,(x,y),(x+w,y+h),(0,254,0),3)

        if x>0 or y>0 or ...