I'm writing a program, subscribing to the topic /ardrone/front/image_raw, but while running the python script its showing the following error -

UnboundLocalError: local variable 'callback' referenced before assignment

#!/usr/bin/env python 

#import library ros 
import rospy 
import time

#import library for send command and receive data navigation from quadcopter
from geometry_msgs.msg import Twist       #all these are msg type
from std_msgs.msg import String 
from std_msgs.msg import Empty 
from ardrone_autonomy.msg import Navdata
from sensor_msgs.msg import Range
from sensor_msgs.msg import Image

#import class status for determine status form quadcopter
#from drone_status import DroneStatus

COMMAND_PERIOD = 1000


class AutonomousFlight():
    def callback(data):
        rospy.loginfo(rospy.get_caller_id() + "I heard %s", data.data)

        def __init__(self):
            self.status = ""
            rospy.init_node('forward', anonymous=False)
            self.rate = rospy.Rate(10)          
            self.pubTakeoff = rospy.Publisher("ardrone/takeoff",Empty, queue_size=10)
            self.pubLand = rospy.Publisher("ardrone/land",Empty, queue_size=10)
            self.pubCommand = rospy.Publisher('cmd_vel',Twist, queue_size=10)
            self.subHeight = rospy.Subscriber('sonar_height',Range, self.callback)
            self.subCamera = rospy.Subscriber('ardrone/front/image_raw',Image, self.callback)
            rospy.spin()
            self.command = Twist()

            self.state_change_time = rospy.Time.now()    
            rospy.on_shutdown(self.SendLand)




        def SendTakeOff(self):
            self.pubTakeoff.publish(Empty()) 
            self.rate.sleep()

        def SendLand(self):
            self.pubLand.publish(Empty())


        def SetCommand(self, linear_x, linear_y, linear_z, angular_x, angular_y, angular_z):
            self.command.linear.x = linear_x
            self.command.linear.y = linear_y
            self.command.linear.z = linear_z
            self.command.angular.x = angular_x
            self.command.angular.y = angular_y
            self.command.angular.z = angular_z
            self.pubCommand.publish(self.command)
            self.rate.sleep()

     #def SetHeight(self):
        #pass   

        def ToggleCam(self):
            self.subCamera.subscribe(Empty())
                self.rate.sleep()   


if __name__ == '__main__': 
    try: 
        i = 0
        uav = AutonomousFlight()

        while not rospy.is_shutdown():
            uav.ToggleCam()
            uav.SendTakeOff()
            if i <= 30 :
                uav.SetCommand(0,0,1,0,0,0)
                i+=1



    except rospy.ROSInterruptException:
        pass