can't see contact sensor data [closed]

edit

hello everyone , i have a question that i follow this tutorial link text,that i create a work dir "$ mkdir ~/gazebo_contact_tutorial; cd ~/gazebo_contact_tutorial" and next make an SDF world file with a box that has a contact sensor. "vim contact.world" ,the code is below ; then i run "$ gazebo contact.world " and i can see a box on the ground ,but when i type "$ gz topic -l" i can't see /gazebo/default/box/link/my_contact

if anyone know why , help me please ~

<pre>

<sdf version="1.4"> <world name="default"> <include> <uri>model://ground_plane</uri> </include>

 <include>
  <uri>model://sun</uri>
</include>

<model name="box">
  <link name="link">
    <pose>0 0 0.5 0 0 0</pose>

    <collision name="box_collision">
      <geometry>
        <box>
          <size>1 1 1</size>
        </box>
      </geometry>
    </collision>

    <visual name="visual">
      <geometry>
        <box>
          <size>1 1 1</size>
        </box>
      </geometry>
    </visual>

    <sensor name='my_contact' type='contact'>
      <contact>
        <collision>box_collision</collision>
      </contact>
    </sensor>
  </link>
</model>

</world> </sdf>