contact sensor is not working in Gazebo8 ,Gazebo9 and Gazebo10

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• In gazebo8 ,gazebo9 and gazebo10 (install from source), the contact atrribute in ContactSensor class is Not work. When I get a ContactSensor by contacts =this->ContactSensor->Contacts(), Then the results of contacts.contact_size(),contacts.contact(0).position_size() and other related values are all zero or empty.

• But method like this->ContactSensor->IsActive(), this->ContactSensor->GetCollisionName(0) , this->ContactSensor->UpdateRate() and this->ContactSensor->Id() is work. Why?

• By the way, gazebo4 and gazebo7 is OK.

Problem Solved.It was the name of the collision cause the problem.I had named my contact link in xacro file like this

<link name="simple"> ... <collision name="simple_collision"> ... </link> and use it like this <gazebo reference="simple"> ... <contact> <collision>simple_collision</collision> <topic>/simple_contact</topic> </contact> ... </gazebo> It would Not work in this case. I should Not add a <collision name="simple_collision"> in xacro file. Just use <collision> in xacro file, it will gives you linkname+_collisionautomaticlly.