roslaunch?does not provide any guarantees about what order nodes start in. This is intentional: there is no way to externally know when a node is fully initialized, so all code that is launched must be robust to launching in any order.
因为launch文件无法规定软件执行的顺序，所以有可能在发出建立机器人模型的时候，gazebo软件还没有完全启动-----然后就会看不到机器人-----虽然命令已经成功发出，但是看不到机器人，有这个可能