ROS中的服务service是一问一答的形式，你来查询了，我就返给你要的信息。
action也有服务的概念，但是它不一样的地方是：不是一问一答，而多了一个反馈，它会不断反馈项目进度。
如navigation下的move_base package，你设定了目标点，反馈信息可能是机器人在规划路径上的即时位姿，
直到机器人到达目标点，返回SUCCEEDED消息。
上面所述的 ActionClient 和 ActionServer 通过 ROS Action Protocol （ROS Action 协议，建立在ROS messages的基础上）来通信。
Client&Server 为用户提供了简单的API，通过函数的调用和回调，用来在client端request一个目标，或者，在server端来执行达成一个目标。
下图说明这个机制如何运行：
Action Specification: Goal, Feedback, & Result
Goal：
为了用actions机制完成一个任务，我们引入了the notion of a goal, 这个goal可以被ActionClient发送到ActionServer. 比如在move base
这个案例中，它的类型是PoseStamped，包含了机器人应该到达的哪里的信息。在激光扫描云台控制案例中，the goal会包含扫描的参数
（min angle, max angle, speed 等）
Feedback
Feedback是server用来告诉ActionClient goal执行过程中的各种情况。在moving_base案例中，它是机器人现在的位姿；
在controlling the tilting laser scanner案例中, this might be the time left until the scan completes（扫描剩余时间）.
Result
执行结果，比如在move_base中的结果和机器人pose；在云台激光扫描中的一个请求的点云数据。等
下面写一个actionlib的c++例程
catkin_create_pkg actionlib_tutorials roscpp rospy actionlib actionlib_msgs message_generation
在包中添加action文件夹在里面新建一个 Fibonacci.action文件
#goal definition int32 order --- #result definition int32[] sequence --- #feedback definition int32[] sequence
修改package.xml添加
<run_depend>message_generation</run_depend>
修改 CMakeLists.txt删除添加；编译cpp文件根据自己的添加
add_action_files(
DIRECTORY action
FILES Fibonacci.action
) generate_messages(DEPENDENCIES actionlib_msgs ) CATKIN_DEPENDS actionlib actionlib_msgs message_generation roscpp rospy
简单的client.cpp
#include <ros/ros.h>
#include <actionlib/client/simple_action_client.h>
#include <actionlib_tutorials/FibonacciAction.h>
using namespace actionlib_tutorials;
typedef actionlib::SimpleActionClient<FibonacciAction> Client;
int main (int argc, char **argv)
{
ros::init(argc, argv, "test_fibonacci_callback");
// Create the action client
Client ac("fibonacci", true);
ROS_INFO("Waiting for action server to start.");
ac.waitForServer();
ROS_INFO("Action server started, sending goal.");
// Send Goal
FibonacciGoal goal;
goal.order = 10;
ac.sendGoal(goal);
bool finish_before_timeout=ac.waitForResult(ros::Duration(15));
if(finish_before_timeout)
{
actionlib::SimpleClientGoalState state = ac.getState();
ROS_INFO("action finish : %s",state.getState().toString().c_str());
}else
{
ROS_INFO("TIMEOUT");
}
ros::spin();
return 0;
}
带回调显示的clientprocess.cpp
#include <ros/ros.h>
#include <actionlib/client/simple_action_client.h>
#include <actionlib_tutorials/FibonacciAction.h>
using namespace actionlib_tutorials;
typedef actionlib::SimpleActionClient<FibonacciAction> Client;
// Called once when the goal completes
void doneCb(const actionlib::SimpleClientGoalState& state,
const FibonacciResultConstPtr& result)
{
ROS_INFO("Finished in state [%s]", state.toString().c_str());
ROS_INFO("Answer: %i", result->sequence.back());
ros::shutdown();
}
// Called once when the goal becomes active
void activeCb()
{
ROS_INFO("Goal just went active");
}
// Called every time feedback is received for the goal
void feedbackCb(const FibonacciFeedbackConstPtr& feedback)
{
ROS_INFO("Got Feedback of length %lu", feedback->sequence.size());
}
int main (int argc, char **argv)
{
ros::init(argc, argv, "test_fibonacci_callback");
// Create the action client
Client ac("fibonacci", true);
ROS_INFO("Waiting for action server to start.");
ac.waitForServer();
ROS_INFO("Action server started, sending goal.");
// Send Goal
FibonacciGoal goal;
goal.order = 20;
ac.sendGoal(goal, &doneCb, &activeCb, &feedbackCb);
ros::spin();
return 0;
}
server.cpp
#include <ros/ros.h>
#include <actionlib/server/simple_action_server.h>
#include <actionlib_tutorials/FibonacciAction.h>
typedef actionlib::SimpleActionServer<actionlib_tutorials::FibonacciAction> Server;
class FibonacciAction
{
protected:
ros::NodeHandle nh_;
Server as_; // NodeHandle instance must be created before this line. Otherwise strange error occurs.
std::string action_name_;
// create messages that are used to published feedback/result
actionlib_tutorials::FibonacciFeedback feedback_;
actionlib_tutorials::FibonacciResult result_;
public:
FibonacciAction(std::string name) :
as_(nh_, name, boost::bind(&FibonacciAction::executeCB, this, _1), false),
action_name_(name)
{
as_.start();
}
~FibonacciAction(void)
{
}
void executeCB(const actionlib_tutorials::FibonacciGoalConstPtr &goal)
{
// helper variables
ros::Rate r(1);
bool success = true;
// push_back the seeds for the fibonacci sequence
feedback_.sequence.clear();
feedback_.sequence.push_back(0);
feedback_.sequence.push_back(1);
// publish info to the console for the user
ROS_INFO("%s: Executing, creating fibonacci sequence of order %i with seeds %i, %i", action_name_.c_str(), goal->order, feedback_.sequence[0], feedback_.sequence[1]);
// start executing the action
for(int i=1; i<=goal->order; i++)
{
// check that preempt has not been requested by the client
if (as_.isPreemptRequested() || !ros::ok())
{
ROS_INFO("%s: Preempted", action_name_.c_str());
// set the action state to preempted
as_.setPreempted();
success = false;
break;
}
feedback_.sequence.push_back(feedback_.sequence[i] + feedback_.sequence[i-1]);
// publish the feedback
as_.publishFeedback(feedback_);
// this sleep is not necessary, the sequence is computed at 1 Hz for demonstration purposes
r.sleep();
}
if(success)
{
result_.sequence = feedback_.sequence;
ROS_INFO("%s: Succeeded", action_name_.c_str());
// set the action state to succeeded
as_.setSucceeded(result_);
}
}
};
int main(int argc, char** argv)
{
ros::init(argc, argv, "fibonacci");
FibonacciAction fibonacci("fibonacci");
ros::spin();
return 0;
}
编译后运行
rosrun actionlib_tutorials client
rosrun actionlib_tutorials sever
在设置时间内服务完成则状态输出完成，否则反馈失败，这种等待模式效率太低
rosrun actionlib_tutorials clientprocess rosrun actionlib_tutorials sever
将动作完成过程实时的将状态反馈回来，直到动作完成或失败