前言
ROS 2中的Launch是一个执行和管理用户定义流程的系统。它负责监控其启动的流程的状态，并报告这些流程的状态变化并作出反应。这些变化被称为事件，可以通过向启动系统注册事件处理程序来处理。可以为特定事件注册事件处理程序，并可用于监视进程的状态。此外，它们还可用于定义一组复杂的规则，这些规则可用于动态修改启动文件。
动动手
我们还是继续在之前的launch_tutorial包中进行。
编写事件句柄launch文件
在launch文件夹中新建example_event_handlers_launch.py，复制以下内容：
from launch_ros.actions import Node
from launch import LaunchDescription
from launch.actions import (DeclareLaunchArgument, EmitEvent, ExecuteProcess,
LogInfo, RegisterEventHandler, TimerAction)
from launch.conditions import IfCondition
from launch.event_handlers import (OnExecutionComplete, OnProcessExit,
OnProcessIO, OnProcessStart, OnShutdown)
from launch.events import Shutdown
from launch.substitutions import (EnvironmentVariable, FindExecutable,
LaunchConfiguration, LocalSubstitution,
PythonExpression)
def generate_launch_description():
turtlesim_ns = LaunchConfiguration('turtlesim_ns')
use_provided_red = LaunchConfiguration('use_provided_red')
new_background_r = LaunchConfiguration('new_background_r')
turtlesim_ns_launch_arg = DeclareLaunchArgument(
'turtlesim_ns',
default_value='turtlesim1'
)
use_provided_red_launch_arg = DeclareLaunchArgument(
'use_provided_red',
default_value='False'
)
new_background_r_launch_arg = DeclareLaunchArgument(
'new_background_r',
default_value='200'
)
turtlesim_node = Node(
package='turtlesim',
namespace=turtlesim_ns,
executable='turtlesim_node',
name='sim'
)
spawn_turtle = ExecuteProcess(
cmd=[[
FindExecutable(name='ros2'),
' service call ',
turtlesim_ns,
'/spawn ',
'turtlesim/srv/Spawn ',
'"{x: 2, y: 2, theta: 0.2}"'
]],
shell=True
)
change_background_r = ExecuteProcess(
cmd=[[
FindExecutable(name='ros2'),
' param set ',
turtlesim_ns,
'/sim background_r ',
'120'
]],
shell=True
)
change_background_r_conditioned = ExecuteProcess(
condition=IfCondition(
PythonExpression([
new_background_r,
' == 200',
' and ',
use_provided_red
])
),
cmd=[[
FindExecutable(name='ros2'),
' param set ',
turtlesim_ns,
'/sim background_r ',
new_background_r
]],
shell=True
)
return LaunchDescription([
turtlesim_ns_launch_arg,
use_provided_red_launch_arg,
new_background_r_launch_arg,
turtlesim_node,
RegisterEventHandler(
OnProcessStart(
target_action=turtlesim_node,
on_start=[
LogInfo(msg='Turtlesim started, spawning turtle'),
spawn_turtle
]
)
),
RegisterEventHandler(
OnProcessIO(
target_action=spawn_turtle,
on_stdout=lambda event: LogInfo(
msg='Spawn request says "{}"'.format(
event.text.decode().strip())
)
)
),
RegisterEventHandler(
OnExecutionComplete(
target_action=spawn_turtle,
on_completion=[
LogInfo(msg='Spawn finished'),
change_background_r,
TimerAction(
period=2.0,
actions=[change_background_r_conditioned],
)
]
)
),
RegisterEventHandler(
OnProcessExit(
target_action=turtlesim_node,
on_exit=[
LogInfo(msg=(EnvironmentVariable(name='USER'),
' closed the turtlesim window')),
EmitEvent(event=Shutdown(
reason='Window closed'))
]
)
),
RegisterEventHandler(
OnShutdown(
on_shutdown=[LogInfo(
msg=['Launch was asked to shutdown: ',
LocalSubstitution('event.reason')]
)]
)
),
])稍微说明。
RegisterEventHandler(
OnProcessStart(
target_action=turtlesim_node,
on_start=[
LogInfo(msg='Turtlesim started, spawning turtle'),
spawn_turtle
]
)
), RegisterEventHandler为事件句柄的注册函数，大家从上面那个完整的launch文件里应该也看到了，一共注册了 OnProcessStart, OnProcessIO, OnExecutionComplete, OnProcessExit, 和OnShutdown 这五个事件句柄。
OnProcessStart事件处理程序用于注册在turtlesim节点启动时执行的回调函数。它将消息输出到控制台，并在turtlesim节点启动时执行spawn_turtle操作。
RegisterEventHandler(
OnProcessIO(
target_action=spawn_turtle,
on_stdout=lambda event: LogInfo(
msg='Spawn request says "{}"'.format(
event.text.decode().strip())
)
)
),OnProcessIO事件处理程序用于注册回调函数，该函数在spawn_turtle操作写入其标准输出时执行。它记录生成请求的结果。
RegisterEventHandler(
OnExecutionComplete(
target_action=spawn_turtle,
on_completion=[
LogInfo(msg='Spawn finished'),
change_background_r,
TimerAction(
period=2.0,
actions=[change_background_r_conditioned],
)
]
)
),OnExecutionComplete事件处理程序用于注册在spawn_turtle操作完成时执行的回调函数。它将一条消息输出到控制台，并在派生操作完成时执行change_background_r和change_background_r_conditioned操作。
RegisterEventHandler(
OnProcessExit(
target_action=turtlesim_node,
on_exit=[
LogInfo(msg=(EnvironmentVariable(name='USER'),
' closed the turtlesim window')),
EmitEvent(event=Shutdown(
reason='Window closed'))
]
)
),OnProcessExit事件处理程序用于注册在turtlesim节点退出时执行的回调函数。它将消息输出到控制台，并在turtlesim节点退出时执行EmitEvent操作以发出Shutdown事件。这意味着当turtlesim窗口关闭时，启动过程将关闭。
RegisterEventHandler(
OnShutdown(
on_shutdown=[LogInfo(
msg=['Launch was asked to shutdown: ',
LocalSubstitution('event.reason')]
)]
)
),最后，OnShutdown事件处理程序用于注册回调函数，该函数在启动文件被要求关闭时执行。它会向控制台输出一条消息，说明为什么要求关闭启动文件。它记录带有关闭原因的消息，如关闭turtlesim窗口或用户发出的ctrl-c信号。
构建功能包
我们回到launch_ws工作空间根路径下进行构建：
$colcon build运行launch
$ros2 launch launch_tutorial example_event_handlers_launch.py turtlesim_ns:='turtlesim3' use_provided_red:='True' new_background_r:=200这个过程会有下面几个部分组成：
- 启动一个turtlesim节点（背景颜色为蓝色）；
- 孵化第二个小海龟；
- 改变背景颜色为紫色；
- 2秒后改变背景颜色为粉色（如果其提供的background_r参数值为200且use_provided_red参数值为True）;
- 当小海龟窗口关闭的时候关掉launch文件；
另外当发生以下的任何一种情况时控制台会输出log消息：
- turtlesim节点启动时；
- 孵化动作被执行时；
- change_background_r动作被执行时；
- change_background_r_conditioned动作被执行时；
- turtlesim节点退出时；
- launch进程被请求要关闭时；
运行之前记得source install/setup.bash,运行最终结果如下：
当我们ctrl-c关闭时，返回如下：
文档资料
官方文档有提供更为详细的可用的事件句柄信息。
本篇完。