Move OnProcessStart from LoadComposableNodes to ComposableNodeContainer (#16)

* Move OnProcessStart from Load to Container

This allows LoadComposableNodes to be triggered by some event long after
the container process starts.

Signed-off-by: Shane Loretz <[email protected]>

* remove unused imports

Signed-off-by: Shane Loretz <[email protected]>

* Document LodeComposableNode must execute after container

Signed-off-by: Shane Loretz <[email protected]>

Author: sloretz

launch_ros/launch_ros/actions/composable_node_container.py

@@ -18,6 +18,8 @@
 from typing import Optional
 
 from launch.action import Action
+from launch.actions import RegisterEventHandler
+from launch.event_handlers.on_process_start import OnProcessStart
 from launch.launch_context import LaunchContext
 from launch.some_substitutions_type import SomeSubstitutionsType
 
@@ -61,10 +63,20 @@ def execute(self, context: LaunchContext) -> Optional[List[Action]]:
         load_actions = None  # type: Optional[List[Action]]
         if self.__composable_node_descriptions is not None:
             from .load_composable_nodes import LoadComposableNodes
-            load_actions = [LoadComposableNodes(
-                composable_node_descriptions=self.__composable_node_descriptions,
-                target_container=self
-            )]
+            # Perform load action once the container has started.
+            load_actions = [
+                RegisterEventHandler(
+                    event_handler=OnProcessStart(
+                        target_action=self,
+                        on_start=[
+                            LoadComposableNodes(
+                                composable_node_descriptions=self.__composable_node_descriptions,
+                                target_container=self
+                            )
+                        ]
+                    )
+                )
+            ]
         container_actions = super().execute(context)  # type: Optional[List[Action]]
         if container_actions is not None and load_actions is not None:
             return container_actions + load_actions

launch_ros/launch_ros/actions/load_composable_nodes.py

@@ -20,9 +20,6 @@
 import composition_interfaces.srv
 
 from launch.action import Action
-from launch.actions import RegisterEventHandler
-from launch.event_handlers.on_process_start import OnProcessStart
-from launch.events.process import ProcessStarted
 from launch.launch_context import LaunchContext
 import launch.logging
 from launch.utilities import ensure_argument_type
@@ -51,6 +48,9 @@ def __init__(
         The container node is expected to provide a `~/_container/load_node` service for
         loading purposes.
         Loading will be performed sequentially.
+        When executed, this action will block until the container's load service is available.
+        Make sure any LoadComposableNode action is executed only after its container processes
+        has started.
 
         :param composable_node_descriptions: descriptions of composable nodes to be loaded
         :param target_container: the container to load the nodes into
@@ -156,15 +156,6 @@ def _load_in_sequence(
                 )
             )
 
-    def _on_container_start(
-        self,
-        event: ProcessStarted,
-        context: LaunchContext
-    ) -> Optional[List[Action]]:
-        """Load nodes on container process start."""
-        self._load_in_sequence(self.__composable_node_descriptions, context)
-        return None
-
     def execute(
         self,
         context: LaunchContext
@@ -176,10 +167,5 @@ def execute(
                 self.__target_container.node_name
             )
         )
-        # Perform load action once the container has started.
-        return [RegisterEventHandler(
-            event_handler=OnProcessStart(
-                target_action=self.__target_container,
-                on_start=self._on_container_start,
-            )
-        )]
+        # Assume user has configured `LoadComposableNodes` to happen after container action
+        self._load_in_sequence(self.__composable_node_descriptions, context)