Add a common interface for describing an EventHandler

A common pattern for describing event handlers has been moved to the parent class.
Subclasses should implement the 'handler_description' and 'matcher_description' properties rather than implementing a describe method.

launch/launch/event_handler.py

@@ -15,7 +15,10 @@
 """Module for EventHandler class."""
 
 from typing import Callable
+from typing import List
 from typing import Optional
+from typing import Text
+from typing import Tuple
 
 from .event import Event
 from .some_actions_type import SomeActionsType
@@ -59,6 +62,8 @@ def __init__(
     @property
     def entities(self):
         """Getter for entities."""
+        # if self.__entities is None:
+        #     return []
         return self.__entities
 
     @property
@@ -75,7 +80,35 @@ def handle_once(self, value):
             )
         self.__handle_once = value
 
-    # TODO(wjwwood): setup standard interface for describing event handlers
+    @property
+    def handler_description(self):
+        """
+        Return the string description of the handler.
+
+        This should be overridden.
+        """
+        return None
+
+    @property
+    def matcher_description(self):
+        """
+        Return the string description of the matcher.
+
+        This should be overridden.
+        """
+        return None
+
+    def describe(self) -> Tuple[Text, List[SomeActionsType]]:
+        """Return the description list with 0 as a string, and then LaunchDescriptionEntity's."""
+        return (
+            "{}(matcher='{}', handler='{}', handle_once={})".format(
+                type(self).__name__,
+                self.matcher_description,
+                self.handler_description,
+                self.handle_once
+            ),
+            self.entities if self.entities is not None else []
+        )
 
     def matches(self, event: Event) -> bool:
         """Return True if the given event should be handled by this event handler."""

launch/launch/event_handlers/on_include_launch_description.py

@@ -14,13 +14,10 @@
 
 """Module for OnIncludeLaunchDescription class."""
 
-from typing import List
 from typing import Text
-from typing import Tuple
 
 from ..event_handler import EventHandler
 from ..events import IncludeLaunchDescription
-from ..launch_description_entity import LaunchDescriptionEntity
 from ..utilities import is_a_subclass
 
 
@@ -38,12 +35,12 @@ def __init__(self, **kwargs):
             **kwargs,
         )
 
-    def describe(self) -> Tuple[Text, List[LaunchDescriptionEntity]]:
-        """Return the description list with 0 as a string, and then LaunchDescriptionEntity's."""
-        return (
-            "OnIncludeLaunchDescription(matcher='{}', handler='{}')".format(
-                'event issubclass of launch.events.IncludeLaunchDescription',
-                'returns the launch_description in the event'
-            ),
-            [],
-        )
+    @property
+    def handler_description(self) -> Text:
+        """Return the string description of the handler."""
+        return 'returns the launch_description in the event'
+
+    @property
+    def matcher_description(self) -> Text:
+        """Return the string description of the matcher."""
+        return 'event issubclass of launch.events.IncludeLaunchDescription'

launch/launch/event_handlers/on_process_exit.py

@@ -17,11 +17,9 @@
 import collections
 from typing import Callable
 from typing import cast
-from typing import List
 from typing import Optional
 from typing import overload
 from typing import Text
-from typing import Tuple
 
 from ..event import Event
 from ..event_handler import EventHandler
@@ -110,24 +108,18 @@ def handle(self, event: Event, context: LaunchContext) -> Optional[SomeActionsTy
         """Handle the given event."""
         return self.__on_exit(cast(ProcessExited, event), context)
 
-    def describe(self) -> Tuple[Text, List[LaunchDescriptionEntity]]:
-        """Return the description list with 0 as a string, and then LaunchDescriptionEntity's."""
+    @property
+    def handler_description(self) -> Text:
+        """Return the string description of the handler."""
+        # TODO(jacobperron): revisit how to describe known actions that are passed in.
+        #                    It would be nice if the parent class could output their desciption
+        #                    via the 'entities' property.
         if self.__actions_on_exit:
-            # A list of resulting actions is already known.
-            return (
-                "OnProcessExit(matcher='{}', handler=<actions>)".format(self.matcher_description),
-                self.__actions_on_exit,
-            )
-        # A callable handler has been provided.
-        return (
-            "OnProcessExit(matcher='{}', handler={})".format(
-                self.matcher_description,
-                self.__on_exit),
-            [],
-        )
+            return '<actions>'
+        return '{}'.format(self.__on_exit)
 
     @property
-    def matcher_description(self):
+    def matcher_description(self) -> Text:
         """Return the string description of the matcher."""
         if self.__target_action is None:
             return 'event == ProcessExited'

launch/launch/event_handlers/on_process_io.py

@@ -16,16 +16,13 @@
 
 from typing import Callable
 from typing import cast
-from typing import List
 from typing import Optional
 from typing import Text
-from typing import Tuple
 
 from ..event import Event
 from ..event_handler import EventHandler
 from ..events.process import ProcessIO
 from ..launch_context import LaunchContext
-from ..launch_description_entity import LaunchDescriptionEntity
 from ..some_actions_type import SomeActionsType
 
 if False:
@@ -78,8 +75,9 @@ def handle(self, event: Event, context: LaunchContext) -> Optional[SomeActionsTy
             return self.__on_stdin(event)
         return None
 
-    def describe(self) -> Tuple[Text, List[LaunchDescriptionEntity]]:
-        """Return the description list with 0 as a string, and then LaunchDescriptionEntity's."""
+    @property
+    def handler_description(self) -> Text:
+        """Return the string description of the handler."""
         handlers = []
         if self.__on_stdin is not None:
             handlers.append("on_stdin: '{}'".format(self.__on_stdin))
@@ -88,15 +86,10 @@ def describe(self) -> Tuple[Text, List[LaunchDescriptionEntity]]:
         if self.__on_stderr is not None:
             handlers.append("on_stderr: '{}'".format(self.__on_stderr))
         handlers_str = '{' + ', '.join(handlers) + '}'
-        return (
-            "OnProcessIO(matcher='{}', handlers={})".format(
-                self.matcher_description, handlers_str
-            ),
-            [],
-        )
+        return handlers_str
 
     @property
-    def matcher_description(self):
+    def matcher_description(self) -> Text:
         """Return the string description of the matcher."""
         if self.__target_action is None:
             return 'event issubclass of ProcessIO'

launch/launch/event_handlers/on_shutdown.py

@@ -16,16 +16,13 @@
 
 from typing import Callable
 from typing import cast
-from typing import List
 from typing import Optional
 from typing import overload
 from typing import Text
-from typing import Tuple
 
 from ..event import Event
 from ..event_handler import EventHandler
 from ..events import Shutdown
-from ..launch_description_entity import LaunchDescriptionEntity
 from ..some_actions_type import SomeActionsType
 from ..utilities import is_a_subclass
 
@@ -70,15 +67,11 @@ def handle(self, event: Event, context: 'LaunchContext') -> Optional[SomeActions
         context.extend_locals({'event': event})
         return self.__on_shutdown(cast(Shutdown, event), context)
 
-    def describe(self) -> Tuple[Text, List[LaunchDescriptionEntity]]:
-        """Return the description list with 0 as a string, and then LaunchDescriptionEntity's."""
+    @property
+    def handler_description(self) -> Text:
+        """Return the string description of the handler."""
         # TODO(dhood): print known actions if they were passed in, like in OnProcessExit
-        return (
-            "OnShutdown(matcher='{}', handler={})".format(
-                self.matcher_description,
-                self.__on_shutdown),
-            [],
-        )
+        return '{}'.format(self.__on_shutdown)
 
     @property
     def matcher_description(self):

launch_ros/launch_ros/event_handlers/on_state_transition.py

@@ -15,14 +15,11 @@
 """Module for OnStateTransition class."""
 
 from typing import Callable
-from typing import List
 from typing import Optional
 from typing import Text
-from typing import Tuple
 
 from launch.event import Event
 from launch.event_handler import EventHandler
-from launch.launch_description_entity import LaunchDescriptionEntity
 from launch.some_actions_type import SomeActionsType
 from launch.some_substitutions_type import SomeSubstitutionsType
 
@@ -84,12 +81,10 @@ def __init__(
         )
         self.__target_lifecycle_node = target_lifecycle_node
 
-    def describe(self) -> Tuple[Text, List[LaunchDescriptionEntity]]:
-        """Return the description list with 0 as a string, and then LaunchDescriptionEntity's."""
-        return (
-            "OnStateTransition(matcher='{}', handler=<actions>)".format(self.matcher_description),
-            self.entities,
-        )
+    @property
+    def handler_description(self) -> Text:
+        """Return the string description of the handler."""
+        return '<actions>'
 
     @property
     def matcher_description(self):