Add a "hello world" style example (#532)

* Added hello_world example and updated README

Signed-off-by: Aditya Pande <[email protected]>

* Added marker

Signed-off-by: Aditya Pande <[email protected]>

* Updated readme, docstrings

Signed-off-by: Aditya Pande <[email protected]>

* Updated readme, added comments to .py file

Signed-off-by: Aditya Pande <[email protected]>

* Updated comment in .py file

Signed-off-by: Aditya Pande <[email protected]>

* Added shell='True' for windows CI

Signed-off-by: Aditya Pande <[email protected]>

* typo fix

Signed-off-by: Aditya Pande <[email protected]>

Author: adityapande-1995

launch_testing/README.md

@@ -201,6 +201,21 @@ add_launch_test(
 
 ## Examples
 
+### `hello_world_launch_test.py`
+
+Usage:
+
+```sh
+launch_test test/launch_testing/examples/hello_world_launch_test.py
+```
+
+This test is a simple example on how to use the ``launch_testing``. 
+
+It launches a process and asserts that it prints "hello_world" to ``stdout`` using ``proc_output.assertWaitFor()``.
+Finally, it checks if the process exits normally (zero exit code).
+
+The ``@launch_testing.markers.keep_alive`` decorator ensures that the launch process stays alive long enough for the tests to run.
+
 ### `good_proc_launch_test.py`
 
 Usage:

launch_testing/test/launch_testing/examples/hello_world_launch_test.py

@@ -0,0 +1,58 @@
+# Copyright 2021 Open Source Robotics Foundation, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import unittest
+
+import launch
+import launch.actions
+import launch_testing.actions
+import launch_testing.markers
+import pytest
+
+
+# This function specifies the processes to be run for our test
+@pytest.mark.launch_test
+@launch_testing.markers.keep_alive
+def generate_test_description():
+    """Launch a simple process to print 'hello_world'."""
+    return launch.LaunchDescription([
+        # Launch a process to test
+        launch.actions.ExecuteProcess(
+            cmd=['echo', 'hello_world'],
+            shell=True
+        ),
+        # Tell launch to start the test
+        launch_testing.actions.ReadyToTest()
+    ])
+
+
+# This is our test fixture. Each method is a test case.
+# These run alongside the processes specified in generate_test_description()
+class TestHelloWorldProcess(unittest.TestCase):
+
+    def test_read_stdout(self, proc_output):
+        """Check if 'hello_world' was found in the stdout."""
+        # 'proc_output' is an object added automatically by the launch_testing framework.
+        # It captures the outputs of the processes launched in generate_test_description()
+        # Refer to the documentation for further details.
+        proc_output.assertWaitFor('hello_world', timeout=10, stream='stdout')
+
+
+# These tests are run after the processes in generate_test_description() have shutdown.
+@launch_testing.post_shutdown_test()
+class TestHelloWorldShutdown(unittest.TestCase):
+
+    def test_exit_codes(self, proc_info):
+        """Check if the processes exited normally."""
+        launch_testing.asserts.assertExitCodes(proc_info)