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Update Pluginlib.rst - use the method of the derived class #5778

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Update Pluginlib.rst - use the method of the derived class #5778

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63 changes: 41 additions & 22 deletions source/Tutorials/Beginner-Client-Libraries/Pluginlib.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -43,7 +43,7 @@ Create a new empty package in your ``ros2_ws/src`` folder with the following com

.. code-block:: console

$ ros2 pkg create --build-type ament_cmake --license Apache-2.0 --dependencies pluginlib --node-name area_node polygon_base
$ ros2 pkg create --build-type ament_cmake --license Apache-2.0 --dependencies pluginlib polygon_base
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This will fail with the following.

root@tomoyafujita:~/ros2_ws/colcon_ws# ros2 pkg create --build-type ament_cmake --license Apache-2.0 --dependencies pluginlib polygon_base
usage: ros2 pkg create [-h] [--package-format {2,3}] [--description DESCRIPTION] [--license LICENSE]
                       [--destination-directory DESTINATION_DIRECTORY] [--build-type {cmake,ament_cmake,ament_python}]
                       [--dependencies DEPENDENCIES [DEPENDENCIES ...]] [--maintainer-email MAINTAINER_EMAIL]
                       [--maintainer-name MAINTAINER_NAME] [--node-name NODE_NAME] [--library-name LIBRARY_NAME]
                       package_name
ros2 pkg create: error: the following arguments are required: package_name



Open your favorite editor, edit ``ros2_ws/src/polygon_base/include/polygon_base/regular_polygon.hpp``, and paste the following inside of it:
Expand Down Expand Up @@ -167,6 +167,8 @@ Let's go through the arguments to the ``PLUGINLIB_EXPORT_CLASS`` macro:
1. The fully-qualified type of the plugin class, in this case, ``polygon_plugins::Square``.
2. The fully-qualified type of the base class, in this case, ``polygon_base::RegularPolygon``.

Note that for the Triangle class, we're implementing a method ``getHeight()`` that isn't in the base class. We'll come back to this method later, when we call it.
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github workflow will fail with this line against one sentence per line rule.


2.2 Plugin Declaration XML
~~~~~~~~~~~~~~~~~~~~~~~~~~

Expand Down Expand Up @@ -220,41 +222,57 @@ The arguments to the ``pluginlib_export_plugin_description_file`` command are:
^^^^^^^^^^^^^^^^^

Now it's time to use the plugins.
This can be done in any package, but here we're going to do it in the base package.
We're going to do it in a new package.

.. code-block:: console

$ ros2 pkg create --build-type ament_cmake --license Apache-2.0 --dependencies pluginlib polygon_base polygon_plugins polygon_area --node-name area_node polygon_area

Open your favorite editor, edit ``ros2_ws/src/polygon_area/src/area_node.cpp``, and paste the following inside of it:



Edit ``ros2_ws/src/polygon_base/src/area_node.cpp`` to contain the following:

.. code-block:: C++

#include <pluginlib/class_loader.hpp>
#include <polygon_base/regular_polygon.hpp>
#include <polygon_plugins/polygon_plugins.hpp>

int main(int argc, char** argv)
{
// To avoid unused parameter warnings
(void) argc;
(void) argv;
// To avoid unused parameter warnings
(void) argc;
(void) argv;
Comment on lines -233 to +247
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indent with 2 spaces are fine, i do not think we need to change those lines.


pluginlib::ClassLoader<polygon_base::RegularPolygon> poly_loader("polygon_base", "polygon_base::RegularPolygon");
pluginlib::ClassLoader<polygon_base::RegularPolygon> poly_loader("polygon_base", "polygon_base::RegularPolygon");

try
{
std::shared_ptr<polygon_base::RegularPolygon> triangle = poly_loader.createSharedInstance("polygon_plugins::Triangle");
triangle->initialize(10.0);
try
{
std::shared_ptr<polygon_base::RegularPolygon> triangle = poly_loader.createSharedInstance("polygon_plugins::Triangle");
triangle->initialize(10.0);

std::shared_ptr<polygon_base::RegularPolygon> square = poly_loader.createSharedInstance("polygon_plugins::Square");
square->initialize(10.0);
std::shared_ptr<polygon_base::RegularPolygon> square = poly_loader.createSharedInstance("polygon_plugins::Square");
square->initialize(10.0);

printf("Triangle area: %.2f\n", triangle->area());
printf("Square area: %.2f\n", square->area());
}
catch(pluginlib::PluginlibException& ex)
{
printf("The plugin failed to load for some reason. Error: %s\n", ex.what());
}
printf("Triangle area: %.2f\n", triangle->area());

auto custom_plugin = std::dynamic_pointer_cast<polygon_plugins::Triangle>(triangle);
if (custom_plugin) printf("Triangle height : %.2f\n", custom_plugin->getHeight());

return 0;
printf("Square area: %.2f\n", square->area());

}
catch(pluginlib::PluginlibException& ex)
{
printf("The plugin failed to load for some reason. Error: %s\n", ex.what());
}

return 0;
}


The ``ClassLoader`` is the key class to understand, defined in the ``class_loader.hpp`` `header file <https://github.com/ros/pluginlib/blob/ros2/pluginlib/include/pluginlib/class_loader.hpp>`_:

* It is templated with the base class, i.e. ``polygon_base::RegularPolygon``.
Expand All @@ -265,8 +283,8 @@ There are a number of ways to instantiate an instance of the class.
In this example, we're using shared pointers.
We just need to call ``createSharedInstance`` with the fully-qualified type of the plugin class, in this case, ``polygon_plugins::Square``.

Important note: the ``polygon_base`` package in which this node is defined does NOT depend on the ``polygon_plugins`` class.
The plugins will be loaded dynamically without any dependency needing to be declared.
Important note: If we only use the methods of the plugin's base class ``polygon_base``, the ``polygon_area`` package in which this node is defined does NOT depend on the ``polygon_plugins`` class. Here the node is using a method specific to one of the a dervied class (``getHeight()`` of the Triangle class). This creates dependency to ``polygon_plugins``.
In the first situation, using only the properties of the base class, the plugins will be loaded dynamically without any dependency needing to be declared.
Furthermore, we're instantiating the classes with hardcoded plugin names, but you can also do so dynamically with parameters, etc.

4 Build and run
Expand Down Expand Up @@ -306,6 +324,7 @@ Now run the node:

$ ros2 run polygon_base area_node
Triangle area: 43.30
Triangle height: 8.66
Square area: 100.00

Summary
Expand Down