This package makes up the basis for custom ROS interfaces and contains all custom msg/srv/act files. Additionally, the special files (_CMakeLists.txt_ and _package.xml_) describe how these interface files are to be used.
This package must be created as a CMake package: `ros2 pkg create --build-type ament-cmake <package_name>`
This will result in an empty package structure:
* msg/srv/act directory:
* This directory contains the custom msg files (eg. CustomMsg1.msg)
* CMakeLists.txt:
* This file describes how to build this package
* Configure this file according to this [instruction](https://index.ros.org/doc/ros2/Tutorials/Custom-ROS2-Interfaces/#cmakelists-txt)
* package.xml:
* This file contains meta information about this package
* Configure this file according to this [instruction](https://index.ros.org/doc/ros2/Tutorials/Custom-ROS2-Interfaces/#package-xml)
### 1. Create CMake Package
* Move to your colcon workspace's src directory: `cd <workspace_path>/src`
* If you already [updated your .bashrc file](https://github.com/patrickw135/pubsub/blob/master/bashrc_addons.txt) you can close all open consoles and start a new console (Ctrl+Alt+T). This will source your workspace automatically, as .bashrc is run every time you start a console.
__Important__: If you use multiple workspaces make sure you have the wanted workspace defined in .bashrc! Otherwise the changes introduced when building will not be available.
### 7. Check functionality
Check functionality of your messages by creataing a topic using your newly created message:
This package contains your scripts, programs and libraries. After building the workspace (`colcon build`) the custom messages are available to all other packages.
When using custom interfaces in python scripts these must be imported into the python script first
```python
from <package_name>.msg import <message_name>
```
replacing `<package_name>` with the package containing the custom message and `<message_name>` with the message file name (excluding the file ending .msg).
However, in order to be able to import the custom message types, `<message_name>` must first be known to the ROS system. This was established when creating the [CMake package](https://github.com/patrickw135/pubsub/blob/master/instructions_custom_topics.md#cmake-package-eg-pubsub_msg) containing the custom message. Additionally, you must add this dependency to the _package.xml_ of this package as stated in the next chapter.
### 3. Configure package.xml
In addition to importing the message type into your python script you must also configure _package.xml_ adding the package dependency of where you inherite the custom message from. Add this line to _package.xml_:
```xml
<depend>std_msgs</depend>
<!-- CUSTOM LINE -->
<!-- This is custom for the package you depend on -->
<exec_depend>package_name</exec_depend>
```
exchanging _package_name_ with the source package of the custom message type (here _pubsub_msg_), e.g.:
```xml
<depend>std_msgs</depend>
<!-- CUSTOM LINE -->
<!-- This is custom for the package you depend on -->
The file setup.py is used to create entry points, meaning you link your python scripts to names which you can be called through `ros2 run ...`. In order for this to work your python scripts must be written using the following mechanism
```python
def main():
# main code
if __name__ == "__main__":
main()
```
in which you put all of your code inside `main()`. This prevents any code from being run, if this script is not run as *"__main__"*.
Linking your scripts to names is done inside the file _setup.py_ by defining entry points:
```python
entry_points={
'console_scripts': [
'talker = pubsub.talker:main',
'listener = pubsub.listener:main'
],
},
```
The _setup.py_-excerp above links the the function `main()` from the scripts _talker.py_ and _listener.py_ from inside `/pubsub` to the names _talker_ and _listener_. This way they can be called using `ros2 run pubsub talker` or `ros2 run pubsub listener`.
* If you already [updated your .bashrc file](https://github.com/patrickw135/pubsub/blob/master/bashrc_addons.txt) you can close all open consoles and start a new console (Ctrl+Alt+T). This will source your workspace automatically, as .bashrc is run every time you start a console.
__Important__: If you use multiple workspaces make sure you have the wanted workspace defined in .bashrc! Otherwise the changes introduced when building will not be available.
