README.md

Node Library

This example illustrates how to create a library of reusable MRC nodes that can be later used to form pipelines. Building a library of reusable nodes can facilitate rapid pipeline development but it requires using a different interface when creating the nodes from the make_source, make_node, and make_sink methods shown in Example 1.

In this example, we will only be creating a single source node that functions identically to the source node in Example 1. If you recall, we created that node using the following code:

auto source = s.make_source<int>("int_source", [](rxcpp::subscriber<int> s) {
    s.on_next(1);
    s.on_next(2);
    s.on_next(3);
    s.on_completed();
});

While this syntax makes it quick to define a one-off node, it can become very verbose when the same node is needed in multiple places. To create a reusable node, we need to inherit from one of the following classes:

• Source: Inherit from mrc::node::RxSource<T>
• Node: Inherit from mrc::node::RxNode<T, R>
• Sink: Inherit from mrc::node::RxSink<R>

Creating the Source

Converting the above source that uses make_source into a class that derives from RxSource looks like the following:

class IntSource : public mrc::node::RxSource<int>
{
  public:
    IntSource() :
      mrc::node::RxSource<int>(rxcpp::observable<>::create<int>([](rxcpp::subscriber<int> s) {
          s.on_next(1);
          s.on_next(2);
          s.on_next(3);
          s.on_completed();
      }))
    {}
};

We can see that the constructor for RxSource takes the same argument that was passed to make_source. The main difference here is that we can now create multiple copies of IntSource without needing to redefine the same lambda each time.

Building the Library

To add this node into a C++ library, we need to create the following CMakeLists.txt file:

add_library(ex01_node_library SHARED
  src/nodes.cpp
)

target_include_directories(ex01_node_library
  PUBLIC
    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
    $<INSTALL_INTERFACE:include>
  PRIVATE
    ${CMAKE_CURRENT_SOURCE_DIR}/src
)

target_link_libraries(ex01_node_library
  PUBLIC
    mrc::libmrc
)

As far as CMake libraries go, there is nothing out of the ordinary that needs to be done to build the component into a library. Simply create a library as usual and be sure to add mrc::libmrc to the library list in target_link_libraries.

Running the Example

In this example, we have only created a reusable library. To see an executable that uses this library, go to the next example, ex02_pipeline_with_library