↓Skip to main content
Frank’s Reich

WirePlumber Object Manager - The C-Cut

·496 words·3 mins
Development Threads Pipewire Wireplumber C

Now that we know how to use the object manager in Lua, let us take a look at how it can be used with the wireplumber C API. The C API is a bit more verbose, but it also allows us to do some things that are not possible in Lua. The first step is to initialize wireplumber by calling wp_init. Afterwards, we need to create the main loop, which communicates with wireplumber. The main loop is an event loop similar to libuv, the event loop used by node.js, provided by glib. The last step in getting wireplumber ready is to create and connect the wireplumber core. We then need to set up the object manager where it currently says ..., and after that run the event loop with g_main_loop_run. All together the code snippet looks like this:

struct WirePlumberControl {
  GMainLoop *loop = nullptr;
  WpCore *core = nullptr;
  WpObjectManager *om = nullptr;
};

int main(int argc, char *argv[]) {
  wp_init(WP_INIT_ALL);
  WirePlumberControl wire_plumber_service;

  wire_plumber_service.loop = g_main_loop_new(nullptr, false);
  wire_plumber_service.core = wp_core_new(nullptr, nullptr, nullptr);
  if (wp_core_connect(wire_plumber_service.core)) {
    ...
    g_main_loop_run(wire_plumber_service.loop);
  }
}

Now that the loop is created and the core is connected, we can setup the object manager. The first step is to create it as follows:

wire_plumber_service.om = wp_object_manager_new();

We then setup the interest, similar to the Lua API, by calling wp_object_manager_add_interest. In our case, we add an interest in pipewire ports, which are identified by the GObject type WP_TYPE_PORT:

wp_object_manager_add_interest(wire_plumber_service.om, WP_TYPE_PORT,
                               nullptr);

We also need to declare how much information and functionality should be activated when the object manager encounters a new object:

wp_object_manager_request_object_features(
    wire_plumber_service.om, WP_TYPE_GLOBAL_PROXY,
    WP_PIPEWIRE_OBJECT_FEATURES_MINIMAL);

The object manager now has to be installed on the core:

wp_core_install_object_manager(wire_plumber_service.core,
                               wire_plumber_service.om);

once the object manager is installed, it looks for objects that match the interest when we run the event loop. The object manager will then emit signals which we can connect to. The code snippet below shows how this can be done with the object-added signal:

g_signal_connect(
    wire_plumber_service.om, "object-added",
    G_CALLBACK(+[](WpObjectManager *object_manager, gpointer object,
                   gpointer user_data) {
      const auto g_object = static_cast<WpPipewireObject *>(object);

      const auto id = std::atoi(
          wp_pipewire_object_get_property(g_object, PW_KEY_PORT_ID));
      const auto name = std::string(
          wp_pipewire_object_get_property(g_object, PW_KEY_PORT_NAME));
      const auto alias = std::string(
          wp_pipewire_object_get_property(g_object, PW_KEY_PORT_ALIAS));

      std::cout << "Object added, id " << id << ", name " << name
                << ", alias " << alias << std::endl;
    }),
    nullptr);

When the object manager emits the signal, our application will get the id, name and alias of the port and print it to the console, producing output like this:

Object added, id 0, name playback_AUX0, alias RME RayDAT:playback_AUX0
Object added, id 0, name monitor_AUX0, alias RME RayDAT:monitor_AUX0
Object added, id 1, name playback_AUX1, alias RME RayDAT:playback_AUX1
Object added, id 1, name monitor_AUX1, alias RME RayDAT:monitor_AUX1
Object added, id 2, name playback_AUX2, alias RME RayDAT:playback_AUX2
Object added, id 2, name monitor_AUX2, alias RME RayDAT:monitor_AUX2

And yes, my audio interface is more expensive than my computer, thanks for asking :)

The exmple code is available on github:

Frank-Krick/wireplumber_example_om

C++
0
0