How-To: Write a Module in C++

DEPRECATION WARNING: The Modular framework is being deprecated in favor of the Session Framework.


A Module is a UI component that can participate in a [Story](link to story doc), potentially composed of many different Modules. A Module's lifecycle is tightly bound to the story to which it was added. In addition to the capabilities provided to all Peridot components via fuchsia::modular::ComponentContext, a Module is given additional capabilities via its fuchsia::modular::ModuleContext.


Mod Initialization

The first step to writing a Module is implementing the initializer.

#include <lib/sys/cpp/component_context.h>
#include <lib/async-loop/cpp/loop.h>
#include <lib/async-loop/default.h>
#include <ui/cpp/fidl.h>

#include "src/modular/lib/app_driver/cpp/module_driver.h"

namespace simple {

class SimpleModule : fuchsia::ui::app::ViewProvider {
			modular::ModuleHost* module_host,
			fidl::InterfaceRequest<fuchsia::ui::app::ViewProvider> view_provider_request)
			: view_provider_binding_(this) {

	modular::ModuleHost* module_host_;
	fidl::Binding<fuchsia::ui::app::ViewProvider> view_provider_binding_;
	std::set<std::unique_ptr<SimpleView>> views_;

}  // namespace simple

The ModuleHost provides SimpleModule with its StartupContext and fuchsia::modular::ModuleContext.

The ViewProvider request allows the system to connect to SimpleModule's view. TODO: Update guide to explain view connections.

Connecting to SimpleAgent

See how to write an agent to learn how to connect to a service provided by an Agent.

Running the Module

int main(int argc, const char** argv) {
  async::Loop loop(&kAsyncLoopConfigAttachToCurrentThread);
  auto context = sys::ComponentContext::CreateAndServeOutgoingDirectory();
  modular::ModuleDriver<simple::SimpleModule> driver(context.get(),
                                                     [&loop] { loop.Quit(); });
  return 0;

ModuleDriver is a helper class that manages the Module's lifecyle. Here it is given a newly created StartupContext and a callback that will be executed when the Module exits. ModuleDriver requires SimpleModule to implement the constructor shown above, as well as a Terminate:

void Terminate(fit::function<void()> done);

The module is responsible for calling done once its shutdown sequence is complete.