Why ?

InfiniTime is a monolithic firmware : everything (core OS, UI lib, BLE stack, apps) is built into a single firmware image. The current implementation does not allow to easily add new applications and select which apps must be built into the firmware.
This PR tries to improve those points.

Select which apps must be built into the firmware:

You can easily select the applications that will be built into the firmware by adding/removing app from UserAppTypes:

    using UserAppTypes = TypeList<Apps::Alarm,
                                  Apps::HeartRate,
                                  Apps::Paint,
                                  Apps::Metronome,
                                  Apps::Music,
                                  Apps::Navigation,
                                  Apps::Paddle,
                                  Apps::Steps,
                                  Apps::StopWatch,
                                  Apps::Timer,
                                  Apps::Twos
                                  >;
  }

All the apps that are passed as template parameters to UserAppTypes will be built into the firmware. Moreover, the order in this list represents the order of the apps in the application menu.

The menu is automatically sized (number of pages and number of apps per page) to fit all the apps:

Add a new app

As always, start by implementing your app in a class that derives from Pinetime::Applications::Screens::Screen.

Add the .CPP file in the list of source files in the CMake file of the project.

Then, add a new enum value in Pinetime::Application::Apps

Additionally, declare a new AppTraits corresponding to your app. Here is an example from the Metronome app: You simply need to define the name, an icon, and a method Create() that creates the application:

    template <>
    struct AppTraits<Apps::Metronome> {
      static constexpr Apps app = Apps::Metronome;
      static constexpr const char* icon = Screens::Symbols::drum;
      static Screens::Screen* Create(AppControllers& controllers) {
        return new Screens::Metronome(controllers.motorController, *controllers.systemTask);
      };
    };

Finally, add the enum of your application in the template parameters of UserAppTypes

How ?

This is made possible by a bit of template meta programming magic in UserApps.h. Basically, the function CreateAppDescriptions() creates a std::array of AppDescription at build time according to the enum values listed in UserAppTypes. It means that this array is created at build time and then stored in flash memory. It doesn't cost anything at runtime.

Applications are now either "System applications" or "User applications". System applications are apps that are expected to be always built in the firmware (notifications, clock, settings,...). InfiniTime does not work properly if those apps are not present. User applications are applications that are optionally built into the firmware.

App creation

When a new app must be loaded, DisplayApp first checks if the app is a System app, in which case it simply creates it.
If it's not a System app, it looks into the list of User applications and creates it if it's found:

const auto* d = std::find_if(userApps.begin(), userApps.end(), [app](const AppDescription& appDescription) {
        return appDescription.app == app;
      });
      if (d != userApps.end())
        currentScreen.reset(d->create(controllers));

App menu

The App menu, ApplicationList now receives the list of apps as parameters. This list is generated by DisplayApp from the array of AppDescription generated at build time. It builds the menu pages according to this list of Apps.

Results

Developers can now add their application more easily than before, and no data is duplicated.

Developers and fork-maintainers can also select the apps they want to build in their firmware by editing a single line of code. This allows for easier customization of the firmware.

Firmware image sizes:

• main branch without any change: 374908B (78.99%)
• this branch with all the original apps enabled : 375012B (79.01%)
• this branch with none of the User apps enabled : 347476 (73.21%)

What's next ?

• We'll be able to merge more PR that add new application in InfiniTime; Not all of them will be built into the official releases of InfiniTime, though (we are still constrained by the available space in flash memory).
• We could add a way to easily build applications from external repo (using git submodules, for example). Those applications won't be maintained by the developers/maintainers of InfiniTime but could be easily built into the firmware by developers and fork maintainers.
• All watchfaces are currently considered as "System" applications. However, the mecanism introduced in this PR could be extended to watchfaces.
• This is one step closer to applications that are dynamically loaded at runtime.