Awesome

goomy

A tiny, experimental ECS framework.

⚠️ Disclaimer ⚠️

This project is a proof-of-concept and is mostly an excuse for me to get familiar with modern C++ and a few experimental features. Don't use this code. (yet?)

Introduction

The framework is heavily template-based, and effectively almost disappears at compile-time. Components of the same type are stored contiguously in memory, and entities only hold indices linking to their associated components. Entities are stored contiguously in memory as well, but all these lower-level details are packaged behind a lightweight wrapper API.

The framework also comes with a signaling/event system that makes use of the detection idiom to facilitate dependency injection.

What it looks like

The following example is extremely simple and really only scratches the surface of what's possible, but it should give you a basic idea of how the framework works.

#include <goomy.hpp>

class Lifetime;
class TestSystem;

using Engine =
    goomy::Engine<goomy::Mount<TestSystem>, goomy::Register<Lifetime>>;

class Lifetime {
  public:
    int lifetime;
    Lifetime(int lifetime) : lifetime(lifetime) {}
};

class TestSystem {
  public:
    void onInit(Engine &engine) {
        for (int i = 0; i < 1000; ++i) {
            engine.entity().with<Lifetime>(i);
        }
    }
    void onUpdate(Engine &engine) {
        if (engine.getEntityCount() == 0) {
            engine.shutdown();
        }
    }
    void onUpdate(goomy::Component<Engine, Lifetime> component) {
        if (--component.data().lifetime <= 0) {
            component.entity().destroy();
        }
    }
};

int main() {
    Engine().loop();
    return 0;
}

You can find this example in the examples/basic directory.

Running the examples

The project has only been tested on Linux but doesn't rely on any platform-specific library so you should be good as long as you have a compliant C++17 compiler on hand. One of the examples demonstrates how the framework could be used in combination with SFML, so you'll need to make sure that the latest stable version is available on your system.

1. Clone this repository

   $ git clone https://github.com/vberlier/goomy.git
   $ cd goomy
   

2. Build the project

   $ mkdir cmake-build-release
   $ cd cmake-build-release
   $ cmake -DCMAKE_BUILD_TYPE=Release ..
   $ make
   

3. Run the examples

   $ ./examples/basic/example_basic
   $ ./examples/sfml/example_sfml
   

The SFML example

This example demonstrates how the framework could be used in combination with SFML. The executable creates a window in which you can spawn particles in a few different ways:

• Clicking and dragging the left mouse button anywhere in the window creates a stream of particles (a particle spawns every frame)
• Dragging the right mouse button and releasing it lets you shoot a single particle in a particular direction
• Pressing the left mouse button while aiming with the right mouse button reactivates the particle stream

The code for this example is in the examples/sfml directory.

Usage

Getting started with goomy is fairly straightforward. You pretty much only need to instantiate the goomy::Engine template and declare your systems and components.

using Engine = goomy::Engine<goomy::Mount<System1, System2, ...>,
                             goomy::Register<Component1, Component2, ...>>;

Instantiating the template effectively generates an entity type, creates registries for your different components and wires all the systems and signals together. Everything then happens through your specialized Engine type.

You will probably need to forward-declare your system classes because you'll often want to reference your custom Engine in their declarations.

After declaring your custom engine type, the only thing you need to do is to instantiate it and call the loop() member function.

int main() {
    Engine().loop();
    return 0;
}

Systems

The framework lets you mount any class as a system, there are no requirements, no specific base class to inherit from. For instance, this makes it possible to mount an SFML window to your engine (see: examples/sfml). There are really no limitations to what can be a system.

class TestSystem {};

After mounting the system to the engine, the framework will take care of instantiating it.

goomy::Mount<TestSystem>

Signals

The distinctive capability of classes that have been mounted to the engine as systems is that they can respond to signals. Dispatching a signal will call the associated member function on all the systems where it is defined.

class TestSystem {
  public:
    void onInit();
    void onUpdate();
};

The onInit signal is dispatched by the engine before entering the main loop. Systems that define the onInit() member function will be able to run code on initialization.

Similarly, the onUpdate signal is also dispatched by the engine. It will call the onUpdate() member function every frame on all the systems where it is defined.

The engine also dispatches onBeforeInit and onAfterInit as well as onBeforeUpdate and onAfterUpdate.

Member functions called by signals support some form of dependency injection. You can get a reference to the engine instance by specifying it as a parameter.

class TestSystem {
  public:
    void onInit(Engine &engine) {
        engine.shutdown();
    }
};

It's possible to create your own signals with the GOOMY_SIGNAL(NAME) macro.

GOOMY_SIGNAL(onCustomEvent);

You don't have to use the macro but declaring a custom signal manually can get a little verbose.

struct onCustomEvent {
    template <typename T, typename... Args>
    using detector =
        decltype(std::declval<T>().onCustomEvent(std::declval<Args>()...));

    template <typename T, typename... Args>
    static void invoke(T &instance, Args &&... args) {
        instance.onCustomEvent(std::forward<Args>(args)...);
    }
};

The macro does make things a little more readable, but any struct with a templated detector type and a static function template invoke() can be used as a signal.

void TestSystem::onUpdate(Engine &engine) {
    engine.dispatch<onCustomEvent>();
}

The engine.dispatch<SignalType>() function lets you pass parameters to the systems handling the signal. In that case, the framework will only call the associated member functions if the signature is compatible with the types of the parameters.

void TestSystem::onUpdate(Engine &engine) {
    engine.dispatch<onCustomEvent>(42);
}

void OtherSystem::onCustomEvent(int number) {}

// Injecting the engine still works
void OtherSystem::onCustomEvent(Engine &engine, int number) {}

// Can omit the parameters if the function doesn't need them
void OtherSystem::onCustomEvent() {}

However, if the types of the parameters are not compatible, the function will not get called.

void TestSystem::onUpdate(Engine &engine) {
    engine.dispatch<onCustomEvent>("will not call OtherSystem::onCustomEvent");
}

void OtherSystem::onCustomEvent(int number) {}

💡 Tip

It's important to understand that signals don't have any runtime footprint. Every call to the engine.dispatch<SignalType>() function effectively results in inlined calls to the corresponding member functions on systems with compatible definitions.

Components

Just like with systems, there are no restrictions when it comes to what kind of class can be a component.

class Dummy {};

Components must be registered when creating your custom Engine type.

goomy::Register<Dummy>;

The engine instance allows you to create and destroy entities and components, and iterate through them. You can check out the API reference for more information.

class Dummy {
  public:
    int number;
    Dummy(int number) : number(number) {}
};

void TestSystem::onUpdate(Engine &engine) {
    engine.entity().with<Dummy>(42);
}

void OtherSystem::onUpdate(Engine &engine) {
    for (auto component : engine.components<Dummy>()) {
        component.data().number++;
    }
}

Since iterating over all the components of a given type is a very common operation, the framework makes it possible to lift the iteration in the dependency injection machinery and ask for a component reference to be injected in any member function called by a signal.

void OtherSystem::onUpdate(Dummy &dummy) {
    dummy.number++;
}

Here, the onUpdate() function will be called each frame for every single Dummy component. This particular example shows that you can ask for the component reference directly, but it's also possible to request a component reference wrapper instead when necessary (more details in the component API reference).

void OtherSystem::onUpdate(goomy::Component<Engine, Dummy> component) {
    if (++component.data().number > 9000) {
        component.entity().destroy();
    }
}

Note that component reference injection is compatible with engine injection and custom parameters.

void TestSystem::onUpdate(Engine &engine) {
    engine.dispatch<onCustomEvent>(42);
}

// All of these will get called for each Dummy component
void OtherSystem::onCustomEvent(Dummy &dummy, int number) {}
void OtherSystem::onCustomEvent(Engine &engine, Dummy &dummy, int number) {}
void OtherSystem::onCustomEvent(Dummy &dummy) {}

API reference

Engine

The engine instance lets you interact with the systems, entities, and components you declared when creating your custom Engine type. You can use it to get a reference to any of your custom systems, dispatch signals, iterate through entities and components and more. Note that the engine is explicitly marked as non-copiable, and cannot be moved.

engine.get<SystemType>()

Instantiating a custom Engine type creates instances of all the mounted system types. You can get a reference to any of your system instances by calling the templated get<SystemType>() member function.

auto &system1 = engine.get<System1>();

Note that the returned reference is a very thin wrapper that simply marks the system as non-copiable to prevent any accident.

engine.dispatch<SignalType>(Args &&... args)

This function lets you dispatch signals throughout your systems. The template parameter of the dispatch<SignalType>() member function must be a signal class/struct, usually generated with the GOOMY_SIGNAL(NAME) macro.

GOOMY_SIGNAL(onCustomEvent);

engine.dispatch<onCustomEvent>(arg1, arg2, ...);

This code will essentially go through every system instance at compile-time and generate a call to the onCustomEvent() member function if it is defined.

engine.entity()

You can create entities with the entity() member function. This function returns a lightweight wrapper around a reference to the newly created internal entity.

auto entity = engine.entity();

For more details check out the entity API reference.

engine.entity(std::size_t id)

This function lets you retrieve an entity by its id.

auto entity = engine.entity(42);

engine.entities()

You can iterate over every entity by using the entities() member function.

for (auto entity : engine.entities()) {
    // ...
}

engine.getEntityCount()

Returns the current number of entities. The function doesn't take into account entities that have been created or destroyed in the current frame.

auto count = engine.getEntityCount();

engine.components<ComponentType>()

This lets you iterate over all the components of a specific type.

for (auto component1 : engine.components<Component1>()) {
    // ...
}

The iterator doesn't yield references to the internal components directly, but lightweight wrappers that provide a few useful member functions to let you access the id of the component or its associated entity for instance.

For more details check out the component API reference.

engine.loop()

This function is usually called in your program's main(). It runs the main execution loop and will block until the engine gets shut down.

engine.loop();

engine.shutdown()

You can stop the main execution loop by calling the shutdown() member function. The engine will cleanly finish executing the current frame and exit.

engine.shutdown();

engine.deltaTime()

This returns the number of seconds that it took for executing the frame.

position += velocity * engine.deltaTime();

Entity

The framework lets you interact with the internal entities through lightweight reference wrappers.

Entities are internally all stored contiguously in memory, in a standard std::vector. The engine guarantees that the vector holding the entities internally will not be mutated during the frame by buffering entity creation and deletion in separate containers, which in turn means that entity reference wrappers are valid for the entire frame.

However, the engine flushes the pending modifications at the end of each frame, which potentially mutates the internal vector and invalidates references and reference wrappers. Unless you know what you're doing, the lifetime of any entity reference wrapper should not exceed the frame it was created in.

entity.id()

Returns the id of the entity. The id can be used to retrieve the entity from the engine instance with the engine.entity(id) member function.

auto id = entity.id();

Note that this id can change over time when other entities are destroyed.

entity.engine()

Returns a reference to the engine instance.

auto &engine = entity.engine();

entity.has<ComponentType>()

Returns whether the entity has a component of the given type.

bool hasComponent1 = entity.has<Component1>();

entity.get<ComponentType>()

This function returns a reference wrapper to the component of the given type attached to the entity.

auto component1 = entity.get<Component1>();

This function should only be called if the component actually exists, so make sure to check for the component with the has<ComponentType>() member function where appropriate.

entity.create<ComponentType>(Args &&... args)

Create and attach a new component of the given type to the entity. The arguments are passed to the constructor of the component. The function returns a component reference wrapper.

auto component1 = entity.create<Component1>(arg1, arg2, ...);

Note that if a component of the given type is already attached to the entity the new component will take its place and the current one will be destroyed.

entity.with<ComponentType>(Args &&... args)

This function is very similar to the entity.create<ComponentType>() member function, but returns the entity itself instead of a reference wrapper to the newly created component.

entity
    .with<Component1>(arg1, arg2, ...)
    .with<Component2>(arg1, arg2, ...)
    .with<Component3>(arg1, arg2, ...);

The fluent API makes it easy to create multiple components at the same time.

entity.destroy()

Destroys the entity. This will also destroy all the components associated with the entity.

entity.destroy();

Component

The framework lets you interact with the internal component instances through lightweight reference wrappers.

Instances of each type of component are internally stored contiguously in memory in separate standard std::vector, in a very similar way to how entities are stored internally as well. The engine guarantees that the vectors holding the components internally will not be mutated during the frame by buffering component creation and deletion in separate containers, which in turn means that component reference wrappers are valid for the entire frame.

However, the engine flushes the pending modifications at the end of each frame, which potentially mutates the internal vectors and invalidates references and reference wrappers. Just like with entity reference wrappers, unless you know what you're doing, the lifetime of any component reference wrapper should not exceed the frame it was created in.

component.id()

Returns the id of the component. Note that this id is only unique for components of the same type, and can change over time when other components of the same type are destroyed.

auto id = component.id();

component.engine()

Returns a reference to the engine instance.

auto &engine = component.engine();

component.entity()

Returns a reference wrapper to the associated entity.

auto entity = component.entity();

component.data()

This function returns a reference to the declared component instance.

auto &data = component.data();

component.destroy()

Destroys the component.

component.destroy();

Credits

This project is partly inspired by a talk by Allan Deutsch:

• Game Engine API Design

Other useful resources:

• Cpu Caches and Why You Care (Scott Meyers)
• Modern Template Metaprogramming: A Compendium, Part I & II (Walter E. Brown)
• Universal References in C++11 (Scott Meyers)
• Data-Oriented Design and C++ (Mike Acton)
• The Detection Idiom: A simpler way to SFINAE (Marshall Clow)

License - MIT