ECS System

An Entity Component System (ECS) written in C++ 20 for fun and learning purposes.

This system is not intended for use in professional projects and it's still under development.

Content

This CMake project contains three subprojects exposed below. Any external dependency (google test, SDL) are automatically downloaded during the cmake build process.

• ecs-core: the core of the ECS implementation. It is compiled as a static library that must be linked to any project meant to make use of it.
• ecs-test: a suite of unit tests implemented with the gtest framework.
• ecs-runner: an SDL application that showcases the basic functionalities of the ECS system, spawning 20000 particles launched in random directions and bouncing on the borders of the window.

Getting started

The project uses CMake as building framework.

cd path/to/project
mkdir build
cd build

cmake ..

cmake --build .

Heart of the system is the ecs::World class, from which you can have access to all the core functionalities of an Entity Component System.

// Create and initialize World instance
std::shared_ptr<ecs::World> world = std::make_shared<ecs::World>();
world->Initialize();

// Create an entity and store the entity ID into entity
const ecs::entity_id entity = world->CreateEntity<comps::Velocity, comps::Rect, comps::Color>();

// Gets a handle to the entity (essential for accessing methods required for components manipulation)
ecs::EntityHandle handle = world->GetEntity(entity);

// Modify component's data
comps::Velocity& velocity = handle.GetComponent<comps::Velocity>();
velocity.x = generateRandomVelocity();
velocity.y = generateRandomVelocity();

// Add systems
world->AddSystem<systems::MovementSystem>();

// run the system
while (s_keepUpdating)
{
	world->Update(deltaTime);
}

Example of a system:

class MovementSystem : public ecs::ISystem 
{
	void Update(std::weak_ptr<ecs::World> world, ecs::real_t deltaTime) override
	{
		ecs::query<comps::Rect, comps::Velocity>::MakeQuery(world).forEach(
			[deltaTime](ecs::EntityHandle entity, comps::Rect& rect, comps::Velocity& velocity)
			{
				rect.rect.x += static_cast<int>(velocity.x * deltaTime);
				rect.rect.y += static_cast<int>(velocity.y * deltaTime);

				// Simple bouncing off the edges of the screen
				if (rect.rect.x < 0 || rect.rect.x > SCREEN_WIDTH)
				{
					velocity.x *= -1.0f;
					if (rect.rect.x < 0)
					{
						rect.rect.x = 3;
					}
					else
					{
						rect.rect.x = SCREEN_WIDTH - 3;
					}
				}

				if (rect.rect.y < 0 || rect.rect.y > SCREEN_HEIGHT)
				{
					velocity.y *= -1.0f;
					if (rect.rect.y < 0)
					{
						rect.rect.y = 3;
					}
					else
					{
						rect.rect.y = SCREEN_HEIGHT - 3;
					}
				}
			});
	}
};