Flecs: Flecs Quickstart
Description: A fast entity component system (ECS) for C & C++

| Flecs v4.0 A fast entity component system (ECS) for C & C++

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Flecs Quickstart

This document provides a quick overview of the different features and concepts in Flecs with short examples. This is a good resource if you're just getting started or just want to get a better idea of what kind of features are available in Flecs!

# Building Flecs

To use Flecs, copy the distr/flecs.c and distr/flecs.h files from the distr folder to your project's source folder. When building, make sure your build system is setup to do the following:

- If it is a C++ project, make sure to compile distr/flecs.c as C code, for example by using `gcc`/`clang` instead of `g++`/`clang++`.
- If you are building on Windows and you're not using the Microsoft Visual Studio compiler, make sure to add `-lWs2_32` to the end(!) of the linker command. The socket API is used for connecting to Flecs explorer.
- When compiling Flecs with `gcc`/`clang`, add `-std=gnu99` to the compiler command. This ensures that addons that rely on time & socket functions are compiled correctly.
- C++ files that use Flecs must be compiled with `-std=c++0x` (C++11) or higher.

## Dynamic linking

To build Flecs as a dynamic library, remove this line from the top of the distr/flecs.h file:

#define flecs_STATIC

When compiling distr/flecs.c, make sure to define `flecs_EXPORTS`, for example by adding `-Dflecs_EXPORTS` to the compiler command.

Alternatively Flecs can also be built as a dynamic library with the cmake, meson, bazel or bake build files provided in the repository. These use the files from `src` to build as opposed to the amalgamated files, which is better suited for development.

## Building with CMake

Locate `flecs` on your system (either by cloning or as a submodule) and use `add_subdirectory` or use `FetchContent` to download the source code from the master branch of the flecs repository. After that, add the following to your `CMakeLists.txt` file:

target_link_libraries(${PROJECT_NAME} flecs::flecs_static)

## Building with Bake

Download or `git clone` the flecs repository and run `bake` from inside the directory. After that, add the following to your `project.json` file's value property:

"use": ["flecs"]

## Running tests (bake)

First make sure you have bake installed (see the bake repository for instructions).

Run the following commands to run all tests (use `-j` to specify the number of threads):

# Core test suite
bake run test/core -- -j 4
# Addon tests
bake run test/addons -- -j 4
# Reflection tests
bake run test/meta -- -j 4
# C++ tests
bake run test/cpp -- -j 4

To run tests with asan enabled, add `--cfg sanitize` to the command:

bake run --cfg sanitize test/api -- -j 4

### Running tests (cmake, experimental)

First make sure to clone bake.

Run the following commands to run all the tests:

# Generate make files for Flecs and tests
cmake -DFLECS_TESTS=ON -DBAKE_DIRECTORY="path to cloned bake repository"
# Build flecs and test suites
cmake --build . -j 4
# Run the tests
ctest -C Debug --verbose

## Emscripten

When building for emscripten, add the following command line options to the `emcc` link command:

-s ALLOW_MEMORY_GROWTH=1
-s STACK_SIZE=1mb
-s EXPORTED_RUNTIME_METHODS=cwrap
-s MODULARIZE=1
-s EXPORT_NAME="my_app"

## Addons

Flecs has a modular architecture that makes it easy to only build the features you really need. By default all addons are built. To customize a build, first define `FLECS_CUSTOM_BUILD`, then add defines for the addons you need. For example:

#define FLECS_CUSTOM_BUILD // Don't build all addons
#define FLECS_SYSTEM // Build FLECS_SYSTEM

Additionally, you can also specify addons to exclude from a build by adding `NO` to the define:

#define FLECS_NO_LOG

The following addons can be configured:

Addon | Description | Define
--- | --- | ---
Cpp | C++11 API | FLECS_CPP
Module | Organize game logic into reusable modules | FLECS_MODULE
System | Create & run systems | FLECS_SYSTEM
Pipeline | Automatically schedule & multithread systems | FLECS_PIPELINE
Timer | Run systems at time intervals or at a rate | FLECS_TIMER
Meta | Flecs reflection system | FLECS_META
Units | Builtin unit types | FLECS_UNITS
JSON | JSON format | FLECS_JSON
Doc | Add documentation to components, systems & more | FLECS_DOC
Http | Tiny HTTP server for processing simple requests | FLECS_HTTP
Rest | REST API for showing entities in the browser | FLECS_REST
Parser | Parser utilities used by script & query DSL | FLECS_PARSER
Query DSL | Query DSL parser | FLECS_QUERY_DSL
Script | DSL for scenes, assets and configuration | FLECS_SCRIPT
Stats | Functions for collecting statistics | FLECS_STATS
Metrics | Create metrics from user-defined components | FLECS_METRICS
Alerts | Create alerts from user-defined queries | FLECS_ALERTS
Log | Extended tracing and error logging | FLECS_LOG
Journal | Journaling of API functions | FLECS_JOURNAL
App | Flecs application framework | FLECS_APP
OS API Impl | Default OS API implementation for Posix/Win32 | FLECS_OS_API_IMPL

# Concepts

This section contains an overview of all the different concepts in Flecs and how they wire together. The sections in the quickstart go over them in more detail and with code examples.

Flecs Overview

## World

The world is the container for all ECS data. It stores the entities and their components, does queries and runs systems. Typically there is only a single world, but there is no limit on the number of worlds an application can create.

- C

  ecs_world_t *world = ecs_init();
  // Do the ECS stuff
  ecs_fini(world);
  ecs_world_t
  struct ecs_world_t ecs_world_t
  A world is the container for all ECS data and supporting features.
  Definition flecs.h:425
  ecs_fini
  int ecs_fini(ecs_world_t *world)
  Delete a world.
  ecs_init
  ecs_world_t * ecs_init(void)
  Create a new world.

- C++

  flecs::world world;
  // Do the ECS stuff
  flecs::world
  The world.
  Definition world.hpp:137

- C#

  using World world = World.Create();
  // Do the ECS stuff

- Rust

  let world = World::new();
  // Do the ECS stuff

## Entity

An entity is a unique thing in the world, and is represented by a 64 bit id. Entities can be created and deleted. If an entity is deleted it is no longer considered "alive". A world can contain up to 4 billion(!) alive entities. Entity identifiers contain a few bits that make it possible to check whether an entity is alive or not.

- C

  ecs_entity_t e = ecs_new(world);
  ecs_is_alive(world, e); // true!
  ecs_delete(world, e);
  ecs_is_alive(world, e); // false!
  ecs_entity_t
  ecs_id_t ecs_entity_t
  An entity identifier.
  Definition flecs.h:381
  ecs_new
  ecs_entity_t ecs_new(ecs_world_t *world)
  Create new entity id.
  ecs_delete
  void ecs_delete(ecs_world_t *world, ecs_entity_t entity)
  Delete an entity.
  ecs_is_alive
  bool ecs_is_alive(const ecs_world_t *world, ecs_entity_t e)
  Test whether an entity is alive.

- C++

  auto e = world.entity();
  e.is_alive(); // true!
  e.destruct();
  e.is_alive(); // false!
  flecs::world::entity
  flecs::entity entity(Args &&... args) const
  Create an entity.
  Definition impl.hpp:206
  flecs::entity_view::is_alive
  bool is_alive() const
  Check if entity is alive.
  Definition entity_view.hpp:70

- C#

  Entity e = world.Entity();
  e.IsAlive(); // true!
  e.Destruct();
  e.IsAlive(); // false!

- Rust

  let e = world.entity();
  e.is_alive(); // true!
  e.destruct();
  e.is_alive(); // false!

Entities can have names which makes it easier to identify them in an application. In C++ the name can be passed to the constructor. If a name is provided during entity creation time and an entity with that name already exists, the existing entity will be returned.

- C

  In C a name can be assigned with the `ecs_entity_init` function or `ecs_entity` macro.

  ecs_entity_t e = ecs_entity(world, { .name = "Bob" });
  printf("Entity name: %s\n", ecs_get_name(world, e));
  ecs_entity
  #define ecs_entity(world,...)
  Shorthand for creating an entity with ecs_entity_init().
  Definition flecs_c.h:235
  ecs_get_name
  const char * ecs_get_name(const ecs_world_t *world, ecs_entity_t entity)
  Get the name of an entity.

- C++

  auto e = world.entity("Bob");
  std::cout << "Entity name: " << e.name() << std::endl;
  flecs::entity_view::name
  flecs::string_view name() const
  Return the entity name.
  Definition entity_view.hpp:78

- C#

  Entity e = world.Entity("Bob");
  Console.WriteLine($"Entity name: {e.Name()}");

- Rust

  let e = world.entity_named("bob");
  println!("Entity name: {}", e.name());

Entities can be looked up by name with the `lookup` function:

- C

  ecs_entity_t e = ecs_lookup(world, "Bob");
  ecs_lookup
  ecs_entity_t ecs_lookup(const ecs_world_t *world, const char *path)
  Lookup an entity by it's path.

- C++

  auto e = world.lookup("Bob");
  flecs::world::lookup
  flecs::entity lookup(const char *name, const char *sep="::", const char *root_sep="::", bool recursive=true) const
  Lookup entity by name.
  Definition world.hpp:90

- C#

  Entity e = world.Lookup("Bob");

- Rust

  let e = world.lookup("bob");

## Id

An id is a 64 bit number that can encode anything that can be added to an entity. In flecs this can be either a component, tag or a pair. A component is data that can be added to an entity. A tag is an "empty" component. A pair is a combination of two component/tag ids which is used to encode entity relationships. All entity/component/tag identifiers are valid ids, but not all ids are valid entity identifier.

The following sections describe components, tags and pairs in more detail.

## Component

A component is a type of which instances can be added and removed to entities. Each component can be added only once to an entity (though not really, see Pair). In C applications components must be registered before use. By default in C++ this happens automatically.

- C

  ECS_COMPONENT(world, Position);
  ECS_COMPONENT(world, Velocity);
  ecs_entity_t e = ecs_new(world);
  // Add a component. This creates the component in the ECS storage, but does not
  // assign it with a value.
  ecs_add(world, e, Velocity);
  // Set the value for the Position & Velocity components. A component will be
  // added if the entity doesn't have it yet.
  ecs_set(world, e, Position, {10, 20});
  ecs_set(world, e, Velocity, {1, 2});
  // Get a component
  const Position *p = ecs_get(world, e, Position);
  // Remove component
  ecs_remove(world, e, Position);
  ECS_COMPONENT
  #define ECS_COMPONENT(world, id)
  Declare & define a component.
  Definition flecs_c.h:145

- C++

  auto e = world.entity();
  // Add a component. This creates the component in the ECS storage, but does not
  // assign it with a value.
  e.add<Velocity>();
  // Set the value for the Position & Velocity components. A component will be
  // added if the entity doesn't have it yet.
  e.set<Position>({10, 20})
  .set<Velocity>({1, 2});
  // Get a component
  const Position *p = e.get<Position>();
  // Remove component
  e.remove<Position>();
  flecs::entity_builder::add
  const Self & add() const
  Add a component to an entity.
  Definition builder.hpp:25
  flecs::entity_view::get
  const T * get() const
  Get component value.
  Definition entity_view.hpp:259

- C#

  Entity e = world.Entity();
  // Add a component. This creates the component in the ECS storage, but does not
  // assign it with a value.
  e.Add<Velocity>();
  // Set the value for the Position & Velocity components. A component will be
  // added if the entity doesn't have it yet.
  e.Set<Position>(new(10, 20))
  .Set<Velocity>(new(1, 2));
  // Get a component
  ref readonly Position p = ref e.Get<Position>();
  // Remove component
  e.Remove<Position>();

- Rust

  let e = world.entity();
  // Add a component. This creates the component in the ECS storage, but does not
  // assign it with a value.
  e.add::<Velocity>();
  // Set the value for the Position & Velocity components. A component will be
  // added if the entity doesn't have it yet.
  e.set(Position { x: 10.0, y: 20.0 })
  .set(Velocity { x: 1.0, y: 2.0 });
  // Get a component
  e.get::<&Position>(|p| {
  println!("Position: ({}, {})", p.x, p.y);
  });
  // Remove component
  e.remove::<Position>();

Each component is associated by a unique entity identifier by Flecs. This makes it possible to inspect component data, or attach your own data to components.

- C

  C applications can use the `ecs_id` macro to get the entity id for a component.

  ECS_COMPONENT(world, Position);
  ecs_entity_t pos_e = ecs_id(Position);
  printf("Name: %s\n", ecs_get_name(world, pos_e)); // outputs 'Name: Position'
  // It's possible to add components like you would for any entity
  ecs_add(world, pos_e, Serializable);
  ecs_id
  FLECS_API const ecs_entity_t ecs_id(EcsDocDescription)
  Component id for EcsDocDescription.

- C++

  C++ applications can use the `world::entity` function.

  flecs::entity pos_e = world.entity<Position>();
  std::cout << "Name: " << pos_e.name() << std::endl; // outputs 'Name: Position'
  // It's possible to add components like you would for any entity
  pos_e.add<Serializable>();
  flecs::entity
  Entity.
  Definition entity.hpp:30

- C#

  C# applications can use the `World.Entity()` function.

  Entity posE = world.Entity<Position>();
  Console.WriteLine($"Name: {posE.Name()}"); // outputs 'Name: Position'
  // It's possible to add components like you would for any entity
  posE.Add<Serializable>();

- Rust

  Rust applications can use the `world::entity_from` function.

  let pos_e = world.entity_from::<Position>();
  println!("Name: {}", pos_e.name()); // outputs 'Name: Position'
  // It's possible to add components like you would for any entity
  pos_e.add::<Serializable>();

The thing that makes an ordinary entity a component is the `EcsComponent` (or `flecs::Component`, in C++) component. This is a builtin component that tells Flecs how much space is needed to store a component, and can be inspected by applications:

- C

  ECS_COMPONENT(world, Position);
  ecs_entity_t pos_e = ecs_id(Position);
  const EcsComponent *c = ecs_get(world, pos_e, EcsComponent);
  printf("Component size: %u\n", c->size);
  EcsComponent
  Component information.
  Definition flecs.h:1523
  EcsComponent::size
  ecs_size_t size
  Component size.
  Definition flecs.h:1524

- C++

  flecs::entity pos_e = world.entity<Position>();
  const EcsComponent *c = pos_e.get<flecs::Component>();
  std::cout << "Component size: " << c->size << std::endl;

- C#

  Entity posE = world.Entity<Position>();
  ref readonly EcsComponent c = ref posE.Get<EcsComponent>();
  Console.WriteLine($"Component size: {c.size}");

- Rust

  let pos_e = world.entity_from::<Position>();
  pos_e.get::<&flecs::Component>(|c| {
  println!("Component size: {}", c.size);
  });

Because components are stored as regular entities, they can in theory also be deleted. To prevent unexpected accidents however, by default components are registered with a tag that prevents them from being deleted. If this tag were to be removed, deleting a component would cause it to be removed from all entities. For more information on these policies, see Relationship cleanup properties.

## Tag

A tag is a component that does not have any data. In Flecs tags can be either empty types (in C++) or regular entities (C & C++) that do not have the `EcsComponent` component (or have an `EcsComponent` component with size 0). Tags can be added & removed using the same APIs as adding & removing components, but because tags have no data, they cannot be assigned a value. Because tags (like components) are regular entities, they can be created & deleted at runtime.

- C

  // Create Enemy tag
  ecs_entity_t Enemy = ecs_new(world);
  // Create entity, add Enemy tag
  ecs_entity_t e = ecs_new(world);
  ecs_add_id(world, e, Enemy);
  ecs_has_id(world, e, Enemy); // true!
  ecs_remove_id(world, e, Enemy);
  ecs_has_id(world, e, Enemy); // false!
  ecs_add_id
  void ecs_add_id(ecs_world_t *world, ecs_entity_t entity, ecs_id_t id)
  Add a (component) id to an entity.
  ecs_remove_id
  void ecs_remove_id(ecs_world_t *world, ecs_entity_t entity, ecs_id_t id)
  Remove a (component) id from an entity.
  ecs_has_id
  bool ecs_has_id(const ecs_world_t *world, ecs_entity_t entity, ecs_id_t id)
  Test if an entity has an id.

- C++

  // Option 1: create Tag as empty struct
  struct Enemy { };
  // Create entity, add Enemy tag
  auto e = world.entity().add<Enemy>();
  e.has<Enemy>(); // true!
  e.remove<Enemy>();
  e.has<Enemy>(); // false!
  // Option 2: create Tag as entity
  auto Enemy = world.entity();
  // Create entity, add Enemy tag
  auto e = world.entity().add(Enemy);
  e.has(Enemy); // true!
  e.remove(Enemy);
  e.has(Enemy); // false!
  flecs::entity_view::has
  bool has(flecs::id_t e) const
  Check if entity has the provided entity.
  Definition entity_view.hpp:635

- C#

  // Option 1: create Tag as empty struct
  public struct Enemy { }
  // Create entity, add Enemy tag
  Entity e = world.Entity().Add<Enemy>();
  e.Has<Enemy>(); // true!
  e.Remove<Enemy>();
  e.Has<Enemy>(); // false!
  // Option 2: create Tag as entity
  Entity Enemy = world.Entity();
  // Create entity, add Enemy tag
  Entity e = world.Entity().Add(Enemy);
  e.Has(Enemy); // true!
  e.Remove(Enemy);
  e.Has(Enemy); // false!

- Rust

  // Option 1: create Tag as empty struct
  #[derive(Component)]
  struct Enemy;
  // Create entity, add Enemy tag
  let e = world.entity().add::<Enemy>();
  e.has::<Enemy>(); // true!
  e.remove::<Enemy>();
  e.has::<Enemy>(); // false!
  // Option 2: create Tag as entity
  let enemy = world.entity();
  // Create entity, add Enemy tag
  let e = world.entity().add_id(enemy);
  e.has_id(enemy); // true!
  e.remove_id(enemy);
  e.has_id(enemy); // false!

Note that both options in the C++ example achieve the same effect. The only difference is that in option 1 the tag is fixed at compile time, whereas in option 2 the tag can be created dynamically at runtime.

When a tag is deleted, the same rules apply as for components (see Relationship cleanup properties).

## Pair

A pair is a combination of two entity ids. Pairs can be used to store entity relationships, where the first id represents the relationship kind and the second id represents the relationship target (called "object"). This is best explained by an example:

- C

  // Create Likes relationship
  ecs_entity_t Likes = ecs_new(world);
  // Create a small graph with two entities that like each other
  ecs_entity_t Bob = ecs_new(world);
  ecs_entity_t Alice = ecs_new(world);
  ecs_add_pair(world, Bob, Likes, Alice); // Bob likes Alice
  ecs_add_pair(world, Alice, Likes, Bob); // Alice likes Bob
  ecs_has_pair(world, Bob, Likes, Alice); // true!
  ecs_remove_pair(world, Bob, Likes, Alice);
  ecs_has_pair(world, Bob, Likes, Alice); // false!

- C++

  // Create Likes relationship as empty type (tag)
  struct Likes { };
  // Create a small graph with two entities that like each other
  auto Bob = world.entity();
  auto Alice = world.entity();
  Bob.add<Likes>(Alice); // Bob likes Alice
  Alice.add<Likes>(Bob); // Alice likes Bob
  Bob.has<Likes>(Alice); // true!
  Bob.remove<Likes>(Alice);
  Bob.has<Likes>(Alice); // false!

- C#

  // Create Likes relationship as empty type (tag)
  public struct Likes { }
  // Create a small graph with two entities that like each other
  Entity Bob = world.Entity();
  Entity Alice = world.Entity();
  Bob.Add<Likes>(Alice); // Bob likes Alice
  Alice.Add<Likes>(Bob); // Alice likes Bob
  Bob.Has<Likes>(Alice); // true!
  Bob.Remove<Likes>(Alice);
  Bob.Has<Likes>(Alice); // false!

- Rust

  // Create Likes relationship as empty type (tag)
  #[derive(Component)]
  struct Likes;
  // Create a small graph with two entities that like each other
  let bob = world.entity();
  let alice = world.entity();
  bob.add_first::<Likes>(alice); // bob likes alice
  alice.add_first::<Likes>(bob); // alice likes bob
  bob.has_first::<Likes>(alice); // true!
  bob.remove_first::<Likes>(alice);
  bob.has_first::<Likes>(alice); // false!

A pair can be encoded in a single 64 bit identifier by using the `ecs_pair` macro in C, or the `world.pair` function in C++:

- C

  ecs_id_t id = ecs_pair(Likes, Bob);
  ecs_id_t
  uint64_t ecs_id_t
  Ids are the things that can be added to an entity.
  Definition flecs.h:374

- C++

  flecs::id id = world.pair<Likes>(Bob);
  flecs::id
  Class that wraps around a flecs::id_t.
  Definition decl.hpp:27
  flecs::world::pair
  flecs::id pair() const
  Get pair id from relationship, object.
  Definition impl.hpp:80

- C#

  Id id = world.Pair<Likes>(bob);

- Rust

  let id = world.id_first::<Likes>(bob);

The following examples show how to get back the elements from a pair:

- C

  if (ecs_id_is_pair(id)) {
  ecs_entity_t relationship = ecs_pair_first(world, id);
  ecs_entity_t target = ecs_pair_second(world, id);
  }
  ecs_id_is_pair
  bool ecs_id_is_pair(ecs_id_t id)
  Utility to check if id is a pair.

- C++

  flecs::id id = ...;
  if (id.is_pair()) {
  auto relationship = id.first();
  auto target = id.second();
  }

- C#

  Id id = ...;
  if (id.IsPair())
  {
  Entity relationship = id.First();
  Entity target = id.Second();
  }

- Rust

  let id = world.id_from::<(Likes, Apples)>();
  if id.is_pair() {
  let relationship = id.first_id();
  let target = id.second_id();
  }

A component or tag can be added multiple times to the same entity as long as it is part of a pair, and the pair itself is unique:

- C

  ecs_add_pair(world, Bob, Eats, Apples);
  ecs_add_pair(world, Bob, Eats, Pears);
  ecs_add_pair(world, Bob, Grows, Pears);
  ecs_has_pair(world, Bob, Eats, Apples); // true!
  ecs_has_pair(world, Bob, Eats, Pears); // true!
  ecs_has_pair(world, Bob, Grows, Pears); // true!

- C++

  flecs::entity bob = ...;
  bob.add(Eats, Apples);
  bob.add(Eats, Pears);
  bob.add(Grows, Pears);
  bob.has(Eats, Apples); // true!
  bob.has(Eats, Pears); // true!
  bob.has(Grows, Pears); // true!

- C#

  Entity Bob = ...;
  Bob.Add(Eats, Apples);
  Bob.Add(Eats, Pears);
  Bob.Add(Grows, Pears);
  Bob.Has(Eats, Apples); // true!
  Bob.Has(Eats, Pears); // true!
  Bob.Has(Grows, Pears); // true!

- Rust

  let bob = world.entity();
  bob.add_id((eats, apples));
  bob.add_id((eats, pears));
  bob.add_id((grows, pears));
  bob.has_id((eats, apples)); // true!
  bob.has_id((eats, pears)); // true!
  bob.has_id((grows, pears)); // true!

The `target` function can be used in C and C++ to get the object for a relationship:

- C

  ecs_entity_t o = ecs_get_target(world, Alice, Likes, 0); // Returns Bob
  ecs_get_target
  ecs_entity_t ecs_get_target(const ecs_world_t *world, ecs_entity_t entity, ecs_entity_t rel, int32_t index)
  Get the target of a relationship.

- C++

  flecs::entity alice = ...;
  auto o = alice.target<Likes>(); // Returns Bob

- C#

  Entity Alice = ...;
  Entity o = Alice.Target<Likes>(); // Returns Bob

- Rust

  let alice = world.entity().add_first::<Likes>(bob);
  let o = alice.target::<Likes>(0); // Returns bob

Entity relationships enable lots of interesting patterns and possibilities. Make sure to check out the Relationships manual.

## Hierarchies

Flecs has builtin support for hierarchies with the builtin `EcsChildOf` (or `flecs::ChildOf`, in C++) relationship. A hierarchy can be created with the regular relationship API, or with the `child_of` shortcut in C++:

- C

  ecs_entity_t parent = ecs_new(world);
  // ecs_new_w_pair is the same as ecs_new_id + ecs_add_pair
  ecs_entity_t child = ecs_new_w_pair(world, EcsChildOf, parent);
  // Deleting the parent also deletes its children
  ecs_delete(world, parent);
  EcsChildOf
  const ecs_entity_t EcsChildOf
  Used to express parent-child relationships.

- C++

  auto parent = world.entity();
  auto child = world.entity().child_of(parent);
  // Deleting the parent also deletes its children
  parent.destruct();
  flecs::entity_builder::child_of
  const Self & child_of(entity_t second) const
  Shortcut for add(ChildOf, entity).
  Definition builder.hpp:230
  flecs::entity::destruct
  void destruct() const
  Delete an entity.
  Definition entity.hpp:362

- C#

  Entity parent = world.Entity();
  Entity child = world.Entity().ChildOf(parent);
  // Deleting the parent also deletes its children
  parent.Destruct();

- Rust

  let parent = world.entity();
  let child = world.entity().child_of_id(parent);
  // Deleting the parent also deletes its children
  parent.destruct();

When entities have names, they can be used together with hierarchies to generate path names or do relative lookups:

- C

  ecs_entity_t parent = ecs_entity(world, {
  .name = "parent"
  });
  ecs_entity_t child = ecs_entity(world, {
  .name = "child"
  });
  ecs_add_pair(world, child, EcsChildOf, parent);
  char *path = ecs_get_path(world, child);
  printf("%s\n", path); // output: 'parent.child'
  ecs_os_free(path);
  ecs_lookup(world, "parent.child"); // returns child
  ecs_lookup_from(world, parent, "child"); // returns child

- C++

  auto parent = world.entity("parent");
  auto child = world.entity("child").child_of(parent);
  std::cout << child.path() << std::endl; // output: 'parent::child'
  world.lookup("parent::child"); // returns child
  parent.lookup("child"); // returns child
  flecs::entity_view::path
  flecs::string path(const char *sep="::", const char *init_sep="::") const
  Return the entity path.
  Definition entity_view.hpp:94
  flecs::entity_view::lookup
  flecs::entity lookup(const char *path, bool search_path=false) const
  Lookup an entity by name.
  Definition impl.hpp:188

- C#

  Entity parent = world.Entity("parent");
  Entity child = world.Entity("child").ChildOf(parent);
  Console.WriteLine(child.Path()); // output: 'parent.child'
  world.Lookup("parent.child"); // returns child
  parent.Lookup("child"); // returns child

- Rust

  let parent = world.entity_named("parent");
  let child = world.entity_named("child").child_of_id(parent);
  println!("Child path: {}", child.path().unwrap()); // output: 'parent::child'
  world.lookup("parent::child"); // returns child
  parent.lookup("child"); // returns child

Queries (see below) can use hierarchies to order data breadth-first, which can come in handy when you're implementing a transform system:

- C

  ecs_query_t *q = ecs_query(world, {
  .terms = {
  { ecs_id(Position) },
  { ecs_id(Position), .src = {
  .flags = EcsCascade, // Breadth-first order
  .trav = EcsChildOf // Use ChildOf relationship for traversal
  }}
  }
  });
  ecs_iter_t it = ecs_query_iter(world, q);
  while (ecs_query_next(&it)) {
  Position *p = ecs_field(&it, Position, 0);
  Position *p_parent = ecs_field(&it, Position, 1);
  for (int i = 0; i < it.count; i++) {
  // Do the thing
  }
  }
  ecs_query
  #define ecs_query(world,...)
  Shorthand for creating a query with ecs_query_cache_init.
  Definition flecs_c.h:278
  ecs_query_next
  bool ecs_query_next(ecs_iter_t *it)
  Progress query iterator.
  EcsCascade
  #define EcsCascade
  Sort results breadth first.
  Definition flecs.h:735
  ecs_query_iter
  ecs_iter_t ecs_query_iter(const ecs_world_t *world, const ecs_query_t *query)
  Create a query iterator.
  ecs_iter_t
  Iterator.
  Definition flecs.h:1136
  ecs_query_t
  Queries are lists of constraints (terms) that match entities.
  Definition flecs.h:813

- C++

  auto q = world.query_builder<Position, Position>()
  .term_at(1).parent().cascade()
  .build();
  q.each([](Position& p, Position& p_parent) {
  // Do the thing
  });
  flecs::world::query_builder
  flecs::query_builder< Comps... > query_builder(Args &&... args) const
  Create a query builder.
  Definition impl.hpp:248

- C#

  Query q = world.QueryBuilder<Position, Position>()
  .TermAt(1).Parent().Cascade()
  .Build();
  q.Each((ref Position p, ref Position pParent) =>
  {
  // Do the thing
  });

- Rust

  let q = world
  .query::<(&Position, &mut Position)>()
  .term_at(1)
  .parent()
  .cascade()
  .build();
  q.each(|(p, p_parent)| {
  // Do the thing
  });

## Type

The type (often referred to as "archetype") is the list of ids an entity has. Types can be used for introspection which is useful when debugging, or when for example building an entity editor. The most common thing to do with a type is to convert it to text and print it:

- C

  ECS_COMPONENT(world, Position);
  ECS_COMPONENT(world, Velocity);
  ecs_entity_t e = ecs_new(world);
  ecs_add(world, e, Position);
  ecs_add(world, e, Velocity);
  const ecs_type_t *type = ecs_get_type(world, e);
  char *type_str = ecs_type_str(world, type);
  printf("Type: %s\n", type_str); // output: 'Position,Velocity'
  ecs_os_free(type_str);
  ecs_get_type
  const ecs_type_t * ecs_get_type(const ecs_world_t *world, ecs_entity_t entity)
  Get the type of an entity.
  ecs_type_str
  char * ecs_type_str(const ecs_world_t *world, const ecs_type_t *type)
  Convert type to string.
  ecs_type_t
  A type is a list of (component) ids.
  Definition flecs.h:398

- C++

  auto e = ecs.entity()
  .add<Position>()
  .add<Velocity>();
  std::cout << e.type().str() << std::endl; // output: 'Position,Velocity'

- C#

  Entity e = ecs.Entity()
  .Add<Position>()
  .Add<Velocity>();
  Console.WriteLine(e.Type().Str()); // output: 'Position,Velocity'

- Rust

  let e = world.entity().add::<Position>().add::<Velocity>();
  println!("Components: {}", e.archetype().to_string().unwrap()); // output: 'Position,Velocity'

A type can also be iterated by an application:

- C

  const ecs_type_t *type = ecs_get_type(world, e);
  for (int i = 0; i < type->count; i++) {
  if (type->array[i] == ecs_id(Position)) {
  // Found Position component!
  }
  }
  ecs_type_t::array
  ecs_id_t * array
  Array with ids.
  Definition flecs.h:399
  ecs_type_t::count
  int32_t count
  Number of elements in array.
  Definition flecs.h:400

- C++

  e.each([&](flecs::id id) {
  if (id == world.id<Position>()) {
  // Found Position component!
  }
  });
  flecs::world::id
  flecs::id id() const
  Get id from a type.
  Definition impl.hpp:70

- C#

  e.Each((Id id) =>
  {
  if (id == world.Id<Position>())
  {
  // Found Position component!
  }
  });

- Rust

  e.each_component(|id| {
  if id == world.component_id::<Position>() {
  // Found Position component!
  }
  });

## Singleton

A singleton is a single instance of a component that can be retrieved without an entity. The functions for singletons are very similar to the regular API:

- C

  // Set singleton component
  ecs_singleton_set(world, Gravity, { 9.81 });
  // Get singleton component
  const Gravity *g = ecs_singleton_get(world, Gravity);

- C++

  // Set singleton component
  world.set<Gravity>({ 9.81 });
  // Get singleton component
  const Gravity *g = world.get<Gravity>();
  flecs::world::get
  const T * get() const
  Get singleton component.
  Definition world.hpp:128
  flecs::world::set
  void set(const T &value) const
  Set singleton component.
  Definition world.hpp:649

- C#

  // Set singleton component
  world.Set<Gravity>(new(9.81));
  // Get singleton component
  ref readonly Gravity g = ref world.Get<Gravity>();

- Rust

  // Set singleton component
  world.set(Gravity { x: 10, y: 20 });
  // Get singleton component
  world.get::<&Gravity>(|g| {
  println!("Gravity: {}, {}", g.x, g.y);
  });

Singleton components are created by adding the component to its own entity id. The above code examples are shortcuts for these regular API calls:

- C

  ecs_set(world, ecs_id(Gravity), Gravity, {10, 20});
  const Gravity *g = ecs_get(world, ecs_id(Gravity), Gravity);

- C++

  flecs::entity grav_e = world.entity<Gravity>();
  grav_e.set<Gravity>({10, 20});
  const Gravity *g = grav_e.get<Gravity>();

- C#

  Entity gravE = world.Entity<Gravity>();
  gravE.Set<Gravity>(new(10, 20));
  ref readonly Gravity g = ref gravE.Get<Gravity>();

- Rust

  let grav_e = world.entity_from::<Gravity>();
  grav_e.set(Gravity { x: 10, y: 20 });
  grav_e.get::<&Gravity>(|g| {
  println!("Gravity: {}, {}", g.x, g.y);
  });

The following examples show how to query for a singleton component:

- C

  // Create query that matches Gravity as singleton
  ecs_query_t *q = ecs_query(ecs, {
  .terms = {
  // Regular component
  { .id = ecs_id(Velocity) },
  // A singleton is a component matched on itself
  { .id = ecs_id(Gravity), .src.id = ecs_id(Gravity) }
  }
  });
  // Create a system using the query DSL with a singleton:
  ECS_SYSTEM(world, ApplyGravity, EcsOnUpdate, Velocity, Gravity($));
  EcsOnUpdate
  const ecs_entity_t EcsOnUpdate
  OnUpdate pipeline phase.
  ECS_SYSTEM
  #define ECS_SYSTEM(world, id, phase,...)
  Declare & define a system.
  Definition system.h:220

- C++

  world.query_builder<Velocity, Gravity>()
  .term_at(1).singleton()
  .build();

- C#

  world.QueryBuilder<Velocity, Gravity>()
  .TermAt(1).Singleton()
  .Build();

- Rust

  world
  .query::<(&Velocity, &Gravity)>()
  .term_at(1)
  .singleton()
  .build();

## Query

Queries are the main mechanism for finding and iterating through entities. Queries are used in many parts of the API, such as for systems and observers. The following example shows a simple query:

- C

  ecs_query_t *q = ecs_query(world, {
  .terms = {
  { ecs_id(Position) },
  { ecs_pair(EcsChildOf, parent) }
  }
  });
  // Iterate the query results. Because entities are grouped by their type there
  // are two loops: an outer loop for the type, and an inner loop for the entities
  // for that type.
  ecs_iter_t it = ecs_query_iter(world, q);
  while (ecs_query_next(&it)) {
  // Each type has its own set of component arrays
  Position *p = ecs_field(&it, Position, 0);
  // Iterate all entities for the type
  for (int i = 0; i < it.count; i++) {
  printf("%s: {%f, %f}\n", ecs_get_name(world, it.entities[i]),
  p[i].x, p[i].y);
  }
  }
  ecs_query_fini(f);
  ecs_query_fini
  void ecs_query_fini(ecs_query_t *query)
  Delete a query.

- C++

  // For simple queries the world::each function can be used
  world.each([](Position& p, Velocity& v) { // flecs::entity argument is optional
  p.x += v.x;
  p.y += v.y;
  });
  // More complex queries can first be created, then iterated
  auto q = world.query_builder<Position>()
  .with(flecs::ChildOf, parent)
  .build();
  // Option 1: the each() callback iterates over each entity
  q.each([](flecs::entity e, Position& p) {
  std::cout << e.name() << ": {" << p.x << ", " << p.y << "}" << std::endl;
  });
  // Option 2: the run() callback offers more control over the iteration
  q.run([](flecs::iter& it) {
  while (it.next()) {
  auto p = it.field<Position>(0);
  for (auto i : it) {
  std::cout << it.entity(i).name()
  << ": {" << p[i].x << ", " << p[i].y << "}" << std::endl;
  }
  }
  });
  flecs::world::each
  void each(Func &&func) const
  Iterate over all entities with components in argument list of function.
  Definition impl.hpp:302
  flecs::iter
  Class for iterating over query results.
  Definition iter.hpp:68
  flecs::iter::field
  flecs::field< A > field(int8_t index) const
  Get readonly access to field data.
  Definition iter.hpp:68
  flecs::iter::next
  bool next()
  Progress iterator.
  Definition iter.hpp:376

- C#

  // For simple queries the each function can be used
  world.Each((ref Position p, ref Velocity v) => // Entity argument is optional
  {
  p.X += v.X;
  p.Y += v.Y;
  });
  // More complex filters can first be created, then iterated
  using Query q = world.QueryBuilder<Position>()
  .With(Ecs.ChildOf, parent)
  .Build();
  // Option 1: The Each() callback that iterates each entity
  q.Each((Entity e, ref Position p) =>
  {
  Console.WriteLine($"{e.Name()}: ({p.X}, {p.Y})")
  });
  // Option 2: The Iter() callback provides more control over the iteration
  q.Iter((Iter it, Field<Position> p) =>
  {
  foreach (int i in it)
  Console.WriteLine($"{it.Entity(i).Name()}: ({p[i].X}, {p[i].Y})")
  });

- Rust

  // For simple queries the world::each function can be used
  world.each::<(&mut Position, &Velocity)>(|(p, v)| {
  // EntityView argument is optional, use each_entity to get it
  p.x += v.x;
  p.y += v.y;
  });
  // More complex queries can first be created, then iterated
  let q = world
  .query::<&Position>()
  .with_id((flecs::ChildOf::ID, parent))
  .build();
  // Option 1: the each() callback iterates over each entity
  q.each_entity(|e, p| {
  println!("{}: ({}, {})", e.name(), p.x, p.y);
  });
  // Option 2: the run() callback offers more control over the iteration
  q.run(|mut it| {
  while it.next() {
  let p = it.field::<Position>(0).unwrap();
  for i in it.iter() {
  println!("{}: ({}, {})", it.entity(i).name(), p[i].x, p[i].y);
  }
  }
  });

Queries can use operators to exclude components, optionally match components or match one out of a list of components. Additionally filters may contain wildcards for terms which is especially useful when combined with pairs.

The following example shows a query that matches all entities with a parent that do not have `Position`:

- C

  ecs_query_t *q = ecs_query(world, {
  .terms = {
  { ecs_pair(EcsChildOf, EcsWildcard) }
  { ecs_id(Position), .oper = EcsNot },
  }
  });
  // Iteration code is the same
  EcsWildcard
  const ecs_entity_t EcsWildcard
  Wildcard entity ("*").
  EcsNot
  @ EcsNot
  The term must not match.
  Definition flecs.h:696

- C++

  flecs::query<> q = world.query_builder()
  .with(flecs::ChildOf, flecs::Wildcard)
  .with<Position>().oper(flecs::Not)
  .build();
  // Iteration code is the same
  flecs::query
  Definition impl.hpp:196

- C#

  using Query q = world.QueryBuilder()
  .With(Ecs.ChildOf, Ecs.Wildcard)
  .With<Position>().Oper(Ecs.Not)
  .Build();
  // Iteration code is the same

- Rust

  let q = world
  .query::<()>()
  .with::<(flecs::ChildOf, flecs::Wildcard)>()
  .with::<Position>()
  .set_oper(OperKind::Not)
  .build();
  // Iteration code is the same

See the query manual for more details.

## System

A system is a query combined with a callback. Systems can be either ran manually or ran as part of an ECS-managed main loop (see Pipeline). The system API looks similar to queries:

- C

  // Option 1, use the ECS_SYSTEM convenience macro
  ECS_SYSTEM(world, Move, 0, Position, Velocity);
  ecs_run(world, Move, delta_time, NULL); // Run system
  // Option 2, use the ecs_system_init function/ecs_system macro
  ecs_entity_t move_sys = ecs_system(world, {
  .query.terms = {
  { ecs_id(Position) },
  { ecs_id(Velocity) },
  },
  .callback = Move
  });
  ecs_run(world, move_sys, delta_time, NULL); // Run system
  // The callback code (same for both options)
  void Move(ecs_iter_t *it) {
  Position *p = ecs_field(it, Position, 0);
  Velocity *v = ecs_field(it, Velocity, 1);
  for (int i = 0; i < it->count; i++) {
  p[i].x += v[i].x * it->delta_time;
  p[i].y += v[i].y * it->delta_time;
  }
  }
  ecs_system
  #define ecs_system(world,...)
  Shorthand for creating a system with ecs_system_init().
  Definition system.h:244
  ecs_run
  FLECS_API ecs_entity_t ecs_run(ecs_world_t *world, ecs_entity_t system, ecs_ftime_t delta_time, void *param)
  Run a specific system manually.
  ecs_iter_t::delta_time
  float delta_time
  Time elapsed since last frame.
  Definition flecs.h:1180
  ecs_iter_t::count
  int32_t count
  Number of entities to iterate.
  Definition flecs.h:1186

- C++

  // Use each() function that iterates each individual entity
  auto move_sys = world.system<Position, Velocity>()
  .each([](flecs::iter& it, size_t, Position& p, Velocity& v) {
  p.x += v.x * it.delta_time();
  p.y += v.y * it.delta_time();
  });
  // Just like with queries, systems have both the run() and
  // each() methods to iterate entities.
  move_sys.run();
  flecs::world::system
  flecs::system system(flecs::entity e) const
  Upcast entity to a system.
  Definition impl.hpp:114

- C#

  // Use Each() function that iterates each individual entity
  System<Position, Velocity> moveSys = world.System<Position, Velocity>()
  .Each((Entity e, ref Position p, ref Velocity v) =>
  {
  p.X += v.X * it.DeltaTime();
  p.Y += v.Y * it.DeltaTime();
  });
  // Just like with queries, systems have both the Iter() and
  // Each() methods to iterate entities.
  moveSys.Run();

- Rust

  // Use each_entity() function that iterates each individual entity
  let move_sys = world
  .system::<(&mut Position, &Velocity)>()
  .each_iter(|it, i, (p, v)| {
  p.x += v.x * it.delta_time();
  p.y += v.y * it.delta_time();
  });
  // Just like with queries, systems have both the run() and
  // each() methods to iterate entities.
  move_sys.run();

Systems are stored as entities with an `EcsSystem` component (`flecs::System` in C++), similar to components. That means that an application can use a system as a regular entity:

- C

  printf("System: %s\n", ecs_get_name(world, move_sys));
  ecs_add(world, move_sys, EcsOnUpdate);
  ecs_delete(world, move_sys);

- C++

  std::cout << "System: " << move_sys.name() << std::endl;
  move_sys.add(flecs::OnUpdate);
  move_sys.destruct();

- C#

  Console.WriteLine($"System: {moveSys.Name()}");
  moveSys.Entity.Add(Ecs.OnUpdate);
  moveSys.Entity.Destruct();

- Rust

  println!("System: {}", move_sys.name());
  move_sys.add::<flecs::pipeline::OnUpdate>();
  move_sys.destruct();

## Pipeline

A pipeline is a list of tags that when matched, produces a list of systems to run. These tags are also referred to as a system "phase". Flecs comes with a default pipeline that has the following phases:

- C

  EcsOnLoad
  EcsPostLoad
  EcsPreUpdate
  EcsOnUpdate
  EcsOnValidate
  EcsPostUpdate
  EcsPreStore
  EcsOnStore
  EcsOnStore
  const ecs_entity_t EcsOnStore
  OnStore pipeline phase.
  EcsPreStore
  const ecs_entity_t EcsPreStore
  PreStore pipeline phase.
  EcsOnLoad
  const ecs_entity_t EcsOnLoad
  OnLoad pipeline phase.
  EcsPostUpdate
  const ecs_entity_t EcsPostUpdate
  PostUpdate pipeline phase.
  EcsOnValidate
  const ecs_entity_t EcsOnValidate
  OnValidate pipeline phase.
  EcsPreUpdate
  const ecs_entity_t EcsPreUpdate
  PreUpdate pipeline phase.
  EcsPostLoad
  const ecs_entity_t EcsPostLoad
  PostLoad pipeline phase.

- C++

  flecs::OnLoad
  flecs::PostLoad
  flecs::PreUpdate
  flecs::OnUpdate
  flecs::OnValidate
  flecs::PostUpdate
  flecs::PreStore
  flecs::OnStore

- C#

  Ecs.OnLoad
  Ecs.PostLoad
  Ecs.PreUpdate
  Ecs.OnUpdate
  Ecs.OnValidate
  Ecs.PostUpdate
  Ecs.PreStore
  Ecs.OnStore

- Rust

  flecs::pipeline::OnLoad;
  flecs::pipeline::PostLoad;
  flecs::pipeline::PreUpdate;
  flecs::pipeline::OnUpdate;
  flecs::pipeline::OnValidate;
  flecs::pipeline::PostUpdate;
  flecs::pipeline::PreStore;
  flecs::pipeline::OnStore;

When a pipeline is executed, systems are ran in the order of the phases. This makes pipelines and phases the primary mechanism for defining ordering between systems. The following code shows how to assign systems to a pipeline, and how to run the pipeline with the `progress()` function:

- C

  ECS_SYSTEM(world, Move, EcsOnUpdate, Position, Velocity);
  ECS_SYSTEM(world, Transform, EcsPostUpdate, Position, Transform);
  ECS_SYSTEM(world, Render, EcsOnStore, Transform, Mesh);
  ecs_progress(world, 0); // run systems in default pipeline
  ecs_progress
  FLECS_API bool ecs_progress(ecs_world_t *world, ecs_ftime_t delta_time)
  Progress a world.

- C++

  world.system<Position, Velocity>("Move").kind(flecs::OnUpdate).each( ... );
  world.system<Position, Transform>("Transform").kind(flecs::PostUpdate).each( ... );
  world.system<Transform, Mesh>("Render").kind(flecs::OnStore).each( ... );
  world.progress();
  flecs::world::progress
  bool progress(ecs_ftime_t delta_time=0.0) const
  Progress world one tick.
  Definition impl.hpp:47
  flecs::entity_view::each
  void each(const Func &func) const
  Iterate (component) ids of an entity.
  Definition impl.hpp:128

- C#

  world.System<Position, Velocity>("Move").Kind(Ecs.OnUpdate).Each( ... );
  world.System<Position, Transform>("Transform").Kind(Ecs.PostUpdate).Each( ... );
  world.System<Transform, Mesh>("Render").Kind(Ecs.OnStore).Each( ... );
  world.Progress();

- Rust

  world
  .system_named::<(&mut Position, &Velocity)>("Move")
  .kind::<flecs::pipeline::OnUpdate>()
  .each(|(p, v)| {});
  world
  .system_named::<(&mut Position, &Transform)>("Transform")
  .kind::<flecs::pipeline::PostUpdate>()
  .each(|(p, t)| {});
  world
  .system_named::<(&Transform, &mut Mesh)>("Render")
  .kind::<flecs::pipeline::OnStore>()
  .each(|(t, m)| {});
  world.progress();

Because phases are just tags that are added to systems, applications can use the regular API to add/remove systems to a phase:

- C

  ecs_remove_id(world, Move, EcsOnUpdate);
  ecs_add_id(world, Move, EcsPostUpdate);

- C++

  move_sys.add(flecs::OnUpdate);
  move_sys.remove(flecs::PostUpdate);

- C#

  moveSys.Add(Ecs.OnUpdate);
  moveSys.Remove(Ecs.PostUpdate);

- Rust

  move_sys.add::<flecs::pipeline::OnUpdate>();
  move_sys.remove::<flecs::pipeline::PostUpdate>();

Inside a phase, systems are guaranteed to be ran in their declaration order.

## Observer

Observers are callbacks that are invoked when one or more events matches the query of an observer. Events can be either user defined or builtin. Examples of builtin events are `OnAdd`, `OnRemove` and `OnSet`.

When an observer has a query with more than one component, the observer will not be invoked until the entity for which the event is emitted satisfies the entire query.

An example of an observer with two components:

- C

  ecs_observer(world, {
  .query.terms = { { ecs_id(Position) }, { ecs_id(Velocity) }},
  .event = { EcsOnSet },
  .callback = OnSetPosition
  });
  // Callback code is same as system
  ecs_entity_t e = ecs_new(world); // Doesn't invoke the observer
  ecs_set(world, e, Position, {10, 20}); // Doesn't invoke the observer
  ecs_set(world, e, Velocity, {1, 2}); // Invokes the observer
  ecs_set(world, e, Position, {20, 40}); // Invokes the observer
  EcsOnSet
  const ecs_entity_t EcsOnSet
  Event that triggers when a component is set for an entity.
  ecs_observer
  #define ecs_observer(world,...)
  Shorthand for creating an observer with ecs_observer_init().
  Definition flecs_c.h:293

- C++

  world.observer<Position, Velocity>("OnSetPosition")
  .event(flecs::OnSet)
  .each( ... ); // Callback code is same as system
  auto e = ecs.entity(); // Doesn't invoke the observer
  e.set<Position>({10, 20}); // Doesn't invoke the observer
  e.set<Velocity>({1, 2}); // Invokes the observer
  e.set<Position>({20, 30}); // Invokes the observer
  flecs::world::observer
  flecs::observer observer(flecs::entity e) const
  Observer builder.
  Definition impl.hpp:41

- C#

  world.Observer<Position, Velocity>("OnSetPosition")
  .Event(Ecs.OnSet)
  .Each( ... ); // Callback code is same as system
  Entity e = ecs.Entity(); // Doesn't invoke the observer
  e.Set<Position>(new(10, 20)); // Doesn't invoke the observer
  e.Set<Velocity>(new(1, 2)); // Invokes the observer
  e.Set<Position>(new(20, 30)); // Invokes the observer

- Rust

  world
  .observer_named::<flecs::OnSet, (&Position, &Velocity)>("OnSetPosition")
  .each(|(p, v)| {}); // Callback code is same as system
  let e = world.entity(); // Doesn't invoke the observer
  e.set(Position { x: 10.0, y: 20.0 }); // Doesn't invoke the observer
  e.set(Velocity { x: 1.0, y: 2.0 }); // Invokes the observer
  e.set(Position { x: 30.0, y: 40.0 }); // Invokes the observer

## Module

A module is a function that imports and organizes components, systems, triggers, observers, prefabs into the world as reusable units of code. A well designed module has no code that directly relies on code of another module, except for components definitions. All module contents are stored as child entities inside the module scope with the `ChildOf` relationship. The following examples show how to define a module in C and C++:

- C

  // Module header (e.g. MyModule.h)
  typedef struct {
  float x;
  float y;
  } Position;
  extern ECS_COMPONENT_DECLARE(Position);
  // The import function name has to follow the convention: <ModuleName>Import
  void MyModuleImport(ecs_world_t *world);
  // Module source (e.g. MyModule.c)
  ECS_COMPONENT_DECLARE(Position);
  void MyModuleImport(ecs_world_t *world) {
  ECS_MODULE(world, MyModule);
  ECS_COMPONENT_DEFINE(world, Position);
  }
  // Import code
  ECS_IMPORT(world, MyModule);
  ECS_MODULE
  #define ECS_MODULE(world, id)
  Create a module.
  Definition module.h:107
  ECS_IMPORT
  #define ECS_IMPORT(world, id)
  Wrapper around ecs_import().
  Definition module.h:119
  ECS_COMPONENT_DEFINE
  #define ECS_COMPONENT_DEFINE(world, id_)
  Define a forward declared component.
  Definition flecs_c.h:122
  ECS_COMPONENT_DECLARE
  #define ECS_COMPONENT_DECLARE(id)
  Forward declare a component.
  Definition flecs_c.h:112

- C++

  struct my_module {
  my_module(flecs::world& world) {
  world.module<my_module>();
  // Define components, systems, triggers, ... as usual. They will be
  // automatically created inside the scope of the module.
  }
  };
  // Import code
  world.import<my_module>();
  flecs::world::import
  flecs::entity import()
  Import a module.
  Definition impl.hpp:113
  flecs::world::module
  flecs::entity module(const char *name=nullptr) const
  Define a module.
  Definition impl.hpp:80

- C#

  public struct MyModule : IFlecsModule
  {
  public void InitModule(ref World world)
  {
  world.Module<MyModule>();
  // Define components, systems, triggers, ... as usual. They will be
  // automatically created inside the scope of the module.
  }
  };
  // Import code
  world.Import<MyModule>();

- Rust

  #[derive(Component)]
  struct MyModule;
  impl Module for MyModule {
  fn module(world: &World) {
  world.module::<MyModule>("MyModule");
  // Define components, systems, triggers, ... as usual. They will be
  // automatically created inside the scope of the module.
  }
  }
  // Import code
  world.import::<MyModule>();