NAME

Game::Entities - A simple entity registry for ECS designs

SYNOPSIS

my $ECS = Game::Entities->new;

# Create an entity with a possibly empty set of components
$guid = $ECS->create(
    Collidable->new,
    Drawable->new,
    Consumable->new,
);

# CRUD operations on an entity's components
$ECS->add( $guid => Equipable->new );      # It is now equipable
$ECS->delete( $guid => 'Consumable' );     # It is no longer consumable
$item = $ECS->get( $guid => 'Equipable' );

# A system operates on sets of entities defined by a view
$view = $ECS->view(qw( Drawable Equipable ));

# You can iterate over the set with a callback
# The callback will get the GUID and the components in the order requested
$view->each( sub ( $guid, $draw, $item ) {
    draw_equipment( $draw, $item );
});

# Or you can do the same iterating over the components
for ( $view->components ) {
    my ( $draw, $item ) = @$_;
    draw_equipment( $draw, $item );
}

# Or over the entity GUIDs
for my $guid ( $view->entities ) {
    my ( $draw, $item ) = $ECS->get( $guid => qw( Drawable Equipable ) );
    draw_equipment( $draw, $item );
}

# Or you can iterate over the view directly and get both
for ( @{ $view } ) {
    my ( $guid, $draw, $item ) = ( $_->[0], @{ $_->[1] } );

    if ( $ECS->get( $guid => 'Consumable' ) ) {
        say 'This is equipment you can eat!';
    }

    ...
}

# Delete the entity and all its components
$ECS->delete($guid);

$ECS->clear; # Delete all entities and components

DESCRIPTION

Game::Entities is a minimalistic entity manager designed for applications using an ECS architecture.

If you don't know what this means, Mick West's Evolve Your Hierarchy might be a good place to start.

On Stability

This distribution is currently experimental, and as such, its API might still change without warning. Any change, breaking or not, will be noted in the change log, so if you wish to use it, please pin your dependencies and make sure to check the change log before upgrading.

CONCEPTS

Throughout this documentation, there are a couple of key concepts that will be used repeatedly:

GUID

Entities are represented by an opaque global unique identifier: a GUID. GUIDs used by Game::Entities are opaque to the user, and represent a particular version of an entity which will remain valid as long as that entity is not deleted (with delete or clear).

Each Game::Entities registry supports up to 1,048,575 (2^20 - 1) simultaneous valid entity GUIDs. They will never be 0, so they should always be truthy values.

Valid entities

An entity's GUID is valid from the time it is created with create until the time it is deleted. An entity's GUID can be stored and used as an identifier anywhere in the program at any point during this time, but must not be used outside it.

Components

As far as Game::Entities is concerned, any reference of any type can be used as a component and added to an entity. This includes blessed and non-blessed references.

Entities can have any number of components attached to them, but they will only ever have one component of any one type (as identified by ref).

Views

A group of components defines a view, which is composed of all entities that have all of that view's components. This means that all the entities for the view for components A, B, and C will have at least all those components, but could of course have any others as well.

The main purpose of views is to make it possible to iterate as fast as possible over any group of entities that have a set of common components.

METHODS

new

$ECS = Game::Entities->new;

Creates a new entity registry. The constructor takes no arguments.

create

$guid = $ECS->create(@components)

Creates a new entity and returns its GUID. If called with a list of components, these will be added to the entity before returning.

add

$ECS->add( $guid, @components );

Takes an entity's GUID and a component, and adds that component to the specified entity. If the entity already had a component of that type, calling this method will silently overwrite it.

Multiple components can be specified, and they will all be added in the order they were provided.

get

$component  = $ECS->get( $guid, $component_name  );
@components = $ECS->get( $guid, @component_names );

Takes an entity's GUID and the name of a component as a string, and retrieves the component with that name for the specified entity, if it exists. If the entity has no component by that name, this method returns undefined instead.

Multiple components names can be specified, and they will all be retrieved and returned in the order they were provided.

When called with a single target component name, this method returns a scalar. When called with a list of components names, it will return a list.

delete

$ECS->delete( $guid );
$ECS->delete( $guid, @component_names );

When called with only an entity's GUID, it deletes all components from the entity, and marks that entity's GUID as invalid.

When called with one or more additional component names,the components by those names will be removed from the specified entity in the order provided. Deleting a component from an entity is an idempotent process.

check

$bool = $ECS->check( $guid => $component_name );

Takes an entity's GUID and a component name and returns a truthy value if the specified entity has a component by that name, or a falsy value otherwise.

valid

$bool = $ECS->valid($guid);

Takes an entity's GUID and returns a truthy value if the specified entity is valid, or a falsy value otherwise. An entity is valid if it has been created and not yet deleted.

created

$count = $ECS->created;

Returns the number of entities that have been created. Calling clear resets this number.

alive

$count = $ECS->alive;

Returns the number of created entities that are still valid. That is: entities that have been created and not yet deleted.

clear

$ECS->clear;

Resets the internal storage of the registry. Calling this method leaves no trace from the previous state.

sort

$ECS->sort( $component_name => $parent_name      );
$ECS->sort( $component_name => sub { $a ... $b } );

Since version 0.006.

Under normal circumstances, the order in which a particular set of components is stored is not guaranteed, and will depend entirely on the additions and deletions of that component type.

However, it will sometimes be useful to impose an order on a component set. This will be the case for example when renderable components need to be drawn back to front, etc.

This function accommodates this use case.

Given a single component name, and a code reference to a comparator function, the specified component will be sorted accordingly. The comparator function can use the global $a and $b variables as with regular sort, which will be assigned to the two components being sorted.

Alternatively, if given the name of another component (B) instead of a comparator function, the order of the first component (A) will follow that of the B. After this, iterating over the entities that have A will return

  • all of the entities that also have B, according to the order in B
  • all of the entities that do not have B, in no particular order

Sorting a component pool invalidates any cached views that use that component.

The imposed order for this component is guaranteed to be stable as long as no components of this type are added or removed.

view

$view = $ECS->view;
$view = $ECS->view(@component_names);

Takes a set of one or more component names, and returns an internal object representing a view for that specific set of components. Entities in the view are in no specific order, and this order is not guaranteed to remain the same.

As a special case, calling this method with no arguments will generate a view which is guaranteed have all entities that were valid at the moment the view was created.

Once a view has been created, it should remain valid as long as none of the components in the view's set are added or deleted from any entity. Once this is done, the data returned by the view object is no longer guaranteed to be accurate. For this reason, it is not recommended to keep hold of view objects for longer than it takes to run an iteration.

The view object can be used as an array reference to iterate over the pair of entity GUIDs and component sets. When used in this way, each value will be an array reference holding the GUID as the first value, and a nested array reference with the list of components as the second value.

for my $pair ( $ECS->view(@component_names)->@* ) {
    my ( $guid, @components ) = ( $pair->[0], $pair->[1]->@* );
    # Do something with the data
}

The pairs returned when used in this way are returned by calling pairs in List::Util.

Apart from this, the interface for the view object is documented below:

each

$view->each( sub ( $guid, @components ) { ... } );

Takes a code reference which will be called once per entity in the view.

The code reference will be called with the GUID for the current entity as the first parameter, and the components in the requested set in the order they were specified.

If the view was created with no components (in other words, if it's a view of all entities), the list of components passed to the code reference will be empty.

Within the callback, it is safe to add or delete entities, as well as to add or remove components from those entities.

first

my ( $guid, @comps ) = $view->first( sub ( $guid, @comps ) { ... } );

Since version 0.009.

This function is similar to each, with the difference that iteration through the view will stop early the first time the provided coderef returns a true value.

If the coderef ever returns true, a flat list with the GUID and the components in the view will be returned. If it never returns true, this method will return an empty list.

This function is equivalent to the following code

my ( $guid, @comps )
    = map { ( $_[0], @{ $_[1] } ) }
    List::Util::first { $coderef->( $_[0], @{ $_[1] } ) } @{ $view };

entities

@guids = $view->entities;

Returns a list of only the GUIDs of the entities in this view.

components

@components = $view->components;

Returns a list of array references, each of which will hold the list of components for a single entity in the view. The components will be in the order provided when the view was created.

Useful for iterating like

for ( $ECS->view(qw( A B C )) )->components ) {
    my ( $a, $b, $c ) = @$_;
    ...
}

PERFORMANCE

Game::Entities aims to implement a simple entity that is as fast as possible. Specifically, this means that it needs to be fast enough to be used in game development, which is the natural use case for ECS designs.

To this end, the library caches component iterators which are invalidated every time one of the components relevant to that iterator is either added or removed from any entity. This should make the common case of systems operating over sets of components that tend to be relatively stable (eg. across game frames) as fast as possible.

The distribution includes two tests in its extended suite to test the performance with iterations over large number of entities (xt/short-loops.t), and many iterations over small numbers of entities (xt/long-loops.t). Please refer to these files for accurate estimations.

SEE ALSO

  • EnTT

    Much of the design and API of this distribution is based on that of the entity registry in EnTT (famously used in Minecraft). A significant part of the credit for the algorithms and data structures used by Game::Entities falls on the EnTT developers and the blog posts they've made to explain how they work.

COPYRIGHT AND LICENSE

Copyright 2021 José Joaquín Atria

This library is free software; you can redistribute it and/or modify it under the Artistic License 2.0.