shiva::ecs

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shiva::ecs
In this page you will find all the information you need on the ecs part of shiva, the api of the different classes, the game loop architecture.
How do the system works ?
Systems
Shiva has 3 different kinds of systems:
PreUpdate: These systems are the first to be updated in the game loop, they are generally used to retrieve user input, or manage network events for example.
LogicUpdate: These systems are the second to be updated in the game loop, they are generally used for game logic such as movement or collisions for example.
PostUpdate: These systems are the last to be updated in the game loop, they are generally used for rendering or interpolation for example.
The pseudo code look like this:
function update()
{
  user defined;
}
​
function update_systems_type(system_type)
{
  for_each_systems;
  call update();
}
​
function update_systems() 
{
  call update_systems_type(pre_logic);
  while (frame_rate) {
    call update_systems_type(logic);
  }
  call update_systems_type(post_logic);
  return nb_systems_updated
}
​
GameLoop;
while (game_running) {
  call update_systems
}
return game_return_value;
This game loop is based on the gafferon on games tutorial: Fix your timestep.
Diagram
complete diagram
simple diagram
system_type
Description
This file contains an enum representing the different types of systems presented previously, and three strong types, later used in template parameter by the system class.
system_type API
Enum
Typedefs
enum system_type
{
  pre_update = 0,
  logic_update = 1,
  post_update = 2,
  size = 3,
};
using system_pre_update = NamedType<system_type,struct system_pre_update_tag>;
using system_post_update = NamedType<system_type, struct system_post_update_tag>;
using system_logic_update = NamedType<system_type, struct system_logic_update_tag>;
system_manager
Description
This class manage the systems of the entity component system. You are able to add, remove, retrieve , update or delete systems through it.
Diagram
system_manager
system_manager API
Signature
Constructor
Functions
class system_manager;
explicit system_manager(entt::dispatcher &dispatcher,
                        entt::entity_registry &registry,
                        plugins_registry_t &plugins_registry) noexcept;
Parameters
entt::dispatcher The dispatcher will be provided to the system when it is created.
entt::entity_registry The entity_registry will be provided to the system when it is created.
​plugins_registry registry of the plugged systems
Function Name
Description
​update​
update the systems
​get_system​
get a single system
​get_systems​
get multiple systems
​has_system​
check if a system is present
​has_systems​
check if multiple systems are present
​mark_system​
mark a single system
​mark_systems​
mark multiple systems
​enable_system​
enable a single system
​enable_systems​
enable multiple systems
​disable_system​
disable a single system
​disable_systems​
disable multiple systems
​create_system​
create a single system
​load_systems​
create multiple systems
​nb_systems​
get the number of systems
​load_plugins​
load plugins and add it as systems
​get_system_by_name​
get a single system by his name
update
size_t update() noexcept
Return value
Possible Name
Description
nb_systems_updated
size_t 
number of systems successfully updated
Example
size_t nb_systems_updated = system_manager.update();
if (nb_systems_updated != 5) {
    /* Oh no, i was expected 5 systems to be executed in this game loop tick */
}
Notes
This is the function that updates your systems. Based on the logic of the different kinds of shiva systems, this function take care of updating your systems in the right order.
If you have not loaded any system into the system_manager the function return 0.
If you decide to mark a system, it will be automatically deleted at the next loop tick through this function.
get_system
template <typename TSystem>
const TSystem &get_system() const noexcept;
​
template <typename TSystem>
TSystem &get_system() noexcept;
Template parameters
Name
Description
TSystem
TSystem 
represents the type of system to get
Return value
Possible name
Description
render_system, log_system
TSystem &
a reference to the system obtained
const TSystem &
a const reference to the system obtained
Example
//! Non const version
auto& render_system = system_manager.get_system<game::render_system>();
​
//! const version
const auto& log_system = system_manager.get_system<game::log_system>();
get_systems
template <typename ...TSystems>
std::tuple<std::add_lvalue_reference_t<TSystems>...> get_systems() noexcept;
​
template <typename ...TSystems>
std::tuple<std::add_lvalue_reference_t<std::add_const_t<TSystems>>...> get_systems() const noexcept;
Template parameters
Name
Description
TSystems
TSystems
represents a list of systems to get
Return value
Possible name
Description
tuple_systems, [system_foo, system_bar]
Tuple<TSystems &>
Tuple of systems obtained.
Tuple<const TSystems &>
Tuple of systems obtained (const ref)
Example
// Called from a const context
auto[system_foo, system_bar] = system_manager.get_systems<system_foo, system_bar>();
​
// Called from a non const context
auto[system_foo_nc, system_bar_nc] = system_manager.get_systems<system_foo, system_bar>();
​
// Get it as a tuple
auto tuple_systems = system_manager.get_systems<system_foo, system_bar>();
This function recursively calls the get_system function
has_system
template <typename TSystem>
bool has_system() const noexcept;
Template parameters
Name
Description
TSystem
TSystem
represents the system that needs to be verified
Return value
Possible name
Description
result
boolean
true if the system has been loaded
false otherwise
Example
bool result = system_manager.has_system<my_game::render_system>();
if (!result) {
    //! Oh no, i don't have a rendering system.
}
has_systems
template <typename ... TSystems>
bool has_systems() const noexcept
Template parameters
Name
Description
TSystems
TSystems
represents a list of systems that needs to be verified
Return value
Possible name
Description
result
boolean
true if the list of systems has been loaded
false otherwise
Example
bool result = system_manager.has_systems<my_game::render_system,
                                         my_game::audio_system>();
​
if (!result) {
  //! Oh no, atleast one of the systems is not present
}
This function recursively calls the has_system function
mark_system
 template <typename TSystem>
 bool mark_system() noexcept
Template parameters
Name
Description
TSystem
TSystem
represents the system that needs to be marked
Return value
Possible name
Description
result
boolean
true if the system has been marked
false otherwise
Example
bool result = system_manager.mark_system<my_game::render>();
​
if (!result) {
    //! Oh no the system has not been marked.
    //! Did you mark a system that is not present in the system_manager?
}
This function marks a system that will be destroyed at the next tick of the game loop.
mark_systems
 template <typename ... TSystems>
 bool mark_systems() noexcept
Template parameters
Name
Description
TSystems
TSystems
represents a list of systems that needs to be marked
Return value
Possible name
Description
result
boolean
true if the list of systems has been marked
false otherwise
Example
bool result = system_manager.mark_systems<my_game::render, my_game::audio>();
​
if (!result) {
    //! Oh no, atleast one of the system has not been marked.
}
This function recursively calls the mark_system function
enable_system
template <typename TSystem>
bool enable_system() noexcept
Template parameters
Name
Description
TSystem
TSystem
represents the system that needs to be enabled
Return value
Possible name
Description
result
boolean
true if the system has been enabled
false otherwise
Example
bool result = system_manager.enable_system<my_game::render>();
​
if (!result) {
    //! Oh no, this system cannot be enabled.
    //! Did you enable a system that is not present in the system_manager?
}
by default, a system is enabled.
enable_systems
template <typename ... TSystems>
bool enable_systems() noexcept
Template parameters
Name
Description
TSystems
TSystems
represents a list of systems that needs to be enabled
Return value
Possible name
Description
result
boolean
true if the list of systems has been enabled
false otherwise
bool result = system_manager.enable_systems<my_game::render, my_game::audio>();
​
if (!result) {
    //! Oh no, atleast one of the requested systems cannot be enabled.
}
This function recursively calls the enable_system function
disable_system
 template <typename TSystem>
 bool disable_system() noexcept
Template parameters
Name
Description
TSystem
TSystem
represents the system that needs to be disabled
Return value
Possible name
Description
result
boolean
true if the the system has been disabled
false otherwise
Example
bool result = system_manager.disable_system<my_game::render>();
​
if (!result) {
    //! Oh no the system_manager cannot disable this system.
}
If you deactivate a system, it will not be destroyed but simply ignore during the game loop
disable_systems
 template <typename ... TSystems>
 bool disable_systems() noexcept
Template parameters
Name
Description
TSystems
TSystems
represents a list of systems that needs to be disabled
Return value
Possible name
Description
result
boolean
true if the list of systems has been disabled
false otherwise
Example
bool result = system_manager.disable_systems<my_game::render, my_game::audio>();
​
if (!result) {
    //! Oh no, atleast one of the requested systems cannot be disabled.
}
This function recursively calls the disable_system function
create_system
template <typename TSystem, typename ... TSystemArgs>
TSystem &create_system(TSystemArgs &&...args)
Template parameters
Name
Description
TSystem
TSystem
represents the type of system to create
args
TSystemArgs
represents the arguments needed to construct the system to create
Return value
Possible name
Description
render_system
TSystem &
a reference to the created system
Example
auto& render_system = system_manager.create_system<my_game::render>(/* args */);
load_systems
template <typename ...TSystems>
std::tuple<std::add_lvalue_reference_t<TSystems>...> load_systems() noexcept;
Template parameters
Name
Description
TSystems
TSystems
represents a list of systems to be loaded
Return value
Possible name
Description
tuple_systems, [system_foo, system_bar]
Tuple<TSystem &>
Tuple of systems loaded.
Example
// Tuple packed
auto tuple_systems = system_manager.load_systems<system_foo, system_bar>();
​
// Tuple unpacked
auto [system_foo, system_bar] = system_manager.load_systems<system_foo, system_bar>();
nb_systems
size_t nb_systems() const noexcept;
​
size_t nb_systems(system_type sys_type) const noexcept;
Parameters
Name
Description
sys_type
​system_type​
represent the type of systems
Return value
Possible name
Description
nb_systems
size_t
(first overload) number of systems
(second overload) number of systems of a specific type
Example
//! Retrieve the number of systems.
size_t nb_systems = system_manager.nb_systems();
​
//! Retrieve the number of systems by a specific system_type.
size_t nb_systems_logic = system_manager.nb_systems(shiva::ecs::system_type::logic_update);
load_plugins
bool load_plugins() noexcept;
Return value
Possible name
Description
result
boolean
true if all the plugins has been successfully loaded
false otherwise
Example
bool result = system_manager.load_plugins();
​
if (!result) {
    // Oh no, atleast one of the plugins could not be loaded.
}
Notes
This function allow you to load the plugins of the plugins_registry and create systems with the creator function of each plugins.
get_system_by_name
const base_system *get_system_by_name(std::string system_name,
                                      shiva::ecs::system_type type) const noexcept;
                                      
base_system *get_system_by_name(std::string system_name,
                                shiva::ecs::system_type type) noexcept;
Parameters
Name
Description
system_name
string
name of the system to get
type
​system_type​
system_type of the system to get
Return value
Possible name
Description
render_system
base_system *
pointer to the base system which match this name, nullptr otherwise.
Example
base_system* render_system = system_manager.get_system_by_name("render_system", shiva::ecs::system_type::post_update);
if (render_system == nullptr) {
    //! Oh no, could not get the system properly.
}
Notes
This function allow you to get a system by his name, used for get a specific plugin for example.
base_system
This class is an abstract class, it is documented but is present only to make type-erasure of the class system which is templated
This class can be manipulated when using plugins to share data between them.
Description
base class of shiva systems
Diagram
base_system
base_system API
Signature
Constructor
Functions
class base_system;
explicit base_system(entt::dispatcher &dispatcher,
                     entt::entity_registry &entity_registry,
                     const float &fixed_delta_time) noexcept
Function Name
Description
update
Pure virtual function, must be overriden by the client. 
update the system.
get_name
Pure virtual function, must be overriden by the client. 
get the system name.
get_system_type_RTTI
Pure virtual function, override by the system class.
get the system type at runtime (for plugins). 
​mark​
mark the system
​unmark​
unmark the system
​is_marked​
check if the system is marked.
​enable​
enable the system
​disable​
disable the system
​is_enabled​
check if the system is enabled.
​im_a_plugin​
defines the system as a plugin.
​is_a_plugin​
check if the system is a plugin.
​get_user_data​
retrieve a user data previously set by set_user_data
​set_user_data​
set a user data for the system
mark
void mark() noexcept
Example
auto& render_system = system_manager.get_system<my_game::render>();
render_system.mark();
This function marks the system, it will be destroyed in the next tick of the game loop by the system_manager.
unmark
void unmark() noexcept
Example
auto& render_system = system_manager.get_system<my_game::render>();
render_system.unmark();
This function unmark the system, allows the prevention of a destruction in the next tick of the game loop by the system_manager.
is_marked
bool is_marked() const noexcept
Return value
Possible name
Description
result
boolean
true if the system is marked
false otherwise
Example
auto& render_system = system_manager.get_system<my_game::render>();
bool result = render_system.is_marked();
​
if (!result) {
    //! render_system is not marked
}
enable
void enable() noexcept
Example
auto& render_system = system_manager.get_system<my_game::render>();
render_system.enable();
disable
void disable() noexcept
Example
auto& render_system = system_manager.get_system<my_game::render>();
render_system.disable();
Take a look @ disable_system​
is_enabled
bool is_enabled() const noexcept
Return value
Possible name
Description
result
boolean
true if the system is enabled
false otherwise
Example
auto& render_system = system_manager.get_system<my_game::render>();
bool result = render_system.is_enabled();
​
if (!result) {
    //! render_system is not enable
}
im_a_plugin
void im_a_plugin() noexcept
Example
auto& render_system = system_manager.get_system<my_game::render>();
render_system.im_a_plugin();
This function defines the system as a plugin, and therefore use more feature in runtime to work properly
By default this function is called on plugins.
is_a_plugin
bool is_a_plugin() const noexcept
Return value
Possible name
Description
result
boolean
true if the system is a plugin
false otherwise
Example
auto& render_system = system_manager.get_system<my_game::render>();
bool result = render_system.is_a_plugin();
​
if (!result) {
    //! render_system is not a plugin
}
get_user_data
void *get_user_data() noexcept;
Return value
Possible name
Description
data
void *
user data of a system (nullptr by default)
Example
auto render_system = system_manager_.get_system_by_name("render_system", shiva::ecs::system_type::post_update);
auto input_system = system_manager_.get_system_by_name("input_system", shiva::ecs::system_type::pre_update);
input_system->set_user_data(render_system->get_user_data());
This function retrieve a user data previously set by set_user_data​
 by default a user_data is a void pointer equal to nullptr.
set_user_data
void set_user_data(void *data) noexcept;
Parameters
Name
Description
data
void *
a void pointer representing the user data
Example
See the example above.
This function set a user data for this system
This function is very useful to transfer (with get_user_data) data between plugins since they are base_class.
This function wcall on_set_user_data callback at the epilogue, by default on_set_user_data is empty and you need to override it if you need it.
user should be aware here, that's manipulating void pointer is as your own risk.
system
Description
This class is the class that you have to inherit to create your systems