Entity System Prototype in Erlang

2d_component.erl

-module ('2d_component').
-behaviour(gen_event).

-export([init/1, handle_event/2, terminate/2]).

init({X, Y, Width, Height, Rotation}) ->
  {ok, {X, Y, Width, Height, Rotation}}.

handle_event({change, {x, NewX}}, {OldX, OldY, W, H, Rotation}) ->
	io:format("Entity moved from (~p, ~p) to (~p, ~p).~n", [OldX, OldY, NewX, OldY]),
	{ok, {NewX, OldY, W, H, Rotation}};

handle_event({change, {y, NewY}}, {OldX, OldY, W, H, Rotation}) ->
	io:format("Entity moved from (~p, ~p) to (~p, ~p).~n", [OldX, OldY, OldX, NewY]),
	{ok, {OldX, NewY, W, H, Rotation}};

% The failsafe callback for other events that this component does not care %
handle_event(_, State) ->
	{ok, State}.

terminate(_Args, _State) ->
    ok.

entity.erl

%% This is an entity manager that creates entity, 
%% routes calls to gen_event methods.
-module (entity).
-export ([init/0, add_component/3, remove_component/3, components/1, notify/2]).

%% A Factory Method that generate a Event Manager with a Pid.
%% This Pid is the ID of the entity.
init() ->
	{ok, Pid} = gen_event:start_link(),
	%% TODO: add each one in Components as a handler
	%% Maybe add a specific handler to keep track of the entities?
	{ok, Pid}.

add_component(Pid, Component, Args) ->
	gen_event:add_handler(Pid, Component, Args).

remove_component(Pid, Component, Args) ->
	gen_event:delete_handler(Pid, Component, turn_off).

components(Pid) ->
	gen_event:which_handlers(Pid).

notify(Pid, Event) ->
	gen_event:notify(Pid, Event).

detroy(Pid) ->
	gen_event:stop(Pid).

health_component.erl

-module (health_component).
-behaviour(gen_event).

-export([init/1, handle_event/2, terminate/2]).

init(HP) ->
  {ok, HP}.

handle_event({hit, Damage}, HP) when Damage =< HP ->
	NewHP = HP - Damage,
	io:format("Entity got hit with ~p damage and has ~p HP left.~n", [Damage, NewHP]),
  {ok, NewHP};

handle_event({hit, _Damage}, 0) ->
	io:format("Entity is dead already -.-.~n"),
	{ok, 0};

handle_event({hit, Damage}, _HP) ->
	io:format("Entity took ~p damage and died.~n", [Damage]),
	{ok, 0};

% The failsafe callback for other events that this component does not care %
handle_event(_, State) ->
	{ok, State}.

terminate(_Args, _State) ->
    ok.

output.1

Erlang R15B01 (erts-5.9.1) [64-bit] [smp:4:4] [async-threads:0]

Eshell V5.9.1  (abort with ^G)
1> cd("D:/Repos/erlang").
D:/Repos/erlang
ok
2> c(entity.erl).
entity.erl:17: Warning: variable 'Args' is unused
entity.erl:26: Warning: function detroy/1 is unused
{ok,entity}
3> c(health_component.erl).
health_component.erl:2: Warning: undefined callback function code_change/3 (behaviour 'gen_event')
health_component.erl:2: Warning: undefined callback function handle_call/2 (behaviour 'gen_event')
health_component.erl:2: Warning: undefined callback function handle_info/2 (behaviour 'gen_event')
{ok,health_component}
4> {ok, Entity} = entity:init().
{ok,<0.44.0>}
5> entity:add_component(Entity, health_component, 100).
ok
6> entity:notify(Entity, {hit, 10}).
Entity got hit with 10 damage and has 90 HP left.
ok
7> entity:notify(Entity, {hit, 95}).
Entity took 95 damage and died.
ok
8> entity:notify(Entity, {hit, 15}).
Entity is dead already -.-.
ok

output.2

9> c("2d_component.erl").
2d_component.erl:2: Warning: undefined callback function code_change/3 (behaviour 'gen_event')
2d_component.erl:2: Warning: undefined callback function handle_call/2 (behaviour 'gen_event')
2d_component.erl:2: Warning: undefined callback function handle_info/2 (behaviour 'gen_event')
{ok,'2d_component'}
10> entity:add_component(Entity, '2d_component', {10, 15, 20, 25, 0}).
ok
11> entity:notify(Entity, {change, {x, 20}}).
Entity moved from (10, 15) to (20, 15).
ok
12> entity:notify(Entity, {change, {y, 100}}).
Entity moved from (20, 15) to (20, 100).
ok