A Filter is a function that selects which Entities apply to a System. Filters take two parameters, the System and the Entity, and return a boolean value indicating if the Entity should be processed by the System. A truthy value includes the entity, while a falsey (nil or false) value excludes the entity.

Filters must be added to Systems by setting the filter field of the System. Filter's returned by tiny-ecs's Filter functions are immutable and can be used by multiple Systems.

local f1 = tiny.requireAll("position", "velocity", "size")
local f2 = tiny.requireAny("position", "velocity", "size")

local e1 = {
    position = {2, 3},
    velocity = {3, 3},
    size = {4, 4}
}

local entity2 = {
    position = {4, 5},
    size = {4, 4}
}

local e3 = {
    position = {2, 3},
    velocity = {3, 3}
}

print(f1(nil, e1), f1(nil, e2), f1(nil, e3)) -- prints true, false, false
print(f2(nil, e1), f2(nil, e2), f2(nil, e3)) -- prints true, true, true

Filters can also be passed as arguments to other Filter constructors. This is a powerful way to create complex, custom Filters that select a very specific set of Entities.

-- Selects Entities with an "image" Component, but not Entities with a
-- "Player" or "Enemy" Component.
filter = tiny.requireAll("image", tiny.rejectAny("Player", "Enemy"))

A System is a wrapper around function callbacks for manipulating Entities. Systems are implemented as tables that contain at least one method; an update function that takes parameters like so:

• function system:update(dt).

There are also a few other optional callbacks:

• function system:filter(entity) - Returns true if this System should include this Entity, otherwise should return false. If this isn't specified, no Entities are included in the System.
• function system:onAdd(entity) - Called when an Entity is added to the System.
• function system:onRemove(entity) - Called when an Entity is removed from the System.
• function system:onModify(dt) - Called when the System is modified by adding or removing Entities from the System.
• function system:onAddToWorld(world) - Called when the System is added to the World, before any entities are added to the system.
• function system:onRemoveFromWorld(world) - Called when the System is removed from the world, after all Entities are removed from the System.
• function system:preWrap(dt) - Called on each system before update is called on any system.
• function system:postWrap(dt) - Called on each system in reverse order after update is called on each system. The idea behind preWrap and postWrap is to allow for systems that modify the behavior of other systems. Say there is a DrawingSystem, which draws sprites to the screen, and a PostProcessingSystem, that adds some blur and bloom effects. In the preWrap method of the PostProcessingSystem, the System could set the drawing target for the DrawingSystem to a special buffer instead the screen. In the postWrap method, the PostProcessingSystem could then modify the buffer and render it to the screen. In this setup, the PostProcessingSystem would be added to the World after the drawingSystem (A similar but less flexible behavior could be accomplished with a single custom update function in the DrawingSystem).

For Filters, it is convenient to use tiny.requireAll or tiny.requireAny, but one can write their own filters as well. Set the Filter of a System like so:

system.filter = tiny.requireAll("a", "b", "c")

or

function system:filter(entity)
    return entity.myRequiredComponentName ~= nil
end

All Systems also have a few important fields that are initialized when the system is added to the World. A few are important, and few should be less commonly used.

• The world field points to the World that the System belongs to. Useful for adding and removing Entities from the world dynamically via the System.
• The active flag is whether or not the System is updated automatically. Inactive Systems should be updated manually or not at all via system:update(dt). Defaults to true.
• The entities field is an ordered list of Entities in the System. This list can be used to quickly iterate through all Entities in a System.
• The interval field is an optional field that makes Systems update at certain intervals using buffered time, regardless of World update frequency. For example, to make a System update once a second, set the System's interval to 1.
• The index field is the System's index in the World. Lower indexed Systems are processed before higher indices. The index is a read only field; to set the index, use tiny.setSystemIndex(world, system).
• The indices field is a table of Entity keys to their indices in the entities list. Most Systems can ignore this.
• The modified flag is an indicator if the System has been modified in the last update. If so, the onModify callback will be called on the System in the next update, if it has one. This is usually managed by tiny-ecs, so users should mostly ignore this, too.

There is another option to (hopefully) increase performance in systems that have items added to or removed from them often, and have lots of entities in them. Setting the nocache field of the system might improve performance. It is still experimental. There are some restriction to systems without caching, however.

• There is no entities table.
• Callbacks such onAdd, onRemove, and onModify will never be called
• Noncached systems cannot be sorted (There is no entities list to sort).

A World is a container that manages Entities and Systems. Typically, a program uses one World at a time.

For all World functions except tiny.world(...), object-oriented syntax can be used instead of the documented syntax. For example, tiny.add(world, e1, e2, e3) is the same as world:add(e1, e2, e3).