No way to get signal that fires once a collection of components are present

[Nimblz]

A common pattern I've dropped into when making systems is listening and checking for multiple components being added. An example of this is an animation system, which requires access to the AI and Rig components of an entity. To do this right now, you have to listen to the addition of all components you need to reason with, then check that all components exist in a component added method.

function AnimationSystem:onDependencyAdded(instance)
    local rig = self.core:getComponent(instance, RecsComponents.Rig)
    local ai = self.core:getComponent(instance, RecsComponents.AI)

    if rig and ai then
        -- do stuff
    end
end

function AnimationSystem:init()
    for instance in self.core:components(RecsComponents.Rig, RecsComponents.AI) do
        self:onDependencyAdded(instance)
    end

    self.core:getComponentAddedSignal(RecsComponents.Rig):connect(function(instance)
        self:onDependencyAdded(instance)
    end)
    self.core:getComponentAddedSignal(RecsComponents.AI):connect(function(instance)
        self:onDependencyAdded(instance)
    end)
end

Some equivalent to getComponentAddedSignal(identifier) that accepts multiple identifiers and only fires when all exist would clean this pattern up.