Premature event listeners in UMG, and why we should try to avoid them in the future.
In UMG, qbuses and evbuses have been a complete godsend.
They have allowed us to create behaviour, and delay the semantics to future systems, so future systems can have more of a say.
Simplified example of this-
Rendering system:
-- rendering mod:
umg.on("@draw", function()
for _, ent in sortedEntities(cameraDimension) do
umg.call("rendering:drawEntity", ent, x, y)
end
end)
And what's super-cool about this, is that we can extend the drawing of entities by simply tagging onto the emitted event:
-- lootplot mod:
umg.on("rendering:drawEntity", function(ent, x, y)
if ent.slot and isHovered(ent) then
drawIndicator(ent, x, y)
end
end)
But wait! Something about this is dumb!
The rendering:drawEntity event is fine...
The issue is actually the @draw event. (For context, @draw is an event that is emitted globally once per frame.)
Why is tagging onto @draw bad?
Well, 2 reasons:
Reason 1: Future systems cannot control as much.
Lets imagine that we had a "lobby/menu" screen; where the player isn't actually in the game; but rather, exists within a menu.
With this, we ideally dont WANT to render the world-entities. But we are forced to, because the system is responding to the @draw event prematurely, and in a global-fashion.
Reason 2:
The game as a whole is slightly harder to understand; since events are sprawled out globally.
A simple alternative:
Just expose the function:
function rendering.drawEntities()
for _, ent in sortedEntities(cameraDimension) do
umg.call("rendering:drawEntity", ent, x, y)
end
end
And then just call it directly whenever, and wherever.
EG:
function GameState:draw()
rendering.drawEntities()
end
And this is amazing too, because we can also apply shaders, do fancy transformations, and do whatever pre/post-processing that we want; since we control the call-site:
function GameState:draw()
applyShader()
doPreProcessing()
rendering.drawEntities()
doPostProcessing()
end
Amazing.
Currently in UMG, most systems are not implemented like this.
But they definitely should be, and it was a mistake not to have them be like this.
Although a big global event tree is minimalistic, elegant, and beautiful at first-sight; yielding extra control to the future-mods is SO VERY IMPORTANT in UMG.