Buses

Event buses and Question buses are the heart of UMG.
Without them, UMG would be worthless.

Event buses:

• Dispatch events with call
• When we dispatch information, we don't care who responds
• Respond to events with on
• (Similar to pub-sub design pattern)

Question buses:

• Request information with ask
• When we gather information, we don't care who gives it
• Provide information with answer
• (Similar to pub-sub design pattern, but in reverse)

Remember that in UMG, we have clientside AND serverside.
Buses are not synced across the network. Both client and server have their own buses.

---

Example usage:

-- `rendering:drawEntity` is an event being emitted by the `rendering` mod.
umg.on("rendering:drawEntity", function(ent)
    -- draw a circle around all drawn entities:
    love.graphics.circle("line", ent.x, ent.y, 50)
end)

Example of question bus usage:

umg.answer("xy:getSpeedMultiplier", function(ent)
    if ent.health and ent.health < 50 then
        -- entities below 50 health move twice as fast!
        return 2
    end
end)

---

Creating our own events:

We can also create our own events:

umg.defineEvent("my_mod:hello")

umg.on("my_mod:hello", function(a,b,c)
    -- creates a function that listens to `hello` events
    print("Hello one: ", a,b,c)
end)


umg.on("my_mod:hello", function(a,b,c)
    -- creates another function that listens to `hello` events
    print("Hello two: ", a,b,c)
end)


-- emits a `hello` event:
umg.call("my_mod:hello", 1, "ent", {})
--[[ 
OUTPUT:

Hello one: 1 "ent" {}
Hello two: 1 "ent" {}
]]

---

Engine events

There are a lot of events emitted by the engine. These events are prexied with @, which means they are special.

For example, @update:
@update is an event called every frame. It's like the game loop.

@update (dt)

-- Example usage:
umg.on("@update", function(dt)
    local p = getPlayer()
    updatePlayer(p, dt)
end)

@update ( dt )   -- called every frame (like love.update)

@tick () -- Represents one tick.
-- Ticks are special, because they are synced between server/client.

@load ()-- called when the mods and entities are done loading


@playerJoin (username) -- called when `username` joins the server
@playerLeave (username) -- called when `username` leaves the server


-- Client-side only:
@draw () -- when stuff should get drawn (like love.draw)
@keypressed ( key, scancode, isrepeat ) -- same as love.keypressed
@mousepressed (x, y, button, istouch, presses) -- same as love.mousepressed

---

Non-engine events:

Here some other example callbacks that are defined by mods:

"rendering:drawEntity" ( ent ) -- an entity is getting drawn
"mortality:entityDeath" ( ent ) -- an entity dies