statemonitor: to register callbacks to state

It takes two functions; one is to check the state and the other is a
callback to execute if the first function evaluates true. It can
automatically handling deregistering functions once they are no longer
needed

statemonitor.lua

@@ -0,0 +1,43 @@
+local statemonitor={}
+
+local callbacks={}
+
+local function ternary(val, t, f)
+	if val then return t else return f end
+end
+
+--- Deregister a callback
+---@param name string Name of callback
+function statemonitor.deregister(name)
+	callbacks[name] = nil
+end
+
+--- Register a callback to a function
+--Registers a callback to run when a check funciton evaluates to true
+---@param name string Name of callback
+---@param check function Function to evaluate
+---@param callback function Callback to run when true
+---@param frequency? integer Frequency to run the check, in ticks (default 5)
+---@param once? boolean Whether the callback should be deregistered after running once, default true
+function statemonitor.register(name, check, callback, frequency, once)
+	frequency=frequency or 5
+	once=ternary(once ~= nil, once, true)
+	callbacks[name] = {check=check, callback=callback, f=frequency,
+	deregister=once}
+end
+
+local function tick()
+	for k, v in pairs(callbacks) do
+		if world.getTimeOfDay() % v.f and v.check() then
+			v.callback()
+			if v.deregister then
+				statemonitor.deregister(k)
+			end
+		end
+	end
+end
+
+events.TICK:register(tick, "nulllib.statemonitor")
+
+
+return statemonitor