completely removed Observers

This change was mostly inspired by this issue, dphfox/Fusion#310

I totally agree with Observers being *completely* redundant, and for the most part unneeded, because most the time when you're creating an Observer it's just to pipe it straight into a `:onBind()` or `:onChange()` call. I've replaced Observers with a function simply called `observe()` which takes the target and the callback as arguments and returns a disconnect function. It adds a set inside the target that holds all of the callbacks and when that target value is evaluate it calls all of those observe tasks, essentially removing the Observer "middleman".

(I know I'm not good at describing these things, you can just look at the changes yourself to see what I mean)

src/Animation/Tween.luau

@@ -7,13 +7,13 @@ local getTweenAlpha = require(script.Parent.getTweenAlpha)
 local getTweenDuration = require(script.Parent.getTweenDuration)
 local lerp = require(script.Parent.lerp)
 
-local Observer = require(package.Graph.Observer)
 local Stopwatch = require(package.Animation.Stopwatch)
 
 local castToState = require(package.State.castToState)
 local depend = require(package.Graph.depend)
 local destructor = require(package.Memory.destructor)
 local evaluate = require(package.Graph.evaluate)
+local observe = require(package.Graph.observe)
 local peek = require(package.State.peek)
 
 local Info = Symbols.named 'Info'
@@ -63,9 +63,7 @@ local function Tween<V>(
 end
 
 function class:onChange(callback)
-	local observer = Observer(self._scope, self)
-
-	return observer:onChange(callback), observer
+	return observe(self, callback)
 end
 
 function class:_evaluate()

src/Graph/evaluate.luau

@@ -42,12 +42,18 @@ local function evaluate(target, forceComputation)
 			targetMeaningfullyChanged = target:_evaluate() or firstEvaluation
 		end
 
+		target[Symbols.Validity] = 'valid'
+
 		if targetMeaningfullyChanged then
+			if target[Symbols.Observers] then
+				for callback in target[Symbols.Observers] do
+					task.spawn(callback, target[Symbols.InternalValue])
+				end
+			end
+
 			target[Symbols.LastChange] = os.clock()
 		end
 
-		target[Symbols.Validity] = 'valid'
-
 		return targetMeaningfullyChanged
 	end
 

src/Graph/observe.luau

@@ -0,0 +1,28 @@
+local package = script.Parent.Parent
+
+local Symbols = require(package.Symbols)
+local Types = require(package.Types)
+
+local castToState = require(package.State.castToState)
+
+local function observe<V>(target: Types.UsedAs<V>, callback: (V) -> ())
+	if not castToState(target) then
+		return
+	end
+
+	local observers = target[Symbols.Observers]
+
+	if observers == nil then
+		observers = {}
+
+		target[Symbols.Observers] = observers
+	end
+
+	observers[callback] = true
+
+	return function()
+		observers[callback] = nil
+	end
+end
+
+return observe

src/Instance/Keys/Attribute.luau

@@ -2,8 +2,9 @@ local package = script.Parent.Parent.Parent
 
 local Types = require(package.Types)
 
-local Observer = require(package.Graph.Observer)
 local castToState = require(package.State.castToState)
+local observe = require(package.Graph.observe)
+local peek = require(package.State.peek)
 
 local keyCache = {}
 
@@ -15,16 +16,18 @@ local function Attribute(attribute: string): Types.SpecialKey
 			kind = 'key',
 			type = 'attribute',
 
-			apply = function(scope: Types.Scope, applyTo: Instance, value: Types.UsedAs<any>)
+			apply = function(
+				_: Types.Scope,
+				applyTo: Instance,
+				value: Types.UsedAs<any>
+			)
 				if castToState(value) then
-					Observer(scope, value):onBind(function(new)
+					observe(value, function(new)
 						applyTo:SetAttribute(attribute, new)
 					end)
-
-					return
 				end
 
-				applyTo:SetAttribute(attribute, value)
+				applyTo:SetAttribute(attribute, peek(value))
 			end,
 		}
 

src/Instance/Keys/Children.luau

@@ -2,17 +2,20 @@ local package = script.Parent.Parent.Parent
 
 local Types = require(package.Types)
 
-local Observer = require(package.Graph.Observer)
-
 local castToState = require(package.State.castToState)
 local doCleanup = require(package.Memory.doCleanup)
+local observe = require(package.Graph.observe)
 local peek = require(package.State.peek)
 
 local Children: Types.SpecialKey = table.freeze {
 	kind = 'key',
 	type = 'children',
 
-	apply = function(scope: Types.Scope, applyTo: Instance, children: Types.Child)
+	apply = function(
+		scope: Types.Scope,
+		applyTo: Instance,
+		children: Types.Child
+	)
 		local newChildren, oldChildren = {}, {}
 		local newScopes, oldScopes = {}, {}
 
@@ -46,7 +49,7 @@ local Children: Types.SpecialKey = table.freeze {
 
 					if not childScope then
 						childScope = {}
-						Observer(childScope, child):onChange(updateChildren)
+						observe(child, updateChildren)
 					else
 						oldScopes[child] = nil
 					end

src/Instance/Keys/Tag.luau

@@ -2,8 +2,8 @@ local package = script.Parent.Parent.Parent
 
 local Types = require(package.Types)
 
-local Observer = require(package.Graph.Observer)
 local castToState = require(package.State.castToState)
+local observe = require(package.Graph.observe)
 local peek = require(package.State.peek)
 
 local keyCache = {}
@@ -16,9 +16,13 @@ local function Tag(tag: string): Types.SpecialKey
 			kind = 'key',
 			type = 'tag',
 
-			apply = function(scope: Types.Scope, instance: Instance, enabled: Types.UsedAs<boolean>)
+			apply = function(
+				_: Types.Scope,
+				instance: Instance,
+				enabled: Types.UsedAs<boolean>
+			)
 				if castToState(enabled) then
-					Observer(scope, enabled):onChange(function(new)
+					observe(enabled, function(new)
 						if new == true then
 							instance:AddTag(tag)
 						elseif instance:HasTag(tag) then

src/Instance/Keys/_Parent.luau

@@ -2,8 +2,9 @@ local package = script.Parent.Parent.Parent
 
 local Types = require(package.Types)
 
-local Observer = require(package.Graph.Observer)
 local castToState = require(package.State.castToState)
+local observe = require(package.Graph.observe)
+local peek = require(package.State.peek)
 
 local function setParent(instance: Instance, parent: Instance?)
 	local success, err = pcall(function()
@@ -20,19 +21,17 @@ local Parent = table.freeze {
 	kind = 'key',
 
 	apply = function(
-		scope: Types.Scope,
+		_: Types.Scope,
 		applyTo: Instance,
 		parent: Types.UsedAs<Instance?>
 	)
 		if castToState(parent) then
-			Observer(scope, parent):onBind(function(new)
+			observe(parent, function(new)
 				setParent(applyTo, new)
 			end)
-
-			return
 		end
 
-		setParent(applyTo, parent)
+		setParent(applyTo, peek(parent))
 	end,
 }
 

src/Instance/applyProperties.luau

@@ -2,10 +2,9 @@ local package = script.Parent.Parent
 
 local Types = require(package.Types)
 
-local Observer = require(package.Graph.Observer)
-
 local Parent = require(package.Instance.Keys._Parent)
 local castToState = require(package.State.castToState)
+local observe = require(package.Graph.observe)
 local peek = require(package.State.peek)
 local sortKeys = require(package.Instance.Keys.sortKeys)
 
@@ -23,20 +22,17 @@ local function setProperty(instance: Instance, property: string, value: any)
 end
 
 local function applyProperty(
-	scope: Types.Scope,
 	instance: Instance,
 	property: string,
 	value: Types.UsedAs<any>
 )
 	if castToState(value) then
-		Observer(scope, value):onBind(function()
-			setProperty(instance, property, peek(value))
+		observe(value, function(new)
+			setProperty(instance, property, new)
 		end)
-
-		return
 	end
 
-	setProperty(instance, property, value)
+	setProperty(instance, property, peek(value))
 end
 
 local function applyProperties(
@@ -55,7 +51,7 @@ local function applyProperties(
 		local keyType = typeof(key)
 
 		if keyType == 'string' then
-			applyProperty(scope, instance, key, value)
+			applyProperty(instance, key, value)
 		elseif kindof(key) == 'key' then
 			table.insert(specialKeys, key)
 		end
@@ -68,7 +64,7 @@ local function applyProperties(
 	end
 
 	if props.Parent then
-		applyProperty(scope, instance, 'Parent', props.Parent)
+		applyProperty(instance, 'Parent', props.Parent)
 	end
 end
 

src/State/Computed.luau

@@ -3,13 +3,12 @@ local package = script.Parent.Parent
 local Symbols = require(package.Symbols)
 local Types = require(package.Types)
 
-local Observer = require(package.Graph.Observer)
-
 local castToState = require(package.State.castToState)
 local depend = require(package.Graph.depend)
 local destructor = require(package.Memory.destructor)
 local evaluate = require(package.Graph.evaluate)
 local isSimilar = require(package.Utility.isSimilar)
+local observe = require(package.Graph.observe)
 local peek = require(package.State.peek)
 
 local Processor = Symbols.named 'Processor'
@@ -44,9 +43,7 @@ local function Computed<V, VO>(
 end
 
 function class:onChange(callback)
-	local observer = Observer(self[Symbols.Scope], self)
-
-	return observer:onChange(callback), observer
+	return observe(self, callback)
 end
 
 function class:map(processor)

src/State/ForPairs.luau

@@ -4,12 +4,12 @@ local Symbols = require(package.Symbols)
 local Types = require(package.Types)
 
 local Computed = require(package.State.Computed)
-local Observer = require(package.Graph.Observer)
 
 local castToState = require(package.State.castToState)
 local depend = require(package.Graph.depend)
 local destructor = require(package.Memory.destructor)
 local evaluate = require(package.Graph.evaluate)
+local observe = require(package.Graph.observe)
 local peek = require(package.State.peek)
 
 local Input = Symbols.named 'Input'
@@ -53,9 +53,7 @@ function class:map(processor)
 end
 
 function class:onChange(callback)
-	local observer = Observer(self[Symbols.Scope], self)
-
-	return observer:onChange(callback), observer
+	return observe(self, callback)
 end
 
 function class:_evaluate()