src/apply.lua

@@ -2,7 +2,10 @@
 -- vide/apply.lua
 ------------------------------------------------------------------------------------------
 
-if not game then script = (require :: any) "test/wrap-require" end
+if not game then
+    script = (require :: any) "test/wrap-require"
+    typeof = require "test/mock".typeof
+end
 
 local graph = require(script.Parent.graph)
 type Node<T> = graph.Node<T>

src/cleanup.lua

@@ -0,0 +1,46 @@
+------------------------------------------------------------------------------------------
+-- vide/cleanup.lua
+------------------------------------------------------------------------------------------
+
+if not game then script = require "test/wrap-require" end
+
+-- todo: verify correct behavior in non-standard usage
+local cleanup_callbacks = {} :: { [string]: () -> () }
+local cleanup_callers = {} :: { [string]: () -> () }
+
+setmetatable(cleanup_callers :: any, { __mode = "vs" })
+
+-- todo: rare case where mem address is reused by another function on same line
+
+local function cleanup(callback: () -> ())
+    local caller = debug.info(2, "f") :: () -> ()
+    local line = debug.info(2, "l") :: number
+    local ref = tostring(caller) .. "\0" .. line
+
+    local fn = cleanup_callbacks[ref]
+    if fn then
+        fn()
+    else
+        cleanup_callers[ref] = caller
+    end
+    cleanup_callbacks[ref] = callback
+end
+
+local buffer = {}
+
+local function clean_garbage()
+    for ref, callback in next, cleanup_callbacks do
+        if cleanup_callers[ref] == nil then -- caller was garbage collected
+            callback()
+            table.insert(buffer, ref)
+        end
+    end
+
+    for _, ref in next, buffer do
+        cleanup_callbacks[ref] = nil
+    end
+
+    table.clear(buffer)
+end
+
+return function() return cleanup, clean_garbage end

src/derive.lua

@@ -9,19 +9,9 @@ local create = graph.create
 local get = graph.get
 local capture_and_link = graph.capture_and_link
 
-local function derive<T>(fn: () -> T, cleanup: (T) -> ()?): () -> T
+local function derive<T>(fn: () -> T): () -> T
     local node = create((nil :: any) :: T)
 
-    if cleanup then
-        local f = fn
-        local last: T? = nil
-        fn = function()
-            if last ~= nil then cleanup(last) end
-            last = f()
-            return last :: T
-        end 
-    end
-
     node.cache = capture_and_link(node, fn)
 
     return function()

src/init.lua

@@ -8,6 +8,7 @@ if not game then script = (require :: any) "test/wrap-require" end
 local create = require(script.create)
 local source = require(script.source)
 local watch = require(script.watch)
+local cleanup, clean_garbage = require(script.cleanup)()
 local derive = require(script.derive)
 local map = require(script.map)
 
@@ -25,9 +26,10 @@ local vide = {
     -- core
     create = create,
     source = source,
+    watch = watch,
+    cleanup = cleanup,
     derive = derive,
     map = map,
-    watch = watch,
 
     -- animations
     spring = spring,
@@ -45,6 +47,7 @@ local vide = {
     -- test
     step = function(dt: number)
         update_springs(dt)
+        clean_garbage()
     end
 }
 

src/spring.lua

@@ -93,14 +93,14 @@ local lerpable: { [string]: Lerp<any> } = {
 local springs: { [SpringData<any>]: Node<any> } = {}
 setmetatable(springs, { __mode = "vs" })
 
-local function spring<T>(target: () -> T, period: number, damping_ratio: number?): () -> T
+local function spring<T>(target: () -> T, period: number?, damping_ratio: number?): () -> T
     local initial_position = target()
     local node = create(initial_position)
 
     local data: SpringData<T> = {
         alpha = 0,
         duration = 0,
-        period = period,
+        period = period or 1,
         damping_ratio = damping_ratio or 1,
         velocity = 0,
         initial_velocity = 0,

src/watch.lua

@@ -9,25 +9,18 @@ local set_effect = graph.set_effect
 local capture = graph.capture
 
 local function watch(effect: () -> ()): () -> ()
-    -- todo: call cleanup on initial debug call when strict
+    local nodes = capture(effect :: () -> nil)
-    local nodes, cleanup = capture(effect :: () -> (() -> ()?), nil)
 
     nodes = table.clone(nodes)
 
-    local function fn()
-        if cleanup then cleanup(); cleanup = nil end
-        cleanup = effect()
-    end
-
     for _, node in next, nodes do
-        set_effect(node, fn, true)  
+        set_effect(node, effect, true)  
     end
 
     local function unwatch()
         for _, node in next, nodes do
-            set_effect(node, fn, nil)    
+            set_effect(node, effect, nil)    
         end
-        if cleanup then cleanup(); cleanup = nil end
     end
 
     return unwatch

test/goodsignal.lua

@@ -1,9 +1,12 @@
+--!nocheck
 -- wrapped task library for use in pure luau
 local task = { spawn = function(thread, ...)
     local ok, err = coroutine.resume(thread, ...)
     if not ok then error(err, 3) end
 end }
 
+export type Type = RBXScriptSignal & { Fire: (Type, ...any)-> () }
+
 ----------------------------------------------------------------------------------------------------
 --               Batched Yield-Safe Signal Implementation                     --
 -- This is a Signal class which has effectively identical behavior to a       --
@@ -106,10 +109,12 @@ setmetatable(Connection, {
 local Signal = {}
 Signal.__index = Signal
 
-function Signal.new()
+Signal.__type = "RBXScriptSignal"
+
+function Signal.new(): Type
 	return setmetatable({
 		_handlerListHead = false,
-	}, Signal)
+	}, Signal) :: any
 end
 
 function Signal:Connect(fn)
@@ -174,14 +179,4 @@ function Signal:Once(fn)
 	return cn
 end
 
--- Make signal strict
-setmetatable(Signal, {
-	__index = function(tb, key)
-		error(("Attempt to get Signal::%s (not a valid member)"):format(tostring(key)), 2)
-	end,
-	__newindex = function(tb, key, value)
-		error(("Attempt to set Signal::%s (not a valid member)"):format(tostring(key)), 2)
-	end
-})
-
 return Signal

test/mock.luau

@@ -1,101 +1,111 @@
 local Instance = {} do
     local Signal = require "test/goodsignal"
+    type Signal = Signal.Type
 
-    type userdata = { __USERDATA__: true }
+    type userdata = { __USERDATA: true }
 
-    type Proxy = {
+    --[[
-        _Userdata: userdata,
+
-        _Data: Data,
+    attempt to mimic roblox engine's method of userdata proxy to actual instance data
+    proxy can gc independantly of actual instance data
+    proxy prevents gc of actual instance data
+    luau code never has direct access to actual instance data, only to proxy
+
+    proxy knows data
+    data does not know proxy
+    separate weak map kept data -> proxy
+    
+    ]]
+
+    type ProxyMT = {
+        proxy: userdata,
+        data: Data,
         __index: any,
         __newindex: any
     }
 
     type Data = {
-        Name: string,
+        name: string,
-        Parent: Data?,
+        parent: Data?,
-        Children: { Data },
+        children: { Data },
-        Changed: { [string]: RBXScriptSignal & { Fire: any } },
+        changed: { [string]: Signal },
-        Table: { [string]: unknown },
+        properties: { [string]: unknown },
-        Destroying: RBXScriptSignal,
+        destroying: Signal,
-        ClassName: string,
+        class: string,
-        Type: "Instance"
+        type: "Instance"
     }
 
-    local function deepclone<T>(template: T & {}): T
+    local function deep_clone<T>(template: T & {}): T
         local t = table.clone(template :: {}) :: {}
 
         for i, v in next, t do
             if type(v) == "table" then
-                t[i] = deepclone(v)
+                t[i] = deep_clone(v)
             end
         end
 
         return t :: T & {}
     end
 
-    --[[
+    
-        attempt to mimic roblox engine's method of userdata proxy to actual instance data
+
-        proxy can gc independantly of actual instance data
-        proxy prevents gc of actual instance data
-        luau code never has direct access to actual instance data, only to proxy
-    ]]
     local proxies = {} :: { [Data]: userdata? }
     setmetatable(proxies :: any, { __mode = "v" })
 
-    local function getdata(userdata: userdata): Data
+    local function get_data(userdata: userdata): Data
-        local function getproxy(userdata: userdata): Proxy
+        local function f(userdata: userdata): ProxyMT
             return getmetatable(userdata :: any)
         end
 
-        return getproxy(userdata)._Data
+        return f(userdata).data
     end
 
-    local function isInstance(value: unknown): boolean
+    local function is_instance(value: unknown): boolean
         local mt = getmetatable(value :: any)
-        return mt and mt._Data and mt._Data.Type == "Instance"
+        return mt and mt.data and mt.data.type == "Instance"
     end
 
     local methods = {}
 
     local function __index(userdata: userdata, property: string): ()
-        local data = getdata(userdata)
+        local data = get_data(userdata)
         return if methods[property] then methods[property]
-            elseif property == "Name" then data.Name
+            elseif property == "Name" then data.name
-            elseif property == "Parent" then data.Parent
+            elseif property == "Parent" then data.parent
-            elseif property == "Destroying" then data.Destroying
+            elseif property == "Destroying" then data.destroying
-            else data.Table[property] 
+            else data.properties[property] 
     end
 
     local function __newindex(userdata: userdata, property: string, value: unknown)
-        local data = getdata(userdata)
+        local data = get_data(userdata)
         if property == "Name" then
-            data.Name = value :: string
+            data.name = value :: string
         elseif property == "Parent" then
-            assert(value == nil or isInstance(value), "attempt to set non-instance as parent")
+            assert(value == nil or is_instance(value), "attempt to set non-instance as parent")
-            local parent = data.Parent
+            local parent = data.parent
             if parent then
-                data.Parent = nil
+                data.parent = nil
-                table.remove(parent.Children, table.find(parent.Children, data))
+                table.remove(parent.children, table.find(parent.children, data))
             end
             if value then
-                data.Parent = getdata(value :: userdata)
+                data.parent = get_data(value :: userdata)
-                table.insert(getdata(value :: userdata).Children, data)
+                table.insert(get_data(value :: userdata).children, data)
             end
         else
-            data.Table[property] = value
+            data.properties[property] = value
         end
 
-        if data.Changed[property] then
+        if data.changed[property] then
-            data.Changed[property]:Fire()
+            data.changed[property]:Fire()
         end
     end
 
-    local function getuserdata(data: Data): userdata
+    local function get_proxy(data: Data): userdata
         return proxies[data] or (function()
             local userdata = newproxy(true)
             local proxy = getmetatable(userdata)
-            proxy._Userdata = userdata
+            proxy.proxy = userdata
-            proxy._Data = data
+            proxy.data = data
             proxy.__index = __index
             proxy.__newindex = __newindex
             proxies[data] = userdata   
@@ -105,29 +115,29 @@ local Instance = {} do
 
     function Instance.new(class: string): Instance
         local data = {
-            Name = "UNNAMED",
+            name = "UNNAMED",
-            Parent = nil,
+            parent = nil,
-            Children = {},
+            children = {},
-            Changed = {},
+            changed = {},
-            Table = {},
+            properties = {},
-            ClassName = class,
+            class = class,
-            Destroying = Signal.new() :: any,
+            destroying = Signal.new() :: any,
-            Type = "Instance" :: "Instance"
+            type = "Instance" :: "Instance"
         }
 
-        return getuserdata(data) :: any
+        return get_proxy(data) :: any
     end
 
-    function Instance.isInstance(value: unknown): boolean
+    function Instance.is_instance(value: unknown): boolean
-        return isInstance(value)
+        return is_instance(value)
     end
 
     function methods.Clone(userdata: userdata): userdata
-        local data = getdata(userdata)
+        local data = get_data(userdata)
         local clone_userdata = (Instance.new("") :: any) :: userdata
-        local clone_data = getdata(clone_userdata)
+        local clone_data = get_data(clone_userdata)
 
-        for i, v in next, deepclone(data) do
+        for i, v in next, deep_clone(data) do
             clone_data[i] = v
         end
 
@@ -135,40 +145,40 @@ local Instance = {} do
     end
 
     function methods.FindFirstChild(userdata: userdata, target: string): userdata?
-        local data = getdata(userdata)
+        local data = get_data(userdata)
-        for _, child in data.Children do
+        for _, child in data.children do
-            if child.Name == target then
+            if child.name == target then
-                return getuserdata(child)
+                return get_proxy(child)
             end
         end
         return nil
     end
 
     function methods.GetChildren(userdata: userdata): { userdata }
-        local children = getdata(userdata).Children
+        local children = get_data(userdata).children
         local userdatas = table.create(#children)
 
         for i, child in next, children do
-            userdatas[i] = getuserdata(child)
+            userdatas[i] = get_proxy(child)
         end
 
         return userdatas
     end
 
     function methods.GetPropertyChangedSignal(userdata: userdata, property: string): RBXScriptSignal
-        local data = getdata(userdata)
+        local data = get_data(userdata)
-        if not data.Changed[property] then
+        if not data.changed[property] then
-            data.Changed[property] = Signal.new() :: any
+            data.changed[property] = Signal.new() :: any
         end
-        return data.Changed[property]
+        return data.changed[property]
     end
 
     function methods.Destroy(userdata: userdata)
-        local data = getdata(userdata);
+        local data = get_data(userdata);
-        (data.Destroying :: any):Fire()
+        data.destroying:Fire()
-        data.Parent = nil
+        data.parent = nil
-        if data.Changed["Parent"] then
+        if data.changed["Parent"] then
-            data.Changed["Parent"]:Fire()
+            data.changed["Parent"]:Fire()
         end
     end
 end
@@ -227,8 +237,10 @@ local Enum = {} :: any do
     end})
 end
 
-local function typeof(v)
+local function typeof(v): string
-    return Instance.isInstance(v) and "Instance" or type(v)
+    return if Instance.is_instance(v) then "Instance"
+        elseif getmetatable(v) and getmetatable(v).__type then getmetatable(v).__type
+        else type(v)
 end
 
 return {

test/syntax.luau

@@ -1,72 +1,43 @@
 local vide = require "src/init"
-local wrap = vide.wrap
+local source = vide.source
 local derive = vide.derive
 local map = vide.map
 local create = vide.create
-local unwrap = vide.unwrap
-local Event = vide.Event
-local Layout = vide.Layout
-
-do
-    local count = wrap(0)
-    local count2 = derive(function(from)
-        local c = from(count) * unwrap(count)
-        return c ^ 2
-    end)
-    local x = count2.value
-
-    local count2b = count + 1
-    local xb = unwrap(count2b)
-end
-
-local count = wrap(0)
-local v = count.value
-
-do
-    local t = map(1, function(i)
-        return ""
-    end)
-
-    local t = map({ true }, function(i, v)
-        return 1
-    end)
-
-    local data = wrap { "" }
-
-    local t = map(data, function(i, v)
-        return 1
-    end)
-end
-
-
-
-create("Frame") {
 
+type Action<T> = {
+    type: T,
+    priority: number,
+    callback: (Instance) -> ()
 }
 
-local Value, Computed, peek, OnEvent = nil :: any, nil :: any, nil :: any, nil :: any
+local function processor(priority: number, fn: (Instance) -> ()): Action<any>
-local New = nil :: any
 
-local function Counter(props)
-    local count, set = wrap(0)
-
-    return create("TextLabel") {
-        Text = "Count: " .. count,
-
-        [Layout] = props[Layout],
-
-        [Event.Activated] = function()
-            set(count + 1)
 end
+
+local function Changed(property: string, callback: () -> ())
+    return processor(1, function(instance)
+        instance:GetPropertyChangedSignal(property):Connect(callback)
+    end) :: Action<"Changed">
+end
+
+local function Cleanup(t: {})
+
+end
+
+function TextInput(p: {
+    OnInput: Action<"Changed">
+})
+    create "TextBox" {
+        Text = "test",
+
+        Changed("Text", function()
+
+        end),
+
+        Cleanup {
+
+        },
+
+        create("Frame") {}
     }
 end
-
-local function Frame(props)
-    return create("Frame") {
-        Position = props.Position
-    }
-end
-
-Frame { Position = UDim2.fromScale(0.5, 0.5) }
-
-Frame { Position = positionState }

test/tests.luau

@@ -363,6 +363,7 @@ end)
 TEST("watch()", function()
     local source = vide.source
     local watch = vide.watch
+    local cleanup = vide.cleanup
 
     do CASE "Capture states"
         local a = source(1)
@@ -405,7 +406,7 @@ TEST("watch()", function()
         local unwatch = watch(function()
             state()
             effect_runcount += 1
-            return function() cleanup_runcount += 1 end
+            cleanup(function() cleanup_runcount += 1 end)
         end)
 
         CHECK(effect_runcount == 1)
@@ -413,7 +414,12 @@ TEST("watch()", function()
         state(2)
         CHECK(effect_runcount == 2)
         CHECK(cleanup_runcount == 1)
+
         unwatch()
+        unwatch = nil :: any
+        gc()
+        vide.step(0)
+
         CHECK(effect_runcount == 2)
         CHECK(cleanup_runcount == 2)
     end
@@ -478,6 +484,117 @@ TEST("watch()", function()
     end
 end)
 
+TEST("cleanup()", function()
+    local source = vide.source
+    local derive = vide.derive
+    local watch = vide.watch
+    local cleanup = vide.cleanup
+
+    do CASE "Cleanup runs for watcher"
+        local state = source(1)
+
+        local watched = 0
+        local cleaned = 0
+
+        local stop = watch(function()
+            state()
+            watched += 1
+            cleanup(function()
+                cleaned += 1  
+            end)
+        end)
+
+        CHECK(watched == 1)
+        CHECK(cleaned == 0)
+
+        state(2)
+
+        CHECK(watched == 2)
+        CHECK(cleaned == 1)
+
+        stop()
+
+        do -- vide detects by iterating through and checking for gc'd refs
+            stop = nil :: any
+            gc()
+            vide.step(0)
+        end
+
+        CHECK(watched == 2)
+        CHECK(cleaned == 2)
+    end
+
+    do CASE "Scoped"
+        local function setup()
+            local state = source(1)
+            local obj = { cleaned = 0 }
+
+            local _stop = watch(function()
+                state()
+                cleanup(function()
+                    obj.cleaned += 1
+                end)
+            end)
+
+            return state, obj
+        end
+
+        local stateA, objA = setup()
+        local stateB, objB = setup()
+
+        CHECK(objA.cleaned == 0)
+        CHECK(objB.cleaned == 0)
+
+        stateA(2)
+
+        CHECK(objA.cleaned == 1)
+        CHECK(objB.cleaned == 0)
+
+        stateB(2)
+
+        CHECK(objA.cleaned == 1)
+        CHECK(objB.cleaned == 1)
+
+        do
+            stateA = nil :: any
+            stateB = nil :: any
+            gc()
+            vide.step(0)
+        end
+
+        CHECK(objA.cleaned == 2)
+        CHECK(objB.cleaned == 2)
+    end
+
+    do CASE "Multiple cleanup"
+        local state = source(1)
+
+        local queue = {}
+    
+        watch(function()
+            state()
+            cleanup(function() table.insert(queue, 1) end)
+            cleanup(function() table.insert(queue, 2) end)
+        end)
+
+        CHECK(testkit.seq(queue, {}))
+        state(2)
+        CHECK(testkit.seq(queue, { 1, 2 }))
+        state(3)
+        CHECK(testkit.seq(queue, { 1, 2, 1, 2 }))
+
+        do
+            state = nil :: any
+            gc()
+            vide.step(0)
+        end
+
+        -- todo: guarantee call order when gc? (currently not)
+        --testkit.print2(queue)
+        --CHECK(testkit.seq(queue, { 1, 2, 1, 2, 1, 2 }))
+    end
+end)
+
 TEST("create()", function()
     local create = vide.create
     local source = vide.source
@@ -894,6 +1011,7 @@ TEST("map()", function()
 end)
 
 TEST("spring()", function()
+    local create = vide.create
     local source = vide.source
     local spring = vide.spring
 
@@ -909,43 +1027,43 @@ TEST("spring()", function()
     end
 
     do CASE "Garbage collection"
-        do -- `spring` should not allow gc of `state`
+        do -- `output` should not allow gc of `input`
-            local state = source(10)
+            local input = source(10)
-            local _springed = spring(state, 1, 1)   
+            local output = spring(input)
 
-            wref.state, state = state, nil :: any
+            local wref = { input }
+            input = nil :: any
 
             gc()
-            CHECK(wref.state)
+            CHECK(wref[1])
-
         end
 
-        do -- `value` should allow gc of `spring`
+        do -- `input` should allow gc of `output`
-            local value = source(10)
+            local input = source(10)
-            local springed = spring(value, 1, 1) :: State?
+            local output = spring(input)
         
-            wref.springed, springed = springed, nil
+            local wref = { output }
+            output = nil :: any
             
             gc()
-            CHECK(not wref.springed)
+            CHECK(not wref[1])
-
         end
     end
 
-    local create = vide.create
 
     do CASE "Garbage collection (binded)"
-        local number = source(10)
+        local input = source(10)
-        local springed = spring(number, 1, 1) :: State?
+        local output = spring(input, 1, 1)
 
         local _label = create "TextLabel" {
-            Text = springed
+            Text = output
         }
 
-        wref.springed, springed = springed, nil
+        local wref = { output }
+        output = nil :: any
 
         gc()
-        CHECK(wref.springed) -- `springed` should not gc
+        CHECK(wref[1]) -- `output` should not gc
     end
 end)
 
@@ -954,9 +1072,11 @@ TEST("Events", function()
 
     local function Thing(props)
         local instance = Instance.new("Thing") :: any
-        instance.Signal = (Signal.new() :: any) :: RBXScriptSignal & { Fire: any }
+        instance.Signal = Signal.new()
-        testkit.print2(getmetatable(instance))
+
-        return create(instance)(props)
+        local clone = create(instance)(props)
+
+        return clone
     end
 
     do CASE "Connect event"
@@ -969,7 +1089,7 @@ TEST("Events", function()
             end
         }
 
-        testkit.print2(getmetatable(val))
+        -- testkit.print2(getmetatable(val))
 
         CHECK(not connected)
         val.Value = 1; val.Signal:Fire(val.Value)
@@ -977,6 +1097,7 @@ TEST("Events", function()
     end
 end)
 
+--[[
 TEST("Changed", function()
     local create = vide.create
     local Changed = vide.Changed
@@ -1023,6 +1144,7 @@ TEST("Changed", function()
         CHECK(Changed.Test == Changed.Test)
     end
 end)
+]]
 
 --[[
 TEST("Created", function()
@@ -1050,7 +1172,6 @@ TEST("strict", function()
     vide.strict = true
 
     local source = vide.source
-    local unsource = vide.unsource
     local derive = vide.derive
     local watch = vide.watch
 
@@ -1058,9 +1179,9 @@ TEST("strict", function()
         local state = source(1)
 
         local ok = pcall(function()
-            local _derived = derive(function(from)
+            local _derived = derive(function()
                 coroutine.yield()
-                return from(state)
+                return state()
             end)
         end)
 
@@ -1071,9 +1192,9 @@ TEST("strict", function()
         local state = source(1)
 
         local ok = pcall(function()
-            local _derived = watch(function(from)
+            local _derived = watch(function()
                 coroutine.yield()
-                local _ = from(state)
+                state()
             end)
         end)
 
@@ -1081,59 +1202,33 @@ TEST("strict", function()
     end
 
     do CASE "Run derived callback twice"
-        local state, set = source(1)
+        local state = source(1)
         local runcount = 0
 
-        local derived = derive(function(from)
+        local _ = derive(function()
             runcount += 1
-            return from(state)
+            return state()
         end)
 
         CHECK(runcount == 2)
-        set(2)
+        state(2)
-        local _ = derived()
         CHECK(runcount == 4)
     end
 
     do CASE "Run watcher callback twice"
-        local state, set = source(1)
+        local state = source(1)
         local runcount = 0
 
-        watch(function(from)
+        watch(function()
             runcount += 1
-            local _ = from(state)
+            state()
         end)
 
         CHECK(runcount == 2)
-        set(2)
+        state(2)
-        vide.step(1/60)
         CHECK(runcount == 4)
     end
 
-    do CASE "Does not allow non-layout properties"
-        local ok = pcall(function()
-            vide.create "Frame" {
-                [vide.Layout] = {
-                    AnchorPoint = Vector2.new(0, 0.5),
-                    BackgroundColor3 = Color3.new(0, 0, 0)
-                }
-            }
-        end)
-        CHECK(not ok)
-    end
-
-    do CASE "Does not allow same table set"
-        local _, set = source()
-
-        local t = {}
-
-        set(t)
-        
-        local ok = pcall(set, t)
-
-        CHECK(not ok)
-    end
-
     -- todo: add case for strict mode bindings
 end)
 

todo.md

@@ -39,6 +39,32 @@ async/loading/suspense
 
 define order with nested properties
 
+```lua
+type Action<T> = {
+    type: T,
+    priority: number,
+    callback: (Instance) -> ()
+}
+
+local function action(priority: number, fn: (Instance) -> ()): Action
+
+end
+
+local function Changed(property: string, callback: () -> ())
+    return action(1, function(instance)
+        instance:GetPropertyChangedSignal(property):Connect(callback)
+    end) :: Action<"Changed">
+end
+
+create "TextBox" {
+    Text = "test",
+
+    Changed "Text" < function(self, data)
+
+    end
+}
+```
+
 ## version 1
 
 ```lua