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Values are objects which store single values. You can read from them with :get(), and write to them with :set().

local health = Value(100)

print(health:get()) --> 100
print(health:get()) --> 25


To use Value in your code, you first need to import it from the Fusion module, so that you can refer to it by name:

local Fusion = require(ReplicatedStorage.Fusion)
local Value = Fusion.Value

To create a new value, call the Value function:

local health = Value() -- this will create and return a new Value object

By default, new Value objects store nil. If you want the Value object to start with a different value, you can provide one:

local health = Value(100) -- the Value will initially store a value of 100

You can retrieve the currently stored value at any time with :get():

print(health:get()) --> 100

You can also set the stored value at any time with :set():

print(health:get()) --> 25

Why Objects?

The Problem

Imagine some UI in your head. Think about what it looks like, and think about the different variables it's showing to you.

An example of a game's UI, with some variables labelled and linked to parts of the UI. An example of a game's UI, with some variables labelled and linked to parts of the UI.

Screenshot: GameUIDatabase (Halo Infinite)

Your UIs are usually driven by a few internal variables. When those variables change, you want your UI to reflect those changes.

Unfortunately, there's no way to listen for those changes in Lua. When you change those variables, it's normally your responsibility to figure out what needs to update, and to send out those updates.

Over time, we've come up with many methods of dealing with this inconvenience. Perhaps the simplest are 'setter functions', like these:

local ammo = 100

local function setAmmo(newAmmo)
    ammo = newAmmo
    -- you need to send out updates to every piece of code using `ammo` here

But this is clunky and unreliable; what if there's another piece of code using ammo that we've forgotten to update here? How can you guarantee we've covered everything? Moreover, why is the code setting the ammo even concerned with who uses it?

Building Better Variables

In an ideal world, anyone using ammo should be able to listen for changes, and get notified when someone sets it to a new value.

To make this work, we need to fundamentally extend what variables can do. In particular, we need two additional features:

  • We need to save a list of dependents - other places currently using our variable. This is so we know who to notify when the value changes.
  • We need to run some code when the variable is set to a new value. If we can do that, then we can go through the list and notify everyone.

To solve this, Fusion introduces the idea of a 'state object'. These are objects that represent a single value, which you can :get() at any time. They also keep a list of dependents; when the object's value changes, it can notify everyone so they can respond to the change.

Value is one such state object. It's specifically designed to act like a variable, so it has an extra :set() method. Using that method, you can change the object's value manually. If you set it to a different value than before, it'll notify anyone using the object.

This means you can use Value objects like variables, with the added benefit of being able to listen to changes like we wanted!

Sharing Variables

There is another benefit to using objects too; you can easily share your objects directly with other code. Every usage of that object will refer to the same underlying value:

-- someObject is a `Value` object
local function printValue(someObject)

local health = Value(100)
printValue(health) --> 100

local myDogsName = Value("Dan")
printValue(myDogsName) --> Dan

This is something that normal variables can't do by default, and is a benefit exclusive to state objects.

In the above code, printValue can operate on any arbitrary variable without knowing what it is, or where it comes from. This is very useful for writing generic, reusable code, and you'll see it used a lot throughout Fusion.

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