ReactObservableValue

React hooks for ObservableValue: read a value and rerender, select part of it, or follow it with a binding.

Full API reference at jdevon.github.io/react-observable-value.

Install

Install with Wally by adding both packages to your wally.toml:

[dependencies]
ObservableValue = "jdevon/observable-value@0.1.0"
ReactObservableValue = "jdevon/react-observable-value@0.1.0"

ReactObservableValue only provides the hooks; you create values with ObservableValue. Wally installs a single copy of ObservableValue when both versions are compatible, so the hooks and your code share the same class.

Usage

local React = require(ReplicatedStorage.Packages.React)
local ObservableValue = require(ReplicatedStorage.Packages.ObservableValue)
local ReactObservableValue = require(ReplicatedStorage.Packages.ReactObservableValue)

local health = ObservableValue.new(100)

local function HealthLabel()
	-- rerenders whenever health changes
	local currentHealth = ReactObservableValue.useObservableValue(health)

	return React.createElement("TextLabel", {
		Text = `{currentHealth} HP`,
	})
end

health:set(80) -- every mounted HealthLabel now shows 80 HP

The second return value increases every time the value changes. Use it as a dependency when a table value may have been changed in place:

local inventory = ObservableValue.new({} :: { [string]: number })

local function ItemCount()
	local items, updateId = ReactObservableValue.useObservableValue(inventory)

	local total = React.useMemo(function()
		local count = 0
		for _, quantity in items do
			count += quantity
		end
		return count
	end, { updateId })

	return React.createElement("TextLabel", {
		Text = `{total} items`,
	})
end

For values that change often, useObservableBinding follows the value with a React binding instead of rerendering:

local progress = ObservableValue.new(0)

local function ProgressBar()
	local progressBinding = ReactObservableValue.useObservableBinding(progress)

	return React.createElement("Frame", {
		Size = progressBinding:map(function(value)
			return UDim2.fromScale(value, 1)
		end),
	})
end

To rerender only when part of a value changes, useObservableSelector returns what your selector picks out:

local blurStack = ObservableValue.new({} :: { [number]: true })

local function BlurOverlay()
	-- rerenders only when the stack goes from empty to not empty, or back
	local isVisible = ReactObservableValue.useObservableSelector(blurStack, function(stack)
		return next(stack) ~= nil
	end)

	return React.createElement("Frame", {
		Visible = isVisible,
	})
end

Results are compared with ==. Pass an isEqual function as the third argument if your selector builds a new table each time.

API

Each member links to its full entry in the API reference.

Functions

• .useObservableValue - returns an ObservableValue's current value and an update id, and rerenders the component whenever the value changes.
• .useObservableBinding - returns a React binding that follows an ObservableValue without rerendering
• .useObservableSelector - returns part of an ObservableValue and rerenders only when that part changes