@tldraw/state

@tldraw/state

@tldraw/state is a powerful and lightweight TypeScript library for managing reactive state using signals. It provides fine-grained reactive primitives that automatically track dependencies and efficiently update only what needs to change.

@tldraw/state powers the reactive system at the heart of tldraw, handling everything from canvas updates to collaborative state synchronization. It's designed to work seamlessly with @tldraw/store and has optional React bindings.

Why @tldraw/state?

• Fine-grained reactivity - Only re-runs computations when their actual dependencies change
• High performance - Lazy evaluation and efficient dependency tracking
• Automatic updates - Derived values and side effects update automatically
• Time travel - Built-in history tracking and transactions with rollback support
• Framework agnostic - Works with any JavaScript framework or vanilla JS
• TypeScript first - Excellent type safety with full TypeScript support

Perfect for building reactive UIs, real-time collaborative apps, and complex state machines where performance and predictability matter.

Installation

npm install @tldraw/state

Quick Start

import { atom, computed, react } from '@tldraw/state'

// Create reactive state
const name = atom('name', 'World')
const count = atom('count', 0)

// Derive values automatically
const greeting = computed('greeting', () => {
	return `Hello, ${name.get()}! Count: ${count.get()}`
})

// React to changes
react('logger', () => {
	console.log(greeting.get())
})
// Logs: "Hello, World! Count: 0"

// Update state - reactions run automatically
name.set('tldraw')
// Logs: "Hello, tldraw! Count: 0"

count.set(42)
// Logs: "Hello, tldraw! Count: 42"

Core Concepts

Atoms - State Containers

Atoms hold raw values and are the foundation of your reactive state:

import { atom } from '@tldraw/state'

// Create atoms with initial values
const user = atom('user', { name: 'Alice', age: 30 })
const theme = atom('theme', 'light')

// Read values
console.log(user.get().name) // 'Alice'

// Update values
user.update((current) => ({ ...current, age: 31 }))
theme.set('dark')

Computed Values - Automatic Derivation

Computed signals derive their values from other signals and update automatically:

import { computed } from '@tldraw/state'

const firstName = atom('firstName', 'John')
const lastName = atom('lastName', 'Doe')

const fullName = computed('fullName', () => {
	return `${firstName.get()} ${lastName.get()}`
})

console.log(fullName.get()) // "John Doe"

firstName.set('Jane')
console.log(fullName.get()) // "Jane Doe" - automatically updated!

Reactions - Side Effects

Reactions run side effects when their dependencies change:

import { react } from '@tldraw/state'

const selectedId = atom('selectedId', null)

// Update UI when selection changes
const stop = react('update-selection-ui', () => {
	const id = selectedId.get()
	document.getElementById('selected').textContent = id || 'None'
})

selectedId.set('shape-123')
// UI automatically updates

// Clean up when no longer needed
stop()

Transactions - Batched Updates

Batch multiple updates to prevent intermediate reactions:

import { transact } from '@tldraw/state'

const x = atom('x', 0)
const y = atom('y', 0)

const position = computed('position', () => `(${x.get()}, ${y.get()})`)

react('log-position', () => console.log(position.get()))
// Logs: "(0, 0)"

transact(() => {
	x.set(10)
	y.set(20)
	// Reaction runs only once after transaction
})
// Logs: "(10, 20)"

Advanced Features

History & Time Travel

Track changes over time for undo/redo functionality:

const canvas = atom(
	'canvas',
	{ shapes: [] },
	{
		historyLength: 100,
		computeDiff: (prev, next) => ({ prev, next }),
	}
)

// Make changes...
canvas.update((state) => ({ shapes: [...state.shapes, newShape] }))

// Get diffs since a point in time
const startTime = getGlobalEpoch()
// ... make more changes ...
const diffs = canvas.getDiffSince(startTime)

Performance Optimization

Use unsafe__withoutCapture to read values without creating dependencies:

const expensiveComputed = computed('expensive', () => {
	const important = importantValue.get()

	// Read this without making it a dependency
	const metadata = unsafe__withoutCapture(() => metadataAtom.get())

	return computeExpensiveValue(important, metadata)
})

Debugging

Use whyAmIRunning() to understand what triggered an update:

react('debug-reaction', () => {
	whyAmIRunning() // Logs dependency tree to console
	// Your reaction code...
})

Integration Examples

With tldraw SDK

// In a tldraw application
const editor = useEditor()

// Create reactive state that works with tldraw
const selectedShapes = computed('selectedShapes', () => {
	return editor.getSelectedShapeIds().map((id) => editor.getShape(id))
})

// React to selection changes
react('update-property-panel', () => {
	const shapes = selectedShapes.get()
	updatePropertyPanel(shapes)
})

With React

Install the React bindings:

npm install @tldraw/state-react

import { useAtom, useComputed } from '@tldraw/state-react'

function Counter() {
	const [count, setCount] = useAtom(countAtom)
	const doubled = useComputed(() => count * 2, [count])

	return (
		<div>
			<p>Count: {count}</p>
			<p>Doubled: {doubled}</p>
			<button onClick={() => setCount(count + 1)}>+</button>
		</div>
	)
}

API Reference

For complete API documentation, see DOCS.md.

Core Functions

• atom(name, initialValue, options?) - Create a reactive state container
• computed(name, computeFn, options?) - Create a derived value
• react(name, effectFn, options?) - Create a side effect
• transact(fn) - Batch state updates

Class-based APIs

• @computed - Decorator for computed class properties

Advanced

• reactor(name, effectFn) - Create a controllable reaction
• unsafe__withoutCapture(fn) - Read state without creating dependencies
• whyAmIRunning() - Debug what triggered an update
• getComputedInstance(obj, prop) - Get underlying computed instance
• getGlobalEpoch() - Get current time for history tracking

Related Packages

• @tldraw/state-react - React bindings for @tldraw/state
• @tldraw/store - Record storage built on @tldraw/state
• @tldraw/editor - The tldraw canvas editor
• @tldraw/tldraw - Complete tldraw UI components

Examples & Patterns

Looking for more examples? Check out:

• tldraw SDK examples - Real-world usage in tldraw applications

Contributing

Please see our contributing guide. Found a bug? Please submit an issue.

License

This project is licensed under the MIT License found here. The tldraw SDK is provided under the tldraw license.

Trademarks

Copyright (c) 2024-present tldraw Inc. The tldraw name and logo are trademarks of tldraw. Please see our trademark guidelines for info on acceptable usage.

Contact

Find us on Twitter/X at @tldraw. You can contact us by email at [e-mail].

Community

Have questions, comments or feedback? Join our discord. For the latest news and release notes, visit tldraw.dev.