onejs-react

React 19 reconciler for Unity's UI Toolkit.

Files

File	Purpose
src/host-config.ts	React reconciler implementation (createInstance, commitUpdate, etc.)
src/renderer.ts	Entry point: render(element, container)
src/components.tsx	Component wrappers: View, Text, Label, Button, TextField, etc.
src/screen.tsx	Responsive design: ScreenProvider, useBreakpoint, useScreenSize, useResponsive
src/types.ts	TypeScript type definitions (includes Vector Drawing types)
src/index.ts	Package exports

Components

Component	UI Toolkit Element	Description
View	VisualElement	Container element
Text	TextElement	Primary text display
Label	Label	Form labels, semantic labeling
Button	Button	Interactive button
TextField	TextField	Text input
Toggle	Toggle	Checkbox/toggle
Slider	Slider	Numeric slider
ScrollView	ScrollView	Scrollable container
Image	Image	Image display
ListView	ListView	Virtualized list

Raw text in JSX (e.g., <View>Hello</View>) creates a TextElement, providing semantic distinction from explicit <Label> components.

Usage

import { render, View, Text, Label, Button } from 'onejs-react';

function App() {
    return (
        <View style={{ padding: 20 }}>
            <Text text="Welcome!" style={{ fontSize: 24 }} />
            <Button text="Click me" onClick={() => console.log('clicked')} />
            <View>Raw text also works</View>
        </View>
    );
}

render(<App />, __root);

Type Usage Guide

OneJS has multiple type sources. Here's when to use each:

React Components (Most Common)

Import types from onejs-react for refs and component props:

import { View, Button, VisualElement, ButtonElement } from "onejs-react"

function MyComponent() {
    const viewRef = useRef<VisualElement>(null)
    const buttonRef = useRef<ButtonElement>(null)

    useEffect(() => {
        buttonRef.current?.Focus()
    }, [])

    return (
        <View ref={viewRef}>
            <Button ref={buttonRef} text="Click me" />
        </View>
    )
}

Imperative Element Creation

For creating elements outside React, use unity-types:

import { Button } from "UnityEngine.UIElements"

const btn = new Button()
btn.text = "Dynamic Button"
__root.Add(btn)

render() Container

The render() function accepts any RenderContainer:

import { render, RenderContainer } from "onejs-react"

// __root is provided by the runtime
render(<App />, __root)

Type Hierarchy

RenderContainer         (minimal: __csHandle, __csType)
    └── VisualElement   (full API: style, hierarchy, events)
        ├── TextElement (+ text property)
        │   ├── LabelElement
        │   └── ButtonElement
        ├── TextFieldElement (+ value, isPasswordField, etc.)
        ├── ToggleElement    (+ value: boolean)
        ├── SliderElement    (+ value, lowValue, highValue)
        └── ScrollViewElement (+ scrollOffset, ScrollTo)

Key Concepts

• Element types: Use ojs- prefix internally (e.g., ojs-view, ojs-button) to avoid conflicts with HTML types
• Style shorthands: padding/margin are expanded to individual properties (UI Toolkit requirement)
• Style cleanup: When props change, removed style properties are cleared (not just new ones applied)
• className updates: Selective add/remove of classes (not full clear + reapply)
• Event handlers: Registered via __eventAPI from QuickJSBootstrap.js
• Instance structure: { element, type, props, eventHandlers: Map, appliedStyleKeys: Set }

Build & Test

npm run typecheck  # TypeScript check (no build output - consumed directly by App)
npm test           # Run test suite
npm run test:watch # Run tests in watch mode

Testing

Test suite uses Vitest with mocked Unity CS globals. Tests are in src/__tests__/:

File	Coverage
host-config.test.ts	Instance creation, style/className management, events, children
renderer.test.tsx	Integration tests: render(), unmount(), React state, effects
components.test.tsx	Component wrappers, prop passing, event mapping
mocks.ts	Mock implementations of Unity UI Toolkit classes
setup.ts	Global test setup for CS, __eventAPI

Vector Drawing

OneJS exposes Unity's Painter2D API for GPU-accelerated vector graphics. Any element can render custom vector content via onGenerateVisualContent.

Basic Usage

import { View, render } from "onejs-react"

function Circle() {
    return (
        <View
            style={{ width: 200, height: 200, backgroundColor: "#333" }}
            onGenerateVisualContent={(mgc) => {
                const p = mgc.painter2D

                // Draw a filled circle
                p.fillColor = new CS.UnityEngine.Color(1, 0, 0, 1) // Red
                p.BeginPath()
                p.Arc(
                    new CS.UnityEngine.Vector2(100, 100), // center
                    80,                                   // radius
                    CS.UnityEngine.UIElements.Angle.Degrees(0),
                    CS.UnityEngine.UIElements.Angle.Degrees(360),
                    CS.UnityEngine.UIElements.ArcDirection.Clockwise
                )
                p.Fill(CS.UnityEngine.UIElements.FillRule.NonZero)
            }}
        />
    )
}

Painter2D Methods

Path operations:

• BeginPath() - Start a new path
• ClosePath() - Close the current subpath
• MoveTo(point) - Move to point without drawing
• LineTo(point) - Draw line to point
• Arc(center, radius, startAngle, endAngle, direction) - Draw arc
• ArcTo(p1, p2, radius) - Draw arc tangent to two lines
• BezierCurveTo(cp1, cp2, end) - Cubic bezier curve
• QuadraticCurveTo(cp, end) - Quadratic bezier curve

Rendering:

• Fill(fillRule) - Fill the current path
• Stroke() - Stroke the current path

Properties:

• fillColor - Fill color (Unity Color)
• strokeColor - Stroke color (Unity Color)
• lineWidth - Stroke width in pixels
• lineCap - Line cap style (Butt, Round, Square)
• lineJoin - Line join style (Miter, Round, Bevel)

Triggering Repaints

Use MarkDirtyRepaint() to trigger a repaint when drawing state changes:

function AnimatedCircle() {
    const ref = useRef<VisualElement>(null)
    const [radius, setRadius] = useState(50)

    useEffect(() => {
        // Trigger repaint when radius changes
        ref.current?.MarkDirtyRepaint()
    }, [radius])

    return (
        <View
            ref={ref}
            style={{ width: 200, height: 200 }}
            onGenerateVisualContent={(mgc) => {
                const p = mgc.painter2D
                p.fillColor = new CS.UnityEngine.Color(0, 0.5, 1, 1)
                p.BeginPath()
                p.Arc(
                    new CS.UnityEngine.Vector2(100, 100),
                    radius,
                    CS.UnityEngine.UIElements.Angle.Degrees(0),
                    CS.UnityEngine.UIElements.Angle.Degrees(360),
                    CS.UnityEngine.UIElements.ArcDirection.Clockwise
                )
                p.Fill(CS.UnityEngine.UIElements.FillRule.NonZero)
            }}
        />
    )
}

Differences from HTML5 Canvas

Feature	Unity Painter2D	HTML5 Canvas
Transforms	Manual point calculation	Built-in translate/rotate/scale
Gradients	Limited (strokeGradient)	Full linear/radial/conic
State Stack	Not built-in	save()/restore()
Text	Via MeshGenerationContext.DrawText()	fillText/strokeText
Shadows	Not available	shadowBlur, shadowColor
Clipping	Via nested VisualElements	clip() path-based

Types

The following types are re-exported from unity-types:

type Vector2 = CS.UnityEngine.Vector2
type Color = CS.UnityEngine.Color
type Angle = CS.UnityEngine.UIElements.Angle
type ArcDirection = CS.UnityEngine.UIElements.ArcDirection
type Painter2D = CS.UnityEngine.UIElements.Painter2D
type MeshGenerationContext = CS.UnityEngine.UIElements.MeshGenerationContext
type GenerateVisualContentCallback = (context: MeshGenerationContext) => void

C# Interop Utilities

toArray<T>(collection): T[]

Converts C# collections (List<T>, arrays) to JavaScript arrays. C# collections exposed through the OneJS proxy have .Count/.Length and indexers but lack .map(), .filter(), and other array methods.

import { toArray } from "onejs-react"

// Convert a C# List for use in JSX
{toArray<Item>(inventory.Items).map(item => <ItemView key={item.Id} item={item} />)}

// Convert a C# array
const resolutions = toArray<Resolution>(Screen.resolutions)

// Safe with null — returns []
const npcs = toArray(currentPlace?.NPCs)

Supports objects with .Count (List, IList) or .Length (C# arrays). Returns [] for null/undefined.

Dependencies

• react-reconciler@0.31.x (React 19 compatible)
• vitest (dev) - Test runner
• Peer: react@18.x || 19.x