Schema Rendering

Object UI's schema rendering system is the core mechanism that transforms JSON configurations into live React components. This guide explains how it works and how to use it effectively.

Overview

The schema rendering engine follows a simple principle:

JSON Schema → SchemaRenderer → React Components → Beautiful UI

Every visual element in Object UI starts as a JSON object that describes what should be rendered, not how it should be rendered.

The SchemaRenderer Component

The SchemaRenderer is the primary component that interprets your JSON schemas:

import { SchemaRenderer } from '@object-ui/react'
import { initializeComponents } from '@object-ui/components'
// Side-effect import: loading the package runs its own field registration.
import '@object-ui/fields'

// Register components once at app initialization
initializeComponents()

function App() {
  const schema = {
    type: "page",
    title: "My Dashboard",
    body: { type: "text", content: "Hello" }
  }
  
  return <SchemaRenderer schema={schema} />
}

Schema Structure

Every schema object must have at minimum a type field:

import type { CSSProperties } from 'react'

interface BaseSchema {
  type: string           // Component type identifier
  id?: string           // Optional unique identifier
  className?: string    // Tailwind CSS classes
  style?: CSSProperties // Inline styles (use sparingly)
  visibleOn?: string    // Expression for conditional visibility
  hiddenOn?: string     // Expression for conditional hiding
  disabledOn?: string   // Expression for conditional disabling
}

Example Schema

{
  "type": "card",
  "id": "stats-card",
  "className": "p-6 shadow-lg",
  "title": "User Statistics",
  "visibleOn": "${user.role === 'admin'}",
  "body": {
    "type": "text",
    "content": "Total Users: ${stats.totalUsers}"
  }
}

Data Context

The SchemaRenderer accepts a data prop that provides context for expressions:

const data = {
  user: { name: "John", role: "admin" },
  stats: { totalUsers: 1234 }
}

<SchemaRenderer schema={schema} data={data} />

Accessing Data in Schemas

Use expression syntax ${} to reference data:

{
  "type": "text",
  "content": "Welcome, ${user.name}!"
}

Component Registry

The schema renderer uses a component registry to map schema types to React components:

import { ComponentRegistry } from '@object-ui/core'

// `ComponentRegistry` is a process-level singleton — import it, do not construct one.
// Register a custom component
ComponentRegistry.register('my-component', MyComponent)

// Now you can use it in schemas
const schema = {
  type: "my-component",
  // ... component props
}

Nested Schemas

Schemas can be nested to create complex UIs:

{
  "type": "page",
  "title": "Dashboard",
  "body": {
    "type": "grid",
    "columns": 2,
    "items": [
      {
        "type": "card",
        "title": "Card 1",
        "body": {
          "type": "text",
          "content": "Nested content"
        }
      },
      {
        "type": "card",
        "title": "Card 2",
        "body": {
          "type": "chart",
          "chartType": "bar",
          "data": "${chartData}"
        }
      }
    ]
  }
}

Array Rendering

Use arrays for multiple items:

{
  "type": "container",
  "body": [
    { "type": "text", "content": "First item" },
    { "type": "text", "content": "Second item" },
    { "type": "text", "content": "Third item" }
  ]
}

Expression System

Object UI includes a powerful expression system for dynamic behavior:

Simple Expressions

{
  "type": "text",
  "content": "${user.firstName} ${user.lastName}"
}

Conditional Expressions

{
  "type": "card",
  "title": "${status === 'active' ? 'Active' : 'Inactive'}",
  "description": "${status === 'active' ? 'This record is in use.' : 'This record is archived.'}"
}

card here rather than badge, because an expression is evaluated only on a key the node's own type carries. expressionBindableTextKeysFor — the lookup SchemaRenderer consumes out of @objectstack/spec — gives card the rows title and description, and gives badge no rows at all, so a ${…} written on a badge reaches the DOM as the characters you typed. Resolve a badge's text before you hand the schema over, and author it on label: text is not a BadgeSchema key.

Visibility Control

{
  "type": "button",
  "label": "Delete",
  "visibleOn": "${user.role === 'admin'}"
}

Complex Logic

{
  "type": "alert",
  "variant": "default",
  "title": "Welcome!",
  "body": {
    "type": "text",
    "content": "${
      user.isNew ? 'Start with the quick tour.' :
      user.tasks.length === 0 ? 'You are all caught up.' :
      'You have tasks waiting.'
    }"
  }
}

The branch sits on the nested text node's content, which SchemaRenderer evaluates on every node type — the escape hatch for a component that carries no expression rows of its own, and alert is one of those. Its severity could not be chosen by expression in any case: AlertSchema.variant is the closed set default | destructive, so info, warning and success are not values it accepts. Pick the variant in the host and author it as a literal.

Event Handling

Components can emit events that you handle in React:

<SchemaRenderer 
  schema={schema}
  onAction={(action, context) => {
    console.log('Action:', action)
    console.log('Context:', context)
  }}
  onSubmit={(data) => {
    console.log('Form submitted:', data)
  }}
/>

Reference actions in schemas:

{
  "type": "action:button",
  "name": "call_api",
  "label": "Click Me",
  "actionType": "api",
  "endpoint": "/api/action",
  "method": "POST"
}

Three things about the shape this replaces. A declarative action is its own NODE TYPE, action:button — a plain button has no authorable handler: ButtonSchema.onClick is a runtime slot for a host-supplied function, refused by name by the zod mirror, and (being on SDUI_DOM_PASS_THROUGH_KEYS) forwarded straight to the DOM listener slot, where React throws on the first click: "Expected onClick listener to be a function, instead got a value of object type." The execution type is actionType, and the built-in vocabulary is script | url | modal | flow | api | form (plus objectui's navigation alias) — anything else must be a handler your host registered on ActionProvider. ajax is neither. And the endpoint key is endpoint, with method; api is not a key any action renderer forwards.

Performance Optimization

Lazy Loading

Large schemas are automatically optimized:

{
  "type": "tabs",
  "lazyLoad": true,
  "tabs": [
    { "title": "Tab 1", "body": { /* Loaded when tab is clicked */ } },
    { "title": "Tab 2", "body": { /* Loaded when tab is clicked */ } }
  ]
}

Memoization

The renderer automatically memoizes components to prevent unnecessary re-renders.

Code Splitting

Use dynamic imports for heavy components:

import { lazy } from 'react'

const HeavyChart = lazy(() => import('./HeavyChart'))

registry.register('heavy-chart', HeavyChart)

Error Handling

The renderer includes built-in error boundaries:

<SchemaRenderer 
  schema={schema}
  onError={(error, errorInfo) => {
    console.error('Rendering error:', error)
    // Log to error tracking service
  }}
/>

TypeScript Support

Full type safety for your schemas:

import type { PageNodeSchema, FormSchema } from '@object-ui/types'

const form: FormSchema = {
  type: "form",
  // TypeScript will validate this entire structure
  fields: []
}

const schema: PageNodeSchema = {
  type: "page",
  title: "Typed Page",
  body: [form]
}

Best Practices

1. Keep Schemas Simple

Break complex UIs into smaller, reusable schemas:

// ❌ Bad: One massive schema
const massiveSchema = { /* 500 lines of JSON */ }

// ✅ Good: Composed schemas
const headerSchema = { /* ... */ }
const contentSchema = { /* ... */ }
const footerSchema = { /* ... */ }

const pageSchema = {
  type: "page",
  body: [headerSchema, contentSchema, footerSchema]
}

2. Use Data Context Effectively

Pass all necessary data upfront:

// ✅ Good
const data = {
  user: userData,
  settings: userSettings,
  stats: dashboardStats
}

<SchemaRenderer schema={schema} data={data} />

3. Leverage Expressions

Move logic to expressions instead of creating conditional schemas:

// ❌ Bad
const schema = user.isAdmin ? adminSchema : userSchema

// ✅ Good
const schema = {
  type: "page",
  body: [
    { 
      type: "admin-panel",
      visibleOn: "${user.isAdmin}"
    },
    {
      type: "user-panel",
      visibleOn: "${!user.isAdmin}"
    }
  ]
}

4. Use TypeScript

Always type your schemas for better IDE support and fewer runtime errors.

Common Patterns

Loading States

{
  "type": "container",
  "body": {
    "type": "spinner",
    "visibleOn": "${loading}"
  }
}

Empty States

{
  "type": "empty",
  "visibleOn": "${items.length === 0}",
  "message": "No items found",
  "action": {
    "type": "button",
    "label": "Create New"
  }
}

Error States

{
  "type": "alert",
  "variant": "destructive",
  "visibleOn": "${error}",
  "title": "Something went wrong",
  "body": { "type": "text", "content": "${error.message}" }
}

visibleOn is a condition key and is evaluated on every node type. The message text is a nested text node because alert carries no expression rows — and message is not an AlertSchema key at all: the alert's own text keys are title and description, and the renderer falls back from description to body. destructive is the variant this state wants; error is not in the closed set.

Next Steps

Component Registry - Learn about component registration
Expression System - Master expressions
Schema Overview - Explore all available schemas

SchemaRenderer - Technical reference for the renderer
Architecture Overview - System architecture
@object-ui/core README - Core package API reference
@object-ui/react README - React package API reference

Schema Rendering

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