Custom Renderer API

What is a Custom Renderer

One of the core features of Vue.js is its virtual DOM system, which is responsible for rendering the component tree into actual DOM elements. The custom renderer API allows developers to override the default DOM rendering logic and achieve rendering targets in non-DOM environments, such as Canvas, WebGL, or even native mobile applications.

import { createRenderer } from 'vue'

const { createApp } = createRenderer({
  patchProp,
  insert,
  remove,
  createElement,
  // ...other node operation methods
})

Core Renderer Methods

A custom renderer needs to implement a set of specific node operation methods, which form the core of the renderer:

1. createElement - Create an element
2. insert - Insert an element
3. patchProp - Update properties
4. remove - Remove an element
5. createText - Create a text node
6. setText - Set text content
7. parentNode - Get the parent node
8. nextSibling - Get the next sibling node

interface RendererOptions<Node, Element> {
  patchProp(
    el: Element,
    key: string,
    prevValue: any,
    nextValue: any
  ): void
  insert(el: Node, parent: Element, anchor?: Node | null): void
  remove(el: Node): void
  createElement(type: string): Element
  createText(text: string): Node
  setText(node: Node, text: string): void
  parentNode(node: Node): Element | null
  nextSibling(node: Node): Node | null
}

Implementing a Canvas Renderer

Here’s a simple example of a Canvas renderer implementation, demonstrating how to render Vue components onto a Canvas:

const { createApp } = createRenderer({
  createElement(type) {
    return { type }
  },
  
  patchProp(el, key, prevValue, nextValue) {
    el[key] = nextValue
  },
  
  insert(child, parent, anchor) {
    if (!parent.children) parent.children = []
    const index = parent.children.indexOf(anchor)
    if (index > -1) {
      parent.children.splice(index, 0, child)
    } else {
      parent.children.push(child)
    }
  },
  
  remove(child) {
    const parent = child.parent
    if (parent) {
      const index = parent.children.indexOf(child)
      if (index > -1) parent.children.splice(index, 1)
    }
  },
  
  createText(text) {
    return { type: 'TEXT', text }
  },
  
  setText(node, text) {
    node.text = text
  },
  
  parentNode(node) {
    return node.parent
  },
  
  nextSibling(node) {
    const parent = node.parent
    if (!parent) return null
    const index = parent.children.indexOf(node)
    return parent.children[index + 1] || null
  }
})

Rendering to the Terminal Console

Custom renderers aren’t limited to graphical interfaces; they can also render Vue components to a terminal console:

const consoleRenderer = createRenderer({
  createElement(tag) {
    return { tag }
  },
  
  patchProp(el, key, prevVal, nextVal) {
    el.props = el.props || {}
    el.props[key] = nextVal
  },
  
  insert(child, parent) {
    if (!parent.children) parent.children = []
    parent.children.push(child)
  },
  
  createText(text) {
    return { type: 'text', text }
  },
  
  // Other necessary methods...
})

function renderToConsole(vnode) {
  if (vnode.type === 'text') {
    process.stdout.write(vnode.text)
  } else {
    process.stdout.write(`<${vnode.tag}`)
    if (vnode.props) {
      for (const [key, value] of Object.entries(vnode.props)) {
        process.stdout.write(` ${key}="${value}"`)
      }
    }
    process.stdout.write('>')
    
    if (vnode.children) {
      vnode.children.forEach(renderToConsole)
    }
    
    process.stdout.write(`</${vnode.tag}>`)
  }
}

Combining with the Composition API

Custom renderers can seamlessly integrate with Vue’s Composition API to create reactive components for non-DOM environments:

const app = createApp({
  setup() {
    const count = ref(0)
    
    function increment() {
      count.value++
    }
    
    return {
      count,
      increment
    }
  },
  
  render() {
    return {
      type: 'button',
      props: {
        onClick: this.increment,
        label: `Clicked ${this.count} times`
      }
    }
  }
})

// The custom renderer will handle this virtual node
app.mount(canvasElement)

Performance Optimization Tips

When implementing a custom renderer, performance is a critical consideration:

1. Batched Updates: Implement a batched update mechanism similar to the DOM.
2. Virtual Node Reuse: Optimize the creation and destruction of virtual nodes.
3. Diff Algorithm Optimization: Tailor the diff algorithm for specific environments.

const queue = []
let isFlushing = false

function queueJob(job) {
  if (!queue.includes(job)) {
    queue.push(job)
    if (!isFlushing) {
      isFlushing = true
      Promise.resolve().then(flushJobs)
    }
  }
}

function flushJobs() {
  try {
    for (let i = 0; i < queue.length; i++) {
      queue[i]()
    }
  } finally {
    queue.length = 0
    isFlushing = false
  }
}

Real-World Use Cases

Custom renderers have various practical applications:

1. Game Development: Render Vue components into game engines.
2. Data Visualization: Directly render to Canvas or WebGL.
3. Server-Side Rendering: Generate output in specific formats.
4. Testing Tools: Validate component behavior without relying on real DOM.

// Three.js renderer example
const threeRenderer = createRenderer({
  createElement(type) {
    if (type === 'mesh') {
      return new THREE.Mesh(
        new THREE.BoxGeometry(),
        new THREE.MeshBasicMaterial()
      )
    }
    // Handle other types...
  },
  
  patchProp(el, key, prevValue, nextValue) {
    if (key === 'position') {
      el.position.set(nextValue.x, nextValue.y, nextValue.z)
    }
    // Handle other properties...
  },
  
  insert(child, parent) {
    parent.add(child)
  }
})

Debugging Custom Renderers

Debugging custom renderers requires specialized tools and techniques:

1. Virtual Node Inspector: Develop dedicated devtools plugins.
2. Logging: Track calls to renderer methods.
3. Snapshot Testing: Verify that rendering results match expectations.

function createDebugRenderer(baseRenderer) {
  return {
    createElement(...args) {
      console.log('createElement', ...args)
      return baseRenderer.createElement(...args)
    },
    patchProp(...args) {
      console.log('patchProp', ...args)
      return baseRenderer.patchProp(...args)
    },
    // Wrap other methods...
  }
}

Integrating with the Existing Ecosystem

Custom renderers need to consider compatibility with the Vue ecosystem:

1. Component Library Support: Ensure third-party components work correctly.
2. Vue Router: Handle routing view rendering.
3. State Management: Maintain consistency in the reactivity system.

// Example of integrating Vue Router
const router = createRouter({
  history: createWebHistory(),
  routes: [...]
})

const app = createApp({
  render() {
    return h(RouterView)
  }
})

app.use(router)
app.mount(customRenderTarget)

Testing Strategies

Writing tests for custom renderers requires consideration of:

1. Unit Tests: Validate individual renderer methods.
2. Integration Tests: Test the entire rendering pipeline.
3. Snapshot Tests: Verify rendering results.

describe('Canvas Renderer', () => {
  it('should create elements', () => {
    const renderer = createCanvasRenderer()
    const rect = renderer.createElement('rect')
    expect(rect.type).toBe('rect')
  })
  
  it('should handle props', () => {
    const renderer = createCanvasRenderer()
    const rect = renderer.createElement('rect')
    renderer.patchProp(rect, 'fill', null, 'red')
    expect(rect.fill).toBe('red')
  })
})

Advanced Topics

Explore advanced uses of custom renderers:

1. Custom Directive Support: Implement directives for specific environments.
2. Transition Animations: Create non-DOM transition effects.
3. Server-Side Rendering: Generate non-HTML output.
4. Mixed Renderers: Use multiple rendering targets simultaneously.

// Example of a custom directive
const myDirective = {
  mounted(el, binding) {
    // Directive logic in a custom renderer environment
    if (el.setAttribute) {
      el.setAttribute('data-custom', binding.value)
    }
  },
  updated(el, binding) {
    // Update logic
  }
}

app.directive('custom', myDirective)