Extension points for custom renderers

Virtual DOM and Renderer Basics

Vue3's rendering system is built on the virtual DOM, a lightweight abstract representation of the real DOM. The core responsibility of the renderer is to convert virtual nodes into real DOM nodes and update them efficiently. In Vue3, the renderer is designed to be pluggable, meaning developers can create custom renderers to support non-DOM environments.

interface VNode {
  type: string | symbol | Component
  props: Record<string, any> | null
  children: VNode[] | string | null
  // ...other properties
}

Renderer Creation and Configuration

The entry point for creating a custom renderer is the createRenderer function, which accepts a configuration object containing node operation methods. This configuration object defines how to create, insert, update, and delete nodes.

const { createRenderer } = require('vue')

const renderer = createRenderer({
  createElement(tag) {
    // Return the element for the corresponding platform
  },
  insert(child, parent, anchor) {
    // Platform-specific implementation for inserting nodes
  },
  // Other required methods...
})

Core Extension Points Explained

Element Creation and Handling

createElement is a mandatory method that defines how to create basic elements. For a DOM renderer, this would call document.createElement, but in a custom renderer, it can be implemented to create native elements for any platform.

function createElement(tag: string) {
  if (platform === 'canvas') {
    return new CanvasElement(tag)
  } else if (platform === 'mobile') {
    return nativeModules.createView(tag)
  }
}

Property Handling

The patchProp method handles updates to element properties, including regular attributes, DOM properties, and event listeners. Custom renderers can implement platform-specific property handling logic here.

function patchProp(el, key, prevValue, nextValue) {
  if (key.startsWith('on')) {
    // Handle events
    const event = key.slice(2).toLowerCase()
    el._eventListeners = el._eventListeners || {}
    if (prevValue) el.removeEventListener(event, prevValue)
    if (nextValue) el.addEventListener(event, nextValue)
  } else {
    // Handle regular attributes
    el.setAttribute(key, nextValue)
  }
}

Child Node Handling

The insert and remove methods control node addition and removal operations. Custom renderers can implement platform-specific layout logic here.

function insert(child, parent, anchor) {
  if (platform === 'terminal') {
    parent.children = parent.children || []
    const index = anchor 
      ? parent.children.indexOf(anchor) 
      : parent.children.length
    parent.children.splice(index, 0, child)
  } else {
    parent.insertBefore(child, anchor || null)
  }
}

Advanced Extension Capabilities

Custom Node Types

In addition to built-in HTML element types, custom node types can be extended. For example, special graphic nodes can be defined in a Canvas renderer.

const Circle = {
  __isVue: true,
  render() {
    return {
      type: 'circle',
      props: { x: 100, y: 100, r: 50, fill: 'red' }
    }
  }
}

// Handle custom types in the renderer
function createElement(type) {
  if (type === 'circle') {
    return new CanvasCircle()
  }
  // ...
}

Platform-Specific Directives

Platform-specific directive systems can be extended. For example, adding native gesture directives for a mobile renderer.

// Custom directive implementation
const vSwipe = {
  mounted(el, binding) {
    el._swipeHandler = (event) => {
      if (isSwipeGesture(event)) {
        binding.value(event)
      }
    }
    el.addEventListener('touchmove', el._swipeHandler)
  },
  unmounted(el) {
    el.removeEventListener('touchmove', el._swipeHandler)
  }
}

// Register the directive in the renderer
renderer.directive('swipe', vSwipe)

Performance Optimization Extensions

Custom Update Strategies

By overriding the patch method, differentiated update strategies can be implemented to optimize performance for specific platforms.

function patch(n1, n2, container) {
  if (n1.type === 'complex-widget') {
    // Use a special optimization path for complex components
    return patchComplexWidget(n1, n2, container)
  }
  // Default path
  return defaultPatch(n1, n2, container)
}

Batch Update Control

Implement platform-specific batch update mechanisms, such as combining multiple draw operations in a Canvas renderer.

let isBatching = false
let batchedUpdates = []

function queueUpdate(fn) {
  if (isBatching) {
    batchedUpdates.push(fn)
  } else {
    fn()
  }
}

function startBatch() {
  isBatching = true
}

function endBatch() {
  isBatching = false
  const updates = batchedUpdates.slice()
  batchedUpdates.length = 0
  updates.forEach(fn => fn())
}

Cross-Platform Implementation Examples

Canvas Renderer Implementation

A simple core implementation example for a Canvas renderer:

const canvasRenderer = createRenderer({
  createElement(type) {
    return { type, operations: [] }
  },
  patchProp(el, key, prevVal, nextVal) {
    el.operations.push({ type: 'prop', key, value: nextVal })
  },
  insert(child, parent) {
    if (parent.type === 'canvas') {
      drawOperations(child)
    } else {
      parent.children = parent.children || []
      parent.children.push(child)
    }
  },
  createText(text) {
    return { type: 'text', value: text }
  }
})

function drawOperations(node) {
  const ctx = canvas.getContext('2d')
  node.operations.forEach(op => {
    if (op.type === 'prop' && op.key === 'fillStyle') {
      ctx.fillStyle = op.value
    }
    // Handle other drawing operations...
  })
  if (node.type === 'rect') {
    ctx.fillRect(0, 0, 100, 100)
  }
}

Terminal Text Rendering Example

An example of a text renderer for command-line terminals:

const terminalRenderer = createRenderer({
  createElement(type) {
    return { type, text: '', children: [], style: {} }
  },
  insert(child, parent) {
    parent.children.push(child)
    if (parent.type === 'root') {
      renderTerminal(parent)
    }
  },
  setElementText(node, text) {
    node.text = text
  }
})

function renderTerminal(root) {
  process.stdout.write('\x1Bc') // Clear screen
  root.children.forEach(child => {
    let output = child.text
    if (child.style.color) {
      output = applyTerminalColor(output, child.style.color)
    }
    process.stdout.write(output + '\n')
  })
}

Integration with Vue Ecosystem

Custom Component Support

Custom renderers need to correctly handle Vue components, including lifecycle and reactivity systems.

function mountComponent(vnode, container) {
  const instance = createComponentInstance(vnode)
  setupComponent(instance)
  setupRenderEffect(instance, container)
}

function setupRenderEffect(instance, container) {
  effect(() => {
    if (!instance.isMounted) {
      // Initial mount
      const subTree = instance.render.call(instance.proxy)
      patch(null, subTree, container)
      instance.isMounted = true
    } else {
      // Update
      const nextTree = instance.render.call(instance.proxy)
      patch(instance.subTree, nextTree, container)
      instance.subTree = nextTree
    }
  })
}

Transition Animation Handling

Custom renderers can implement platform-specific transition animation effects.

function handleTransition(vnode, container) {
  if (platform === 'web') {
    // Use CSS transitions
    vnode.el.classList.add('enter-from')
    nextFrame(() => {
      vnode.el.classList.remove('enter-from')
      vnode.el.classList.add('enter-to')
    })
  } else if (platform === 'mobile') {
    // Use native animation APIs
    nativeModules.animateView(
      vnode.el,
      { opacity: 0 },
      { opacity: 1 },
      300
    )
  }
}

Debugging and Testing Support

Custom Debugging Tools

Implement development tool integration for custom renderers to facilitate debugging.

function initDevTools() {
  if (process.env.NODE_ENV === 'development') {
    const devToolsHook = {
      on: (event, fn) => {
        // Listen to renderer events
      },
      emit: (event, ...args) => {
        // Send events to DevTools
      }
    }
    attachRendererToDevTools(renderer, devToolsHook)
  }
}

Testing Tool Integration

Implement special methods required for testing tools to facilitate unit testing.

function createTestRenderer() {
  const renderer = createRenderer({
    // ...Basic methods
    createTextNode(text) {
      return { type: 'TEST_TEXT', text }
    },
    // Test-specific methods
    getRenderedTree() {
      return container._vnode
    }
  })
  return renderer
}