Optimization strategies for the diff algorithm

The Core Idea of the Diff Algorithm

The diff algorithm is the key to the efficient updates of the virtual DOM. In Vue 3, the diff algorithm is optimized based on Snabbdom, primarily employing same-level comparison and a double-ended comparison strategy to minimize DOM operations. When a component's state changes, Vue does not directly manipulate the real DOM. Instead, it first compares the differences between the old and new virtual DOM trees and then updates only the necessary parts.

// Simplified virtual DOM structure example
interface VNode {
  tag: string
  props: Record<string, any>
  children: Array<VNode | string>
  key?: string | number
  el?: HTMLElement
}

Issues with Traditional Diff Algorithms

The time complexity of traditional tree-structured diff algorithms is O(n^3), which is completely unacceptable for web applications. Vue 3 reduces the complexity to O(n) by making the following assumptions:

1. Components of the same type produce similar DOM structures.
2. Components of different types produce different DOM structures.
3. A group of nodes at the same level can be distinguished using keys.

// Inefficient list rendering example
<ul>
  <li v-for="item in items">{{ item.text }}</li>
</ul>

// Optimized version with keys
<ul>
  <li v-for="item in items" :key="item.id">{{ item.text }}</li>
</ul>

Double-Ended Comparison Algorithm

The double-ended comparison algorithm adopted by Vue 3 is a significant improvement over traditional diff algorithms. The algorithm starts comparing from both ends of the old and new child nodes and consists of four steps:

1. Compare the starting nodes of the old and new child nodes (oldStart/newStart).
2. Compare the ending nodes of the old and new child nodes (oldEnd/newEnd).
3. Compare the old starting node with the new ending node (oldStart/newEnd).
4. Compare the old ending node with the new starting node (oldEnd/newStart).

function patchChildren(
  oldChildren: VNode[],
  newChildren: VNode[],
  container: HTMLElement
) {
  let oldStartIdx = 0
  let oldEndIdx = oldChildren.length - 1
  let newStartIdx = 0
  let newEndIdx = newChildren.length - 1
  // ...Double-ended comparison implementation
}

Longest Increasing Subsequence Optimization

For nodes that cannot be processed through double-ended comparison, Vue 3 uses the Longest Increasing Subsequence (LIS) algorithm to find the solution with the fewest DOM operations. This optimization is particularly suitable for handling cases where only a few nodes in a list have moved.

// Calculate the longest increasing subsequence
function getSequence(arr: number[]): number[] {
  const p = arr.slice()
  const result = [0]
  let i, j, u, v, c
  const len = arr.length
  // ...LIS algorithm implementation
  return result
}

Static Hoisting and Patch Flags

Vue 3 uses compile-time static analysis to hoist static nodes, avoiding the creation of new VNodes during each render. It also uses patch flags to mark dynamic content, allowing the diff process to skip static parts.

// Example of compiled code
const _hoisted_1 = /*#__PURE__*/_createVNode("div", null, "static content", -1 /* HOISTED */)

function render() {
  return (_openBlock(), _createBlock("div", null, [
    _hoisted_1,
    _createVNode("p", null, _toDisplayString(_ctx.dynamic), 1 /* TEXT */)
  ]))
}

Event Caching Optimization

Vue 3 caches event handlers to avoid unnecessary updates. When the event handler remains unchanged, it reuses the previous function, reducing unnecessary diff operations.

// Event caching example
const render = () => {
  return h('div', {
    onClick: _cache[1] || (_cache[1] = ($event) => _ctx.handleClick($event))
  })
}

Component-Level Optimization

Vue 3 implements more granular update control at the component level. Through improvements in the reactivity system, updates are triggered only when the component's dependent state changes, avoiding unnecessary subtree diffing.

// Simplified component update logic example
function updateComponent(n1: VNode, n2: VNode) {
  if (n1.props !== n2.props) {
    // Update only if props change
    updateProps(instance, n2.props)
  }
  // ...Other update logic
}

Compile-Time and Runtime Optimization Integration

Vue 3's compiler analyzes templates and generates optimized render function code. These optimizations include static node hoisting, patch flag generation, block marking, and more, working in tandem with the runtime diff algorithm to achieve optimal performance.

// Optimized render function after compilation
export function render(_ctx, _cache) {
  return (_openBlock(), _createBlock("div", null, [
    _createVNode("span", { class: "foo" }, _toDisplayString(_ctx.text), 1 /* TEXT */),
    _hoisted_2
  ]))
}

Hydration Optimization for SSR

In server-side rendering (SSR) scenarios, Vue 3's hydration process also includes diff optimizations. The algorithm strives to reuse the server-rendered DOM structure, only adding event listeners and updating necessary dynamic content, avoiding full DOM reconstruction.

function hydrate(
  vnode: VNode,
  container: HTMLElement,
  parentComponent: ComponentInternalInstance | null
) {
  // ...Hydration implementation
  if (shouldUpdate) {
    patch(/* ... */)
  } else {
    // Reuse existing DOM nodes
  }
}

Performance Monitoring and Tuning

Vue 3 provides more detailed performance monitoring APIs, allowing developers to measure the execution time of the diff algorithm and optimize based on actual conditions. For example, virtual scrolling techniques can be used for large lists to reduce the number of nodes that need diffing.

// Performance measurement example
import { startMeasure, stopMeasure } from 'vue'

startMeasure('patch')
patch(oldVNode, newVNode)
stopMeasure('patch')