Principle of Static Root Node Hoisting in Vue3's SFC Compilation Optimization

Problem Description
During the compilation phase of Vue3's SFC (Single File Component), the template undergoes static analysis to identify static root nodes that will never change. These nodes are then hoisted outside the component's render function to avoid repeated rendering. Please explain in detail the identification criteria for static root node hoisting, the hoisting strategy, and the runtime optimization effects.

Solution Process

1. Definition and Identification of Static Nodes

   • Static Node: A node in the template that contains no dynamic bindings (e.g., {{}}, v-bind, v-if). For example, <div class="header">Title</div>.
   • The compiler marks a node's patchFlag property by parsing the template's AST (Abstract Syntax Tree). If the patchFlag is 0 or the node has no dynamic properties, it is determined to be a static node.

2. Criteria for Determining a Static Root Node

   • The root node must be an element node (not a text/comment node).
   • This node and all its child nodes must be static nodes.
   • If the root node contains only a single text child node (e.g., <div>Static Text</div>), Vue3 will directly optimize it into a text node and not consider it a static root node (due to minimal hoisting benefit).
   • Example:

     <!-- Static Root Node -->
     <div class="footer">
       <p>Contact Email: contact@example.com</p>
       <span>All Rights Reserved</span>
     </div>
     
     <!-- Non-Static Root Node (contains dynamic binding) -->
     <div :class="dynamicClass">{{ dynamicText }}</div>
     

3. Hoisting Strategy and Code Generation

   • The compiler serializes the AST subtree corresponding to the static root node into a string, generating a hoist variable:

     const _hoisted_1 = /*#__PURE__*/_createStaticVNode("<div class=\"footer\"><p>Contact Email: contact@example.com</p><span>All Rights Reserved</span></div>", 1)
     

   • In the render function, the hoisted static node is directly referenced to avoid being recreated each time:

     function render(_ctx, _cache) {
       return (_openBlock(), _createBlock(_ctx.component, null, {
         default: _withCtx(() => [
           _hoisted_1,  // Direct use of the hoisted static node
           _createVNode(_ctx.dynamicComponent)
         ])
       }))
     }
     

4. Runtime Optimization Effects

   • Reduced Virtual DOM Creation Overhead: Static nodes are created only once during initialization and directly reused in subsequent renders.
   • Optimized Diff Algorithm: During the Patch process, static nodes are completely skipped, eliminating the need to compare child nodes.
   • Cached Event Handlers: If a static node contains event listeners (e.g., @click), they are cached via the CacheHandler mechanism to avoid repeated generation.

5. Difference from Static Node Hoisting (Hoist Static)

   • Static Node Hoisting targets individual static nodes (e.g., <span>Static Text</span>) and directly inlines them as strings.
   • Static Root Node Hoisting targets root nodes containing subtrees, generating independent static VNodes to ensure the stability of the subtree structure.

Summary
Static root node hoisting extracts immutable parts of the template as constants through compile-time analysis. Combined with runtime reuse mechanisms, it effectively reduces the performance overhead of component updates. This optimization is particularly beneficial for components containing large amounts of static content (such as headers, footers) and is a crucial part of Vue3's performance optimization chain.