Vue3's Compilation Optimization: Block and Block Tree

Problem Description
Vue3's compilation optimization introduces the concepts of Block and Block Tree, which represent a significant improvement over the traditional Virtual DOM Diff algorithm. Please explain in detail what a Block is, what a Block Tree is, and how they enhance Vue3's rendering performance.

Solution Process

1. Performance Bottleneck of Traditional Virtual DOM Diff

   • In Vue 2 or React, when a component's state changes, a new Virtual DOM tree is generated.
   • The Diff algorithm needs to recursively compare the old and new complete Virtual DOM trees, identify the differences, and update the real DOM.
   • This comparison is "blind"; the algorithm does not know which part of the data actually changed, so it must traverse the entire tree structure. For templates containing a large number of dynamic nodes (i.e., nodes whose attributes or children may change), this full comparison incurs unnecessary performance overhead.

2. Vue3's Solution: Dynamic Node Collection

   • Vue3's compiler performs static analysis during template compilation. It identifies which nodes in the template are static (will never change) and which are dynamic (may change due to reactive data).
   • The core idea is: since the data is reactive and we know what changed, there's no need to compare parts that cannot possibly change.
   • Therefore, Vue3's compiler actively collects all dynamic nodes within a block in the generated render function. This marked "block" is the Block.

3. What is a Block?

   • Definition: A Block is essentially a special virtual node. In addition to the information of a normal virtual node, it has an extra property (often called dynamicChildren).
   • dynamicChildren Array: This property is an array that holds references to all direct dynamic child nodes within the current Block's scope.
   • Root of a Block: Typically, nodes with structural directives (like v-if, v-for) become the root node of a Block because the node structure inside them changes dynamically. For a component template, its outermost node is usually also a Block root.
   • Example:

     <div> <!-- This div is a Block root node -->
       <span>Static Title</span> <!-- Static node, not collected -->
       <span :id="dynamicId">{{ dynamicText }}</span> <!-- Dynamic node, collected -->
       <p v-if="show">Dynamic Paragraph</p> <!-- Dynamic node, collected -->
     </div>
     

     • After compilation, the outermost div virtual node is a Block.
     • Its dynamicChildren array will contain two references: one for the span with the dynamic :id and text, and another for the p controlled by v-if.
     • The purely static span node will not be collected into dynamicChildren.

4. How Does Block Optimize the Diff (i.e., Patch) Process?

   • When this component needs to update, Vue executes an optimized Block Patch algorithm.
   • Traditional Diff: Compares all child nodes under the div (3 spans and possibly a p).
   • Block Diff:
     1. Vue directly locates this Block node.
     2. It only compares the dynamic nodes collected in the dynamicChildren array.
     3. It skips the comparison of that static span node because it is not in the list that needs comparison.
   • This drastically reduces the number of nodes to compare. Especially in scenarios where static content far outweighs dynamic content, the performance improvement is significant. This comparison is targeted updating.

5. What is a Block Tree?

   • Complex templates have nested structures, e.g., a v-for containing a v-if. This forms nested Blocks.
   • Block Tree is a tree structure composed of these nested Blocks.
   • Parent-Child Block Relationship: Each child Block is a dynamic child node of its parent Block. That is, the root node of the child Block is collected into the parent Block's dynamicChildren array.
   • Example:

     <div> <!-- Root Block -->
       <div v-for="item in list" :key="item.id"> <!-- This is a child Block (v-for) -->
         <span>{{ item.name }}</span> <!-- This span is a dynamic node of the child Block -->
         <p v-if="item.showDesc">{{ item.desc }}</p> <!-- This p is another dynamic node of the child Block -->
       </div>
     </div>
     

     • The outermost div is the root Block.
     • The div with v-for is itself a child Block (let's call it Block_vfor).
     • The root Block's dynamicChildren array contains only one element: Block_vfor. It doesn't care about the specific spans or ps inside Block_vfor.
     • Block_vfor's own dynamicChildren array collects its internal dynamic nodes: the span corresponding to {{ item.name }} and the p controlled by v-if.

6. Update Process of a Block Tree

   • When list changes, the update process is:
     1. First, perform the Diff for the root Block. The root Block finds that its dynamic child node (Block_vfor) needs updating.
     2. Then, recursively perform the Diff for the child Block Block_vfor. Block_vfor efficiently updates its internal dynamic nodes based on its own dynamicChildren array (e.g., adding, deleting, or moving list items according to key, or toggling the p tag's visibility).
   • This tree structure allows Vue to "localize" the Diff process. When updating a deeply nested component, only the Blocks along the path from the root Block to the target Block need to be updated, while other Blocks outside this path are completely unaffected.

Summary
Block and Block Tree are key optimizations made by Vue3 during the compilation phase. Through static analysis, the compiler divides the template into different Blocks and collects the dynamic nodes within each Block. At runtime, Vue's renderer executes a targeted Diff algorithm by directly traversing a Block's dynamicChildren array, completely skipping comparisons of static content, and performs efficient localized updates along the Block Tree, thereby significantly improving rendering performance.