Vue3's Compilation Optimizations: Block and Block Tree

Vue3's Compilation Optimizations: Block and Block Tree

Description
Block and Block Tree are among the core optimization strategies in Vue3's compilation phase. They collectively address the "blind recursion" problem inherent in traditional virtual DOM Diff algorithms. When a component's internal structure is dynamic (e.g., containing v-if, v-for), Vue2's Diff algorithm needs to recursively compare the entire subtree, even if most nodes are static. Vue3 introduces the Block mechanism, which partitions dynamic nodes into distinct blocks and establishes a dependency tree (Block Tree) between parent and child blocks, enabling precise targeted updates.

Explanation

1. Background: Limitations of Traditional Diff

  • In Vue2 or React, when a component's state changes trigger a re-render, a recursive comparison (Diff) between the new and old virtual DOM trees is required.
  • If a component's template contains conditional branches (e.g., v-if) or loops (v-for), the entire subtree must be compared even if changes affect only a small portion.
  • Example: 
    <div>
  <div v-if="show">Dynamic Node 1</div>
  <p>Static Node</p>
  <div v-for="item in list">{{ item }}</div>
</div>
  • When the value of show changes, ideally only the conditional branch should be updated, but the traditional Diff also checks the static nodes and loop sections.

2. Basic Concept of Block

  • Definition: A Block is a special virtual node that collects all potentially changing dynamic child nodes (including descendant nodes) within it.
  • Types of Dynamic Nodes:
    • Nodes containing reactive bindings (e.g., {{ value }}).
    • Conditional branch nodes (v-if/v-else).
    • Loop nodes (v-for).
    • Nodes with dynamic attributes.
  • Block Identification: Each Block has a dynamicChildren array that specifically stores its internal dynamic nodes in sequential order.

3. Rules for Creating Blocks

  • Root Block: The top-level node of each component template automatically becomes a Block.
  • Conditional Block: v-if, v-else, v-else-if directives create new Blocks.
  • Loop Block: The v-for directive creates independent Blocks for each iteration item.
  • Nested Blocks: A Block can contain other child Blocks, forming a hierarchical structure.

4. Process of Building a Block Tree

<!-- Example Template -->
<div>
  <p>Static Text</p>
  <div v-if="show">
    <span>{{ dynamicText }}</span>
  </div>
  <ul>
    <li v-for="item in list" :key="item.id">{{ item.name }}</li>
  </ul>
</div>

Step 1: Parse Template Structure

  • The compiler analyzes the template and identifies:
    • Root Block (corresponding to the entire <div>).
    • Conditional Block (corresponding to the <div> with v-if).
    • Loop Blocks (corresponding to each <li> with v-for).

Step 2: Collect Dynamic Nodes

  • The Root Block's dynamicChildren contains:
    • The Conditional Block node.
    • The Loop Block nodes.
  • The Conditional Block's dynamicChildren contains:
    • The <span> node (because it contains {{ dynamicText }}).
  • Each Loop Block's dynamicChildren contains:
    • Its text node ({{ item.name }}).

Step 3: Establish the Block Tree

Root Block
├── Conditional Block (v-if)
│   └── Dynamic Text Node
└── Loop Block (v-for)
    ├── Loop Item Block 1
    │   └── Dynamic Text Node
    ├── Loop Item Block 2
    │   └── Dynamic Text Node
    └── ...

5. Update Advantages of Block Tree

  • Precise Updates: When the value of show changes, only the Conditional Block and its child nodes need to be compared, without checking the loop sections.
  • Efficient Diff: When performing Diff within a Block, only the dynamicChildren array needs to be compared, skipping all static nodes.
  • Structural Stability: The boundaries of Blocks are determined by the template structure, ensuring stable comparison paths during updates.

6. Synergy with PatchFlag

  • Each dynamic node also has a PatchFlag identifying the specific type of change (text, attribute, class name, etc.).
  • The Block mechanism is responsible for determining "which nodes need to be updated," while PatchFlag determines "what specifically needs to be updated in those nodes."
  • For example: When a text node within a Conditional Block changes, the Block locates the node, and PatchFlag indicates that only the text content needs updating.

Summary
Block and Block Tree optimize the traditional "tree-level Diff" into a "block-level Diff." By performing static analysis during compilation to establish the organizational structure of dynamic nodes, the runtime can precisely locate the blocks that need updating based on data changes. This significantly reduces unnecessary virtual node comparisons and is one of the key architectural designs behind Vue3's performance improvements.