探讨 Vue 3 源码中 `v-memo` 指令的编译时和运行时实现，以及它如何实现对特定 VNode 子树的跳过更新。

Alright, everyone, settle in! Welcome to today’s deep dive. We’re going to crack open Vue 3’s v-memo directive and see what makes it tick, both during compilation and runtime. Think of this as a magic trick – we’ll learn how Vue cleverly skips updating parts of the DOM, making our apps snappier. Grab your virtual coffee, and let’s get started!

The Problem: Unnecessary Updates

Before v-memo, imagine you had a complex component rendering a large list. Even if only a tiny piece of data changed that didn’t affect the list itself, Vue would still go through the process of diffing and potentially re-rendering the entire list. That’s wasteful! We want to tell Vue, "Hey, if these specific things haven’t changed, don’t bother updating this whole section." That’s where v-memo comes in.

v-memo – The Short Story

v-memo is a directive that allows you to tell Vue: "Only update this part of the Virtual DOM if these specific dependencies have changed." It essentially creates a memoization point in your component tree. If the dependencies haven’t changed since the last render, Vue skips patching the corresponding VNode subtree, drastically improving performance.

Part 1: Compilation Time – From Markup to Instructions

Vue’s compiler is responsible for transforming your template code (HTML-like) into JavaScript instructions that the Vue runtime can understand. This is where v-memo gets its initial transformation.

• The Template:

  Let’s start with a simple example in our Vue template:

  <template>
    <div>
      <p>Counter: {{ count }}</p>
      <div v-memo="[count % 2 === 0]">
        <ExpensiveComponent />
      </div>
    </div>
  </template>
  
  <script>
  import { ref } from 'vue';
  import ExpensiveComponent from './ExpensiveComponent.vue';
  
  export default {
    components: {
      ExpensiveComponent
    },
    setup() {
      const count = ref(0);
  
      setInterval(() => {
        count.value++;
      }, 1000);
  
      return {
        count
      }
    }
  }
  </script>

  In this example, ExpensiveComponent will only re-render when count is an even number. This is a contrived example, but it highlights the core principle.

• Compiler’s Role:

  The Vue compiler sees the v-memo directive and translates it into something the runtime can use. It doesn’t just magically skip updates; it generates code to make it skip updates. The key function involved is part of the directive transforms. I’ll describe the general idea and some pseudo-code instead of extracting the exact source code line by line, since it can be quite verbose and harder to follow without the complete context.

• Directive Transform

  When the compiler encounters the v-memo directive, it triggers a directive transform. This transform modifies the Abstract Syntax Tree (AST) of the template to include the memoization logic. The main tasks are:

  1. Extract the Dependencies: The compiler analyzes the expression provided to v-memo (e.g., [count % 2 === 0]). It identifies the reactive dependencies used in that expression (in this case, count). The result is usually an array of expressions that represent these dependencies.

  2. Wrap the VNode Creation: The compiler modifies the code that creates the VNode for the element with the v-memo directive. It wraps this VNode creation with logic that checks if the dependencies have changed. If they haven’t changed, it reuses the previously cached VNode.

  Simplified Pseudo-Code of the Transform (conceptual):

  function transformMemoDirective(node, directive, context) {
      // 1. Extract Dependencies
      const dependenciesExpression = directive.exp; // e.g., `[count % 2 === 0]`
  
      // In reality, the compiler would parse the expression to find the real dependencies
  
      // 2. Wrap VNode Creation
      const originalCreateVNodeCall = node.codegenNode; // Assume this holds the original VNode creation code
  
      node.codegenNode = {
          type: "JS_CALL_EXPRESSION",
          callee: "resolveMemo",  // A helper function that will be injected in runtime.
          arguments: [
              dependenciesExpression,  // The expression for the dependencies
              originalCreateVNodeCall  // The original VNode creation
          ]
      };
  }

  What this pseudo-code does is replace the original VNode creation code with a call to resolveMemo. resolveMemo is a helper function that will be available in the Vue runtime. It’s responsible for checking the dependencies and either reusing a cached VNode or creating a new one.

  Key Output: The compiler effectively inserts a call to a runtime helper function (resolveMemo or something similar) that will handle the actual memoization logic. This function receives the dependencies and the code to create the VNode.

Part 2: Runtime – The Actual Memoization

Now, let’s see how Vue handles v-memo during runtime when your component is actually being rendered.

• The resolveMemo (or Equivalent) Function:

  This function is the heart of the v-memo implementation. It’s responsible for:

  1. Evaluating the Dependencies: It evaluates the expression provided to v-memo in the context of the current component instance. This gives us the current values of the dependencies.

  2. Comparing with Previous Values: It compares these values with the values from the previous render. This is where the magic happens.

  3. Returning the Cached VNode or Creating a New One:

     • If the dependencies have not changed (i.e., the current values are the same as the previous values), it returns the previously cached VNode. This tells Vue to skip updating that part of the DOM.
     • If the dependencies have changed, it creates a new VNode by executing the original VNode creation code. It also updates the cached dependency values for the next render.

• Conceptual Runtime Implementation (simplified):

  // This is a simplified representation, actual implementation might differ slightly
  function resolveMemo(dependencies, createVNode) {
      const currentInstance = getCurrentInstance(); // Get the current component instance
      const memoCache = currentInstance.__memoCache || (currentInstance.__memoCache = []);
      const index = findMemoCacheIndex(memoCache, dependencies); // Find the cached VNode and dependencies for this v-memo.
  
      if (index > -1) {
          const [cachedVNode, prevDependencies] = memoCache[index];
  
          // Compare dependencies:
          if (shallowCompare(dependencies, prevDependencies)) {
              return cachedVNode; // Dependencies haven't changed, return the cached VNode
          } else {
              // Dependencies have changed, create a new VNode and update the cache
              const newVNode = createVNode();
              memoCache[index] = [newVNode, dependencies];
              return newVNode;
          }
      } else {
          // No cached VNode found, create a new one and add it to the cache
          const newVNode = createVNode();
          memoCache.push([newVNode, dependencies]);
          return newVNode;
      }
  }
  
  function shallowCompare(arr1, arr2) {
      if (arr1.length !== arr2.length) {
          return false;
      }
      for (let i = 0; i < arr1.length; i++) {
          if (arr1[i] !== arr2[i]) {
              return false;
          }
      }
      return true;
  }
  
  function findMemoCacheIndex(memoCache, dependencies) {
    for (let i = 0; i < memoCache.length; i++) {
      if (memoCache[i][1] === dependencies) { // Comparing the dependency expression directly.
        return i;
      }
    }
    return -1;
  }

  Explanation:

  • getCurrentInstance(): Gets the current component instance. This is how Vue knows which component is currently rendering.
  • __memoCache: Each component instance has a __memoCache property (or something similar) to store the cached VNodes and their corresponding dependencies.
  • findMemoCacheIndex: This function looks in the __memoCache to see if there’s already a cached VNode for this v-memo directive (based on the dependency expression).
  • shallowCompare: This function performs a shallow comparison between the current dependencies and the previously cached dependencies. If they are the same, it means the dependencies haven’t changed.
  • The if (shallowCompare(...)) block is the core of the memoization logic. If the dependencies haven’t changed, it returns the cached VNode, skipping the VNode creation and subsequent patching.
  • If the dependencies have changed or if there’s no cached VNode, it creates a new VNode, updates the cache, and returns the new VNode.

Putting it All Together – The Flow

1. Compilation: The Vue compiler transforms the v-memo directive in your template into a call to a runtime helper function (like resolveMemo). This function receives the dependencies and the original VNode creation code.

2. First Render: During the component’s first render, the resolveMemo function evaluates the dependencies, creates a new VNode, and stores the VNode and the dependency values in the component’s memoization cache.

3. Subsequent Renders: On subsequent renders, the resolveMemo function again evaluates the dependencies. It compares the current dependency values with the values stored in the cache.

   • If the dependencies haven’t changed: resolveMemo returns the cached VNode. Vue skips patching that part of the DOM, saving time and resources.

   • If the dependencies have changed: resolveMemo creates a new VNode, updates the cache with the new VNode and dependency values, and returns the new VNode. Vue then patches the DOM as usual.

Important Considerations

• Shallow Comparison: The shallowCompare function (or its equivalent) performs a shallow comparison of the dependency values. This means it only compares primitive values directly. If your dependencies are objects or arrays, it only checks if the references are the same. If you need a deep comparison, you’ll have to handle it yourself. This is usually a performance killer though, so think twice about deep comparisons.

• Dependency Tracking: Vue’s reactivity system automatically tracks the dependencies used in the expression provided to v-memo. This ensures that the resolveMemo function only re-renders when the relevant dependencies actually change.

• Careful Usage: Using v-memo incorrectly can actually hurt performance. If you’re memoizing small, inexpensive components or if your dependencies change frequently, the overhead of checking the dependencies can outweigh the benefits of skipping updates. Use it strategically for expensive components that don’t need to be updated often.

• Key Difference from shouldComponentUpdate (React): v-memo is more fine-grained than React’s shouldComponentUpdate. shouldComponentUpdate applies to the entire component. v-memo applies to a specific subtree within a component. This gives you more control over what gets re-rendered.

Practical Examples and Scenarios

• Large Lists: Memoizing parts of a large list where only a few items change. For example, memoizing the rendering of each individual list item.

• Complex Components: Memoizing complex components that depend on a small set of props.

• Optimizing Animations: Memoizing parts of the DOM that are not directly involved in an animation.

Example: Memoizing a List Item

<template>
  <ul>
    <li
      v-for="item in items"
      :key="item.id"
      v-memo="[item.name, item.isActive]"
    >
      {{ item.name }} - {{ item.isActive ? 'Active' : 'Inactive' }}
    </li>
  </ul>
</template>

<script>
import { ref } from 'vue';

export default {
  setup() {
    const items = ref([
      { id: 1, name: 'Apple', isActive: true },
      { id: 2, name: 'Banana', isActive: false },
      { id: 3, name: 'Orange', isActive: true },
    ]);

    // Simulate changing an item's isActive status
    setTimeout(() => {
      items.value[1].isActive = true;
    }, 2000);

    return {
      items,
    };
  },
};
</script>

In this example, each list item will only re-render if its name or isActive property changes. If we change isActive of the second item, only the second li will be updated. The rest will be skipped.

Table Summarizing Key Concepts

Concept	Description
v-memo	A directive that allows you to skip updates to a VNode subtree if specific dependencies haven’t changed.
Compilation	The Vue compiler transforms the v-memo directive into a call to a runtime helper function.
Runtime	The runtime helper function evaluates the dependencies, compares them to previous values, and either returns a cached VNode or creates a new one.
Memoization Cache	A cache stored on the component instance that holds the cached VNodes and their corresponding dependency values.
Shallow Comparison	A comparison method used to determine if the dependency values have changed. It only compares primitive values directly or checks object/array references.
Dependencies	The values that the v-memo directive depends on. If these values change, the VNode subtree will be updated.
Performance	Using v-memo can improve performance by reducing unnecessary DOM updates, but it’s important to use it strategically and avoid over-memoizing small, inexpensive components.

Common Pitfalls

• Incorrect Dependencies: If you don’t include all the necessary dependencies in the v-memo expression, your component might not update correctly.

• Over-Memoization: Memoizing too much can actually hurt performance. The overhead of checking the dependencies can outweigh the benefits of skipping updates if the components are cheap to render.

• Deep Object Comparisons: Avoid deep object comparisons in the v-memo expression. They can be very expensive. If you need to compare objects, consider using a computed property to derive a simple value that represents the object’s state.

Conclusion

v-memo is a powerful tool for optimizing Vue 3 applications by reducing unnecessary DOM updates. By understanding how it works during compilation and runtime, you can use it effectively to improve the performance of your components. Remember to use it strategically, and always consider the trade-offs between memoization and the overhead of dependency checking.

Now go forth and memoize…responsibly! Any questions?