Vue3 Compilation Optimization: CacheHandler Event Caching

Event caching is a crucial optimization technique during the compilation phase in Vue3. When event handlers exist in templates, the compiler employs a caching mechanism to avoid unnecessary function regeneration, thereby improving performance.

Problem Analysis
Consider this template:

<button @click="handleClick">Click</button>

In Vue2, a new handleClick function would be recreated every time the component updates, causing unnecessary re-renders of child components. Vue3 solves this problem through event caching.

Implementation Principle

Step 1: Identifying Cacheable Events
The compiler analyzes event handlers in the template:

• Normal function calls: @click="handleClick"
• Inline expressions: @click="count++"

For simple function calls, the compiler generates caching code.

Step 2: Generating Cached Code
For the template above, the compiler generates:

// Compilation result
export function render(_ctx, _cache) {
  return (_openBlock(), _createBlock("button", {
    onClick: _cache[0] || (_cache[0] = (...args) => _ctx.handleClick(...args))
  }, "Click"))
}

Step 3: Cache Mechanism Analysis

1. _cache is the cache array passed to the render function.
2. _cache[0] checks if there is a cached function at index 0.
3. If not (short-circuit by ||), a new function is created and cached in _cache[0].
4. Subsequent renders directly use the cached function, avoiding recreation.

Step 4: Handling Complex Scenarios
For inline expressions, the compiler also caches them:

<button @click="count++">Increment</button>

Compilation result:

onClick: _cache[0] || (_cache[0] = ($event) => _ctx.count++)

Step 5: Cache Index Optimization
The compiler assigns different indices to different cache positions:

<button @click="handleClick">Button 1</button>
<button @click="handleSubmit">Button 2</button>

Generated code:

onClick: _cache[0] || (_cache[0] = (...args) => _ctx.handleClick(...args)),
onSubmit: _cache[1] || (_cache[1] = (...args) => _ctx.handleSubmit(...args))

Performance Advantages

1. Reduce Function Creation Overhead: Avoid creating new functions on every render.
2. Stabilize Child Component Props: Cached functions maintain stable references, preventing unnecessary child component updates.
3. Memory Optimization: Reduce long-term memory usage.

This caching mechanism embodies Vue3's design philosophy of "compile-time optimization + runtime caching," where the compiler performs static analysis to generate optimal runtime code.