这是 Composition API 的常见疑问：
比较项computedwatch用途派生值监听副作用返回值有无（需手动触发）响应式依赖自动收集手动指定执行时机初始化不执行副作用可设 immediate
简单判断：
- 需要“值”用 computed（如格式化、筛选）
- 需要“动作”用 watch（如发请求、写缓存）
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