Vue 源码阅读（三）Special Attributes

Special Attributes

包括以下：key ref slot v-*

key

https://vuejs.org/v2/api/#key

The key special attribute is primarily used as a hint for Vue’s virtual DOM algorithm to identify VNodes when diffing the new list of nodes against the old list. Without keys, Vue uses an algorithm that minimizes element movement and tries to patch/reuse elements of the same type in-place as much as possible. With keys, it will reorder elements based on the order change of keys, and elements with keys that are no longer present will always be removed/destroyed.

Children of the same common parent must have unique keys. Duplicate keys will cause render errors.

源码

src/compiler/parser/index.js

function processKey (el) {
  const exp = getBindingAttr(el, 'key')
  if (exp) {
    if (process.env.NODE_ENV !== 'production' && el.tag === 'template') {
      warn(`<template> cannot be keyed. Place the key on real elements instead.`)
    }
    el.key = exp
  }
}

parseHTML(template, {
    warn,
    expectHTML: options.expectHTML,
    isUnaryTag: options.isUnaryTag,
    shouldDecodeNewlines: options.shouldDecodeNewlines,
    start (tag, attrs, unary) {
        ...
          if (inVPre) {
            processRawAttrs(element)
          } else {
            processFor(element)
            processIf(element)
            processOnce(element)
            processKey(element)
        
            // determine whether this is a plain element after
            // removing structural attributes
            element.plain = !element.key && !attrs.length
        
            processRef(element)
            processSlot(element)
            processComponent(element)
            for (let i = 0; i < transforms.length; i++) {
              transforms[i](element, options)
            }
            processAttrs(element)
          }
        ...
    }   
)

如何获取key值

{
   mounted() {
       const key = this.vnode.key
   }
}