Vue 组件上的v-model双向绑定原理解析

export default function model (
  el: ASTElement,
  dir: ASTDirective,
  _warn: Function
): ?boolean {
  warn = _warn
  // 解析指令对象的值、修饰符、标签、类型
  const value = dir.value
  const modifiers = dir.modifiers
  const tag = el.tag
  const type = el.attrsMap.type
  ......
  } else if (tag === 'input' || tag === 'textarea') {
    // 本案例进入这个逻辑，我们分析一下
    genDefaultModel(el, value, modifiers)
  } else if (!config.isReservedTag(tag)) {
    // 非保留标签，说明是组件节点，执行genComponentModel
    genComponentModel(el, value, modifiers)
    // component v-model doesn't need extra runtime
    return false
  } 
  ......
  
  return true
}

export function genComponentModel (
  el: ASTElement,
  value: string,
  modifiers: ?ASTModifiers
): ?boolean {
  // 解析修饰符
  const { number, trim } = modifiers || {}

  const baseValueExpression = '$$v'
  let valueExpression = baseValueExpression
  
  // 有trim修饰符，进入下面逻辑，生成value表达式
  if (trim) {
    valueExpression =
      `(typeof ${baseValueExpression} === 'string'` +
      `? ${baseValueExpression}.trim()` +
      `: ${baseValueExpression})`
  }
  // 有number修饰符，生成下面表达式
  if (number) {
    valueExpression = `_n(${valueExpression})`
  }
  // 解析 value，生成解析规范后的表达式
  const assignment = genAssignmentCode(value, valueExpression)

  // AST element上挂载model对象
  el.model = {
    value: `(${value})`,
    expression: `"${value}"`,
    callback: `function (${baseValueExpression}) {${assignment}}`
  }
}

export function genData (el: ASTElement, state: CodegenState): string {
  let data = '{'

  // directives may mutate the el's other properties before they are generated.
  // 解析model指令
  const dirs = genDirectives(el, state)
  if (dirs) data += dirs + ','

  ......
  
  // component v-model
  // 组件上的v-model，进入该逻辑，拼接生成下面代码字符串
  if (el.model) {
    data += `model:{value:${
      el.model.value
    },callback:${
      el.model.callback
    },expression:${
      el.model.expression
    }},`
  }

export function createComponent (
 Ctor: Class<Component> | Function | Object | void,
 data: ?VNodeData,
 context: Component,
 children: ?Array<VNode>,
 tag?: string
): VNode | Array<VNode> | void {
 // ...
 // transform component v-model data into props & events
 // 如果data上有model属性
 if (isDef(data.model)) {
   // 调用transformModel，传入的参数为组件构造器的options配置项
   transformModel(Ctor.options, data)
 }

 // ...
 const vnode = new VNode(
   `vue-component-${Ctor.cid}${name ? `-${name}` : ''}`,
   data, undefined, undefined, undefined, context,
   { Ctor, propsData, listeners, tag, children },
   asyncFactory
 )
 
 return vnode
}

// transform component v-model info (value and callback) into
// prop and event handler respectively.
function transformModel (options, data: any) {
  // 找到model上的prop属性
  const prop = (options.model && options.model.prop) || 'value'
  // 找到model上的event事件
  const event = (options.model && options.model.event) || 'input'
  // 在data的props属性对象上添加prop属性值
  ;(data.props || (data.props = {}))[prop] = data.model.value
  // 在data的on属性对象上添加event事件
  const on = data.on || (data.on = {})
  if (isDef(on[event])) {
    on[event] = [data.model.callback].concat(on[event])
  } else {
    on[event] = data.model.callback
  }
}