vue-v-model

v-model是怎么实现双向绑定的？
看源码：
src/platforms/web/compiler/directives/model.js

if (el.component) {
    genComponentModel(el, value, modifiers)
    // component v-model doesn't need extra runtime
    return false
  } else if (tag === 'select') {
    genSelect(el, value, modifiers)
  } else if (tag === 'input' && type === 'checkbox') {
    genCheckboxModel(el, value, modifiers)
  } else if (tag === 'input' && type === 'radio') {
    genRadioModel(el, value, modifiers)
  } else if (tag === 'input' || tag === 'textarea') {
    genDefaultModel(el, value, modifiers)
  } else if (!config.isReservedTag(tag)) {
    genComponentModel(el, value, modifiers)
    // component v-model doesn't need extra runtime
    return false

以上，有分场景
自定义组件上的：
select：change事件

function genSelect (
  el: ASTElement,
  value: string,
  modifiers: ?ASTModifiers
) {
  const number = modifiers && modifiers.number
  const selectedVal = `Array.prototype.filter` +
    `.call($event.target.options,function(o){return o.selected})` +
    `.map(function(o){var val = "_value" in o ? o._value : o.value;` +
    `return ${number ? '_n(val)' : 'val'}})`

  const assignment = '$event.target.multiple ? $$selectedVal : $$selectedVal[0]'
  let code = `var $$selectedVal = ${selectedVal};`
  code = `${code} ${genAssignmentCode(value, assignment)}`
  addHandler(el, 'change', code, null, true)
}

input-checkbox：checked + change

function genCheckboxModel (
  el: ASTElement,
  value: string,
  modifiers: ?ASTModifiers
) {
  const number = modifiers && modifiers.number
  const valueBinding = getBindingAttr(el, 'value') || 'null'
  const trueValueBinding = getBindingAttr(el, 'true-value') || 'true'
  const falseValueBinding = getBindingAttr(el, 'false-value') || 'false'
  addProp(el, 'checked',
    `Array.isArray(${value})` +
    `?_i(${value},${valueBinding})>-1` + (
      trueValueBinding === 'true'
        ? `:(${value})`
        : `:_q(${value},${trueValueBinding})`
    )
  )
  addHandler(el, 'change',
    `var $$a=${value},` +
        '$$el=$event.target,' +
        `$$c=$$el.checked?(${trueValueBinding}):(${falseValueBinding});` +
    'if(Array.isArray($$a)){' +
      `var $$v=${number ? '_n(' + valueBinding + ')' : valueBinding},` +
          '$$i=_i($$a,$$v);' +
      `if($$el.checked){$$i<0&&(${genAssignmentCode(value, '$$a.concat([$$v])')})}` +
      `else{$$i>-1&&(${genAssignmentCode(value, '$$a.slice(0,$$i).concat($$a.slice($$i+1))')})}` +
    `}else{${genAssignmentCode(value, '$$c')}}`,
    null, true
  )
}

input-radio： checked + change

function genRadioModel (
  el: ASTElement,
  value: string,
  modifiers: ?ASTModifiers
) {
  const number = modifiers && modifiers.number
  let valueBinding = getBindingAttr(el, 'value') || 'null'
  valueBinding = number ? `_n(${valueBinding})` : valueBinding
  addProp(el, 'checked', `_q(${value},${valueBinding})`)
  addHandler(el, 'change', genAssignmentCode(value, valueBinding), null, true)
}

input-text | textarea：value + (input | change(设置了lazy))

function genDefaultModel (
  el: ASTElement,
  value: string,
  modifiers: ?ASTModifiers
): ?boolean {
  const type = el.attrsMap.type

  // warn if v-bind:value conflicts with v-model
  // except for inputs with v-bind:type
  if (process.env.NODE_ENV !== 'production') {
    const value = el.attrsMap['v-bind:value'] || el.attrsMap[':value']
    const typeBinding = el.attrsMap['v-bind:type'] || el.attrsMap[':type']
    if (value && !typeBinding) {
      const binding = el.attrsMap['v-bind:value'] ? 'v-bind:value' : ':value'
      warn(
        `${binding}="${value}" conflicts with v-model on the same element ` +
        'because the latter already expands to a value binding internally',
        el.rawAttrsMap[binding]
      )
    }
  }

  const { lazy, number, trim } = modifiers || {}
  const needCompositionGuard = !lazy && type !== 'range'
  const event = lazy
    ? 'change'
    : type === 'range'
      ? RANGE_TOKEN
      : 'input'

  let valueExpression = '$event.target.value'
  if (trim) {
    valueExpression = `$event.target.value.trim()`
  }
  if (number) {
    valueExpression = `_n(${valueExpression})`
  }

  let code = genAssignmentCode(value, valueExpression)
  if (needCompositionGuard) {
    code = `if($event.target.composing)return;${code}`
  }

  addProp(el, 'value', `(${value})`)
  addHandler(el, event, code, null, true)
  if (trim || number) {
    addHandler(el, 'blur', '$forceUpdate()')
  }
}