就是使用Vue.mixin方法（混入），其提供了一種非常靈活的方式，來分發 Vue 元件中的可複用功能。一個混入物件可以包含任意元件選項。當元件使用混入物件時，所有混入物件的選項將被“混合”進入該元件本身的選項。說白了就是給每個生命週期，函式等等中間加入一些公共邏輯。

//vue/src/core/global-api/mixin.js 

import { mergeOptions } from '../util/index'

export function initMixin (Vue: GlobalAPI) {
  Vue.mixin = function (mixin: Object) {
    
 
this.options = mergeOptions(this.options, mixin)
    return this
  }
}

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//vue/src/core/util/options.js /
/**
 * Merge two option objects into a new one.
 * Core utility used in both instantiation and inheritance.
 */
export function mergeOptions (
  parent: Object,
  child: Object,
  vm
 
?: Component
): Object {
  if (process.env.NODE_ENV !== 'production') {
    checkComponents(child)
  }

  if (typeof child === 'function') {
    child = child.options
  }

  normalizeProps(child, vm)
  normalizeInject(child, vm)
  normalizeDirectives(child)

  // Apply extends and mixins on the child options,
  
 
// but only if it is a raw options object that isn't
  // the result of another mergeOptions call.
  // Only merged options has the _base property.
  if (!child._base) {
    if (child.extends) { //遞迴合併extends
      parent = mergeOptions(parent, child.extends, vm)
    }
    if (child.mixins) { //遞迴合併mixin
      for (let i = 0, l = child.mixins.length; i < l; i++) {
        parent = mergeOptions(parent, child.mixins[i], vm)
      }
    }
  }

  const options = {} //屬性和生命週期的合併
  let key
  for (key in parent) {
    mergeField(key)
  }
  for (key in child) {
    if (!hasOwn(parent, key)) {
      mergeField(key)
    }
  }
  function mergeField (key) {
    const strat = strats[key] || defaultStrat
    options[key] = strat(parent[key], child[key], vm, key)
  }
  return options
}