第一阶段:创造Vue类 /src/core/instance/index.js Vue = { prototype:{ _init, $data, $props, $set, $delete, $watch, $on, $once, $off, $emit, _update, $forceUpdate, $destroy, install runtime convenience helpers, $nextTick, _rende } } /src/core/index.js Vue = { config, util, set, delete, nextTick, observable, options, use,