init_vue.js

// vue constructor
function Vue (options) {
  //...
  this._init(options)
}

Vue.prototype._init = function (options?: Object) {
    const vm: Component = this
    // a uid
    vm._uid = uid++

    // ...
  
    // a flag to avoid this being observed
    vm._isVue = true
    // merge options
    if (options && options._isComponent) {
      // optimize internal component instantiation
      // since dynamic options merging is pretty slow, and none of the
      // internal component options needs special treatment.
      initInternalComponent(vm, options)
    } else {
      vm.$options = mergeOptions(
        resolveConstructorOptions(vm.constructor),
        options || {},
        vm
      )
    }
    /* istanbul ignore else */
    if (process.env.NODE_ENV !== 'production') {
      initProxy(vm)
    } else {
      vm._renderProxy = vm
    }
    // expose real self
    vm._self = vm
    initLifecycle(vm)
    initEvents(vm)
    initRender(vm)
    callHook(vm, 'beforeCreate')
    initInjections(vm) // resolve injections before data/props
    // 初始化props、methods、data、computed与watch
    initState(vm)
    initProvide(vm) // resolve provide after data/props
    callHook(vm, 'created')

    if (vm.$options.el) {
      // 编译过程
      vm.$mount(vm.$options.el)
    }
  }

// public mount method
Vue.prototype.$mount = function (el, hydrating) {
  return mountComponent(this, el, hydrating)
};

function mountComponent (vm, el, hydrating) {
  vm.$el = el;
  // ...
  callHook(vm, 'beforeMount');

  var updateComponent;  
  updateComponent = function () {
    //vm._render返回一个vnode
    //vm._update用于拿到新的vnode然后与旧的vnode进行patch操作
    vm._update(vm._render(), hydrating);
  };

  // we set this to vm._watcher inside the watcher's constructor
  // since the watcher's initial patch may call $forceUpdate (e.g. inside child
  // component's mounted hook), which relies on vm._watcher being already defined
  new Watcher(vm, updateComponent, noop, {
    before: function before () {
      if (vm._isMounted) {
        callHook(vm, 'beforeUpdate');
      }
    }
  }, true /* isRenderWatcher */);
  hydrating = false;

  // manually mounted instance, call mounted on self
  // mounted is called for render-created child components in its inserted hook
  if (vm.$vnode == null) {
    vm._isMounted = true;
    callHook(vm, 'mounted');
  }
  return vm
}

// 如果不存在render函数，则把template编译成render函数
// function render() {
//     with(this){
//         return _c('div',{attrs:{"id":"app"}},[_c('child')],1)
//     }
// }
var mount = Vue.prototype.$mount;
Vue.prototype.$mount = function (el, hydrating) {
  el = el && query(el);
  var options = this.$options;
  // resolve template/el and convert to render function
  if (!options.render) {
    var template = options.template;
    if (template) {
      if (typeof template === 'string') {
        if (template.charAt(0) === '#') {
          template = idToTemplate(template);
        }
      } else if (template.nodeType) {
        template = template.innerHTML;
      } else {
        {
          warn('invalid template option:' + template, this);
        }
        return this
      }
    } else if (el) {
      template = getOuterHTML(el);
    }
    if (template) { 
      // compile分为三步，parse, generate, optimise
      var ref = compileToFunctions(template, {
        shouldDecodeNewlines: shouldDecodeNewlines,
        shouldDecodeNewlinesForHref: shouldDecodeNewlinesForHref,
        delimiters: options.delimiters,
        comments: options.comments
      }, this);
      var render = ref.render;
      var staticRenderFns = ref.staticRenderFns;
      options.render = render;
      options.staticRenderFns = staticRenderFns;
    }
  }
  return mount.call(this, el, hydrating)
};