mounted

• app.mounted /packages/runtime-core/src/apiCreateApp

   export function createAppAPI<HostElement>(
  render: RootRenderFunction<HostElement>,  // baseCreateRenderer 传进来的 render方法
  hydrate?: RootHydrateFunction,
): CreateAppFunction<HostElement> {
  return function createApp(rootComponent, rootProps = null) {
    // 处理根组件
    if (!isFunction(rootComponent)) {
      rootComponent = extend({}, rootComponent)
    }

    if (rootProps != null && !isObject(rootProps)) {
      __DEV__ && warn(`root props passed to app.mount() must be an object.`)
      rootProps = null
    }
    // 创建App上下文
    const context = createAppContext()

    // 已安装的插件
    const installedPlugins = new WeakSet()

    let isMounted = false

    // 创建app对象
    const app: App = (context.app = {
      _uid: uid++,
      _component: rootComponent as ConcreteComponent,
      _props: rootProps,
      _container: null,
      _context: context, // 这里是循环引用，暂时不清楚意图
      _instance: null,

      version,

      get config() {
        return context.config
      },

      set config(v) {
        if (__DEV__) {
          warn(
            `app.config cannot be replaced. Modify individual options instead.`,
          )
        }
      },
     // 中间件
      use(plugin: Plugin, ...options: any[]) {
        if (installedPlugins.has(plugin)) {
          __DEV__ && warn(`Plugin has already been applied to target app.`)
        } else if (plugin && isFunction(plugin.install)) {
          installedPlugins.add(plugin)
          plugin.install(app, ...options)
        } else if (isFunction(plugin)) {
          installedPlugins.add(plugin)
          plugin(app, ...options)
        } else if (__DEV__) {
          warn(
            `A plugin must either be a function or an object with an "install" ` +
              `function.`,
          )
        }
        return app
      },
      // 混入
      mixin(mixin: ComponentOptions) {
        if (__FEATURE_OPTIONS_API__) {
          if (!context.mixins.includes(mixin)) {
            context.mixins.push(mixin)
          } else if (__DEV__) {
            warn(
              'Mixin has already been applied to target app' +
                (mixin.name ? `: ${mixin.name}` : ''),
            )
          }
        } else if (__DEV__) {
          warn('Mixins are only available in builds supporting Options API')
        }
        return app
      },
      // 注册组件
      component(name: string, component?: Component): any {
        if (__DEV__) {
          validateComponentName(name, context.config)
        }
        if (!component) {
          return context.components[name]
        }
        if (__DEV__ && context.components[name]) {
          warn(`Component "${name}" has already been registered in target app.`)
        }
        context.components[name] = component
        return app
      },
      // 注册指令
      directive(name: string, directive?: Directive) {
        if (__DEV__) {
          validateDirectiveName(name)
        }

        if (!directive) {
          return context.directives[name] as any
        }
        if (__DEV__ && context.directives[name]) {
          warn(`Directive "${name}" has already been registered in target app.`)
        }
        context.directives[name] = directive
        return app
      },
      // 挂载
      mount(
        rootContainer: HostElement,
        isHydrate?: boolean,
        namespace?: boolean | ElementNamespace,
      ): any {
        if (!isMounted) {
          // #5571
          if (__DEV__ && (rootContainer as any).__vue_app__) {
            warn(
              `There is already an app instance mounted on the host container.\n` +
                ` If you want to mount another app on the same host container,` +
                ` you need to unmount the previous app by calling \`app.unmount()\` first.`,
            )
          }
          // 使用根组件创建vnode
          const vnode = createVNode(rootComponent, rootProps)
          // store app context on the root VNode.
          // this will be set on the root instance on initial mount.
          /* 
          在root VNode上存储app context。这将在初始mount时在root实例上设置。
          */
          vnode.appContext = context

          if (namespace === true) {
            namespace = 'svg'
          } else if (namespace === false) {
            namespace = undefined
          }

          // HMR root reload
          if (__DEV__) {
            context.reload = () => {
              // casting to ElementNamespace because TS doesn't guarantee type narrowing
              // over function boundaries
              /* 
              强制转换为ElementNamespace，因为TS不能保证类型原型越过函数范围
              */
              render(
                cloneVNode(vnode),
                rootContainer,
                namespace as ElementNamespace,
              )
            }
          }

          if (isHydrate && hydrate) {
            hydrate(vnode as VNode<Node, Element>, rootContainer as any)
          } else {
            // 渲染vnode
            render(vnode, rootContainer, namespace)
          }
          isMounted = true
          // 这里有一个循环引用，暂时不清楚意图
          app._container = rootContainer
          // for devtools and telemetry
          ;(rootContainer as any).__vue_app__ = app

          if (__DEV__ || __FEATURE_PROD_DEVTOOLS__) {
            app._instance = vnode.component
            // 初始化devtools
            devtoolsInitApp(app, version)
          }

          return getExposeProxy(vnode.component!) || vnode.component!.proxy
        } else if (__DEV__) {
          warn(
            `App has already been mounted.\n` +
              `If you want to remount the same app, move your app creation logic ` +
              `into a factory function and create fresh app instances for each ` +
              `mount - e.g. \`const createMyApp = () => createApp(App)\``,
          )
        }
      },
     // 卸载
      unmount() {
        if (isMounted) {
          render(null, app._container)
          if (__DEV__ || __FEATURE_PROD_DEVTOOLS__) {
            app._instance = null
            devtoolsUnmountApp(app)
          }
          delete app._container.__vue_app__
        } else if (__DEV__) {
          warn(`Cannot unmount an app that is not mounted.`)
        }
      },

      provide(key, value) {
        if (__DEV__ && (key as string | symbol) in context.provides) {
          warn(
            `App already provides property with key "${String(key)}". ` +
              `It will be overwritten with the new value.`,
          )
        }

        context.provides[key as string | symbol] = value

        return app
      },

      runWithContext(fn) {
        currentApp = app
        try {
          return fn()
        } finally {
          currentApp = null
        }
      },
    })

    if (__COMPAT__) {
      installAppCompatProperties(app, context, render)
    }

    return app
  }
}

整体代码不算复杂，主要核心是render函数，后面和createVnode一起分析