【VUE3源码学习】Vue3在CreateApp时，做了什么？

写在前面

每个 Vue 应用都是通过用
createApp
函数创建一个新的应用实例开始的，一般在Vue项目中
main.js
进行实例化：

const app = Vue.createApp({
  /* 选项 */
  data() {
      return { count: 4 }
  }
})
app.use(store).use(router).mount('#app')

传递给 createApp 的选项用于配置根组件。当我们挂载应用时，该组件被用作渲染的起点。

那么
createApp
在实例化过程中里面都干了什么呢？

先看一下
createApp
函数源码：

// \packages\runtime-dom\src\index.ts

export const createApp = ((...args) => {
  // 变量app由ensureRenderer生成
  const app = ensureRenderer().createApp(...args)

  if (__DEV__) {
    injectNativeTagCheck(app)
    injectCompilerOptionsCheck(app)
  }

  const { mount } = app
  app.mount = (containerOrSelector: Element | ShadowRoot | string): any => {
    const container = normalizeContainer(containerOrSelector)
    if (!container) return

    const component = app._component
    if (!isFunction(component) && !component.render && !component.template) {
      // __UNSAFE__
      // Reason: potential execution of JS expressions in in-DOM template.
      // The user must make sure the in-DOM template is trusted. If it's
      // rendered by the server, the template should not contain any user data.
      component.template = container.innerHTML
      // 2.x compat check
      if (__COMPAT__ && __DEV__) {
        for (let i = 0; i < container.attributes.length; i++) {
          const attr = container.attributes[i]
          if (attr.name !== 'v-cloak' && /^(v-|:|@)/.test(attr.name)) {
            compatUtils.warnDeprecation(
              DeprecationTypes.GLOBAL_MOUNT_CONTAINER,
              null
            )
            break
          }
        }
      }
    }

    // clear content before mounting
    container.innerHTML = ''
    const proxy = mount(container, false, container instanceof SVGElement)
    if (container instanceof Element) {
      container.removeAttribute('v-cloak')
      container.setAttribute('data-v-app', '')
    }
    return proxy
  }

  return app
}) as CreateAppFunction<Element>

我们可以看到重点在于
ensureRenderer
，

// \packages\runtime-dom\src\index.ts

/* patchProp: 处理匹配的props属性 
*  nodeOps: DOM 节操作
*/
const rendererOptions = extend({ patchProp }, nodeOps)

// lazy create the renderer - this makes core renderer logic tree-shakable
// in case the user only imports reactivity utilities from Vue.
let renderer: Renderer<Element | ShadowRoot> | HydrationRenderer

let enabledHydration = false

function ensureRenderer() {
  return (
    renderer ||
    (renderer = createRenderer<Node, Element | ShadowRoot>(rendererOptions))
  )
}

调用
createRenderer

// packages\runtime-core\src\renderer.ts 305行

export function createRenderer<
  HostNode = RendererNode,
  HostElement = RendererElement
>(options: RendererOptions<HostNode, HostElement>) {
  return baseCreateRenderer<HostNode, HostElement>(options)
}

调用
baseCreateRenderer
，·baseCreateRenderer·这个函数简直可以用庞大来形容，
vnode
diff
patch
均在这个方法中实现，回头我们再来细看实现，现在我们只需要关心他最后返回的什么

// packages\runtime-core\src\renderer.ts  322~2351行

// overload 1: no hydration
function baseCreateRenderer<
  HostNode = RendererNode,
  HostElement = RendererElement
>(options: RendererOptions<HostNode, HostElement>): Renderer<HostElement>

// overload 2: with hydration
function baseCreateRenderer(
  options: RendererOptions<Node, Element>,
  createHydrationFns: typeof createHydrationFunctions
): HydrationRenderer

// implementation
function baseCreateRenderer(
  options: RendererOptions,
  createHydrationFns?: typeof createHydrationFunctions
): any {
  // compile-time feature flags check
  if (__ESM_BUNDLER__ && !__TEST__) {
    initFeatureFlags()
  }

  const target = getGlobalThis()
  target.__VUE__ = true
  if (__DEV__ || __FEATURE_PROD_DEVTOOLS__) {
    setDevtoolsHook(target.__VUE_DEVTOOLS_GLOBAL_HOOK__, target)
  }

  const {
    insert: hostInsert,
    remove: hostRemove,
    patchProp: hostPatchProp,
    createElement: hostCreateElement,
    createText: hostCreateText,
    createComment: hostCreateComment,
    setText: hostSetText,
    setElementText: hostSetElementText,
    parentNode: hostParentNode,
    nextSibling: hostNextSibling,
    setScopeId: hostSetScopeId = NOOP,
    cloneNode: hostCloneNode,
    insertStaticContent: hostInsertStaticContent
  } = options

  // ....此处省略近两千行，我们先不管

  return {
    render,
    hydrate,
    createApp: createAppAPI(render, hydrate)
  }

从源码中我们看到
baseCreateRenderer
最终返回
render
hydrate
createApp
3个函数, 但在
createApp
这个函数中我们本质上只需要返回
createApp
这个函数就好，这里返回了3个，说明其它两个会在别处用到，具体哪里能用到，后面我们再回头看

接着将生成的
render
传给
createAppAPI
这个真正的
createApp
方法，
hydrate
为可选参数，ssr 的场景下会用到，这边我们也先跳过

// packages\runtime-core\src\apiCreateApp.ts 177行

export function createAppAPI<HostElement>(
  render: RootRenderFunction,
  hydrate?: RootHydrateFunction
): CreateAppFunction<HostElement> {
  return function createApp(rootComponent, rootProps = null) {
    if (rootProps != null && !isObject(rootProps)) {
      __DEV__ && warn(`root props passed to app.mount() must be an object.`)
      rootProps = null
    }

	// 创建默认APP配置context
    const context = createAppContext()
    const installedPlugins = new Set()

    let isMounted = false

    const app: App = (context.app = {
      _uid: uid++,
      _component: rootComponent as ConcreteComponent, // 根节点组件
      _props: rootProps, // 根节点的props
      _container: null,
      _context: context,
      _instance: null,

      version,

      get config() {
        return context.config
      },

      set config(v) {
        if (__DEV__) {
          warn(
            `app.config cannot be replaced. Modify individual options instead.`
          )
        }
      },

	  /* 应用实例暴露的大多数方法
	   * use() mixin() component() directive() mount() unmount() provide()
	  */
	  
      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
      },

      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
      },

      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
      },

      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
      },
	  
	  // 实例挂载到DOM方法
      mount(
        rootContainer: HostElement,
        isHydrate?: boolean,
        isSVG?: boolean
      ): any {
        if (!isMounted) {
          const vnode = createVNode(
            rootComponent as ConcreteComponent,
            rootProps
          )
          // store app context on the root VNode.
          // this will be set on the root instance on initial mount.
          vnode.appContext = context

          // HMR root reload
          if (__DEV__) {
            context.reload = () => {
              render(cloneVNode(vnode), rootContainer, isSVG)
            }
          }

          if (isHydrate && hydrate) {
            hydrate(vnode as VNode<Node, Element>, rootContainer as any)
          } else {
            render(vnode, rootContainer, isSVG)
          }
          isMounted = true
          app._container = rootContainer
          // for devtools and telemetry
          ;(rootContainer as any).__vue_app__ = app

          if (__DEV__ || __FEATURE_PROD_DEVTOOLS__) {
            app._instance = vnode.component
            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)\``
          )
        }
      },

      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.`
          )
        }
        // TypeScript doesn't allow symbols as index type
        // https://github.com/Microsoft/TypeScript/issues/24587
        context.provides[key as string] = value

        return app
      }
    })

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

    return app
  }
}

createAppContext
实现：返回默认
AppContent
对象

// packages\runtime-core\src\apiCreateApp.ts 148行

export function createAppContext(): AppContext {
  return {
    app: null as any,
    config: {
      isNativeTag: NO,
      performance: false,
      globalProperties: {},
      optionMergeStrategies: {},
      errorHandler: undefined,
      warnHandler: undefined,
      compilerOptions: {}
    },
    mixins: [],
    components: {},
    directives: {},
    provides: Object.create(null),
    optionsCache: new WeakMap(),
    propsCache: new WeakMap(),
    emitsCache: new WeakMap()
  }
}

到这里，整个
createApp
流程就结束了，在整个环节中，我们故意忽略了很多细节，源码学习第一步，先熟悉实例化流程，后面继续细化了解。

原文链接：https://blog.csdn.net/qq_38987146/article/details/122843445

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