CreateApp

顾名思义，CreateApp 作为 vue 的启动函数，返回一个应用实例

从一个例子开始

const HelloVueApp = {
  data() {
    return {
      message: 'Hello Vue!'
    }
  }
}

Vue.createApp(HelloVueApp).mount('#hello-vue')

亲自试一试

那么 createApp 里面都干了什么呢？我们接着往下看

export const createApp = ((...args) => {
  const app = ensureRenderer().createApp(...args)

  if (__DEV__) {
    injectNativeTagCheck(app)
  }

  const { mount } = app
  app.mount = (containerOrSelector: Element | string): any => {
    const container = normalizeContainer(containerOrSelector)
    if (!container) return
    const component = app._component
    if (!isFunction(component) && !component.render && !component.template) {
      component.template = container.innerHTML
    }
    // clear content before mounting
    container.innerHTML = ''
    const proxy = mount(container)
    container.removeAttribute('v-cloak')
    return proxy
  }

  return app
}) as CreateAppFunction<Element>

我们可以看到重点在于 ensureRenderer ，

const rendererOptions = {
  patchProp,  // 处理 props 属性 
  ...nodeOps // 处理 DOM 节点操作
}

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

let enabledHydration = false

function ensureRenderer() {
  return renderer || (renderer = createRenderer(rendererOptions))
}

调用 createRenderer

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

function baseCreateRenderer(
  options: RendererOptions,
  createHydrationFns?: typeof createHydrationFunctions
): any {
  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

看了 baseCreateRenderer 这个函数，再看 createAppAPI 就会觉得太轻松了。。。毕竟不是一个量级的

createAppAPI 首先判断

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
    const context = createAppContext()
    const installedPlugins = new Set()

    let isMounted = false

    const app: App = {
      _component: rootComponent as Component,
      _props: rootProps,
      _container: null,
      _context: context,

      get config() {
        return context.config
      },

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

      // 都是一些眼熟的方法
      use() {},
      mixin() {},
      component() {},
      directive() {},

      // mount 我们拎出来讲
      mount() {},
      unmount() {},
      // ...
    }

    return app
  }
}

createAppContext 实现

export function createAppContext(): AppContext {
  return {
    config: {
      isNativeTag: NO,
      devtools: true,
      performance: false,
      globalProperties: {},
      optionMergeStrategies: {},
      isCustomElement: NO,
      errorHandler: undefined,
      warnHandler: undefined
    },
    mixins: [],
    components: {},
    directives: {},
    provides: Object.create(null)
  }
}

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，不过不重要 ，这个篇幅我们只需要createApp在主流程做了什么了就好

你可能会有很多疑问，比如:

模板是怎么编绎的?
生命周期是怎么挂载的?
组件是怎么注册的?
响应式怎么做到的?

我们先记着，后面有篇幅单独拎出来