Vue中Slot的渲染过程

Vue在通过compiler解析模版中的slot, slot是组件中的插槽，通过解析slot，把slot的name属性值赋值给元素的slotName，作用是知道需要渲染哪些组件，后面会通过slotName获取要渲染的内容

function processSlotOutlet (el) {
  if (el.tag === 'slot') {
    el.slotName = getBindingAttr(el, 'name')
    if (process.env.NODE_ENV !== 'production' && el.key) {
      warn(
        `\`key\` does not work on <slot> because slots are abstract outlets ` +
        `and can possibly expand into multiple elements. ` +
        `Use the key on a wrapping element instead.`,
        getRawBindingAttr(el, 'key')
      )
    }
  }
}

下面的代码省略了部分，作用是解析插槽的真正内容，获取slotTarget (插槽名)、slotTargetDynamic（插槽名是否是动态的）、slotScope(传递到插槽组件中的属性).获取的这些属性赋值给元素，方便生成代码的时候使用

function processSlotContent (el) {
    if (el.tag === 'template') {
      // v-slot on <template>
      const slotBinding = getAndRemoveAttrByRegex(el, slotRE)
      if (slotBinding) {
        if (process.env.NODE_ENV !== 'production') {
          if (el.slotTarget || el.slotScope) {
            warn(
              `Unexpected mixed usage of different slot syntaxes.`,
              el
            )
          }
          if (el.parent && !maybeComponent(el.parent)) {
            warn(
              `<template v-slot> can only appear at the root level inside ` +
              `the receiving component`,
              el
            )
          }
        }
        const { name, dynamic } = getSlotName(slotBinding)
        el.slotTarget = name
        el.slotTargetDynamic = dynamic
        el.slotScope = slotBinding.value || emptySlotScopeToken // force it into a scoped slot for perf
      }
    } else {
      // v-slot on component, denotes default slot
      const slotBinding = getAndRemoveAttrByRegex(el, slotRE)
      if (slotBinding) {
        if (process.env.NODE_ENV !== 'production') {
          if (!maybeComponent(el)) {
            warn(
              `v-slot can only be used on components or <template>.`,
              slotBinding
            )
          }
          if (el.slotScope || el.slotTarget) {
            warn(
              `Unexpected mixed usage of different slot syntaxes.`,
              el
            )
          }
          if (el.scopedSlots) {
            warn(
              `To avoid scope ambiguity, the default slot should also use ` +
              `<template> syntax when there are other named slots.`,
              slotBinding
            )
          }
        }
        // add the component's children to its default slot
        const slots = el.scopedSlots || (el.scopedSlots = {})
        const { name, dynamic } = getSlotName(slotBinding)
        const slotContainer = slots[name] = createASTElement('template', [], el)
        slotContainer.slotTarget = name
        slotContainer.slotTargetDynamic = dynamic
        slotContainer.children = el.children.filter((c: any) => {
          if (!c.slotScope) {
            c.parent = slotContainer
            return true
          }
        })
        slotContainer.slotScope = slotBinding.value || emptySlotScopeToken
        // remove children as they are returned from scopedSlots now
        el.children = []
        // mark el non-plain so data gets generated
        el.plain = false
      }
    }

}

接下来看compiler的生成代码部分，_t就是renderSlot（渲染slot生成node），可以查看core/instance/render-helpers/index.js的文件

function genSlot (el: ASTElement, state: CodegenState): string {
  const slotName = el.slotName || '"default"'
  const children = genChildren(el, state)
  let res = `_t(${slotName}${children ? `,function(){return ${children}}` : ''}`
  const attrs = el.attrs || el.dynamicAttrs
    ? genProps((el.attrs || []).concat(el.dynamicAttrs || []).map(attr => ({
        // slot props are camelized
        name: camelize(attr.name),
        value: attr.value,
        dynamic: attr.dynamic
      })))
    : null
  const bind = el.attrsMap['v-bind']
  if ((attrs || bind) && !children) {
    res += `,null`
  }
  if (attrs) {
    res += `,${attrs}`
  }
  if (bind) {
    res += `${attrs ? '' : ',null'},${bind}`
  }
  return res + ')'
}

renderSlot的作用就是把插槽渲染成node, 方法中的$scopedSlots是一个包含slot的名字和对应的渲染方法或者节点的对象，渲染的对象是slot对应的组件。scopedSlotFn方法不存在的时候，渲染插槽的默认内容

export function renderSlot (
  name: string,
  fallbackRender: ?((() => Array<VNode>) | Array<VNode>),
  props: ?Object,
  bindObject: ?Object
): ?Array<VNode> {
  const scopedSlotFn = this.$scopedSlots[name]
  let nodes
  if (scopedSlotFn) {
    // scoped slot
    props = props || {}
    if (bindObject) {
      if (process.env.NODE_ENV !== 'production' && !isObject(bindObject)) {
        warn('slot v-bind without argument expects an Object', this)
      }
      props = extend(extend({}, bindObject), props)
    }
    nodes =
      scopedSlotFn(props) ||
      (typeof fallbackRender === 'function' ? fallbackRender() : fallbackRender)
  } else {
    nodes =
      this.$slots[name] ||
      (typeof fallbackRender === 'function' ? fallbackRender() : fallbackRender)
  }

  const target = props && props.slot
  if (target) {
    return this.$createElement('template', { slot: target }, nodes)
  } else {
    return nodes
  }
}

slots是当前组件需要的插槽，通过组件的children得到。

  Vue.prototype._render = function (): VNode {
    const vm: Component = this
    const { render, _parentVnode } = vm.$options

    if (_parentVnode) {
      vm.$scopedSlots = normalizeScopedSlots(
        _parentVnode.data.scopedSlots,
        vm.$slots,
        vm.$scopedSlots
      )
    }

    // set parent vnode. this allows render functions to have access
    // to the data on the placeholder node.
    vm.$vnode = _parentVnode
    // render self
    let vnode
    try {
      // There's no need to maintain a stack because all render fns are called
      // separately from one another. Nested component's render fns are called
      // when parent component is patched.
      currentRenderingInstance = vm
      vnode = render.call(vm._renderProxy, vm.$createElement)
    } catch (e) {
      handleError(e, vm, `render`)
      // return error render result,
      // or previous vnode to prevent render error causing blank component
      /* istanbul ignore else */
      if (process.env.NODE_ENV !== 'production' && vm.$options.renderError) {
        try {
          vnode = vm.$options.renderError.call(vm._renderProxy, vm.$createElement, e)
        } catch (e) {
          handleError(e, vm, `renderError`)
          vnode = vm._vnode
        }
      } else {
        vnode = vm._vnode
      }
    } finally {
      currentRenderingInstance = null
    }
    // if the returned array contains only a single node, allow it
    if (Array.isArray(vnode) && vnode.length === 1) {
      vnode = vnode[0]
    }
    // return empty vnode in case the render function errored out
    if (!(vnode instanceof VNode)) {
      if (process.env.NODE_ENV !== 'production' && Array.isArray(vnode)) {
        warn(
          'Multiple root nodes returned from render function. Render function ' +
          'should return a single root node.',
          vm
        )
      }
      vnode = createEmptyVNode()
    }
    // set parent
    vnode.parent = _parentVnode
    return vnode
  }