Vue3源码解析之 compiler（三）

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本篇是 Vue3 源码解析系列第 21 篇，关注专栏

前言

前面我们分别分析了 compiler 编译器中 ast 对象、transform 函数，接下来我们就来分析下编译器的最后一部分 generate 函数，该函数主要将 AST 转换为 render 函数。

案例

首先引入 compile、h、render 三个函数，声明了一个 template 模板，通过 compile 函数将模板编译成 render 函数，之后创建一个 component 组件，再通过 h 函数生成 vnode 对象，最后执行 render 函数进行渲染。

html 复制代码

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <script src="../../../dist/vue.global.js"></script>
  </head>
  <body>
    <div id="app"></div>
    <script>
      const { compile, h, render } = Vue

      const template = `<div>hello world</div>`
      
      const renderFn = compile(template)

      const component = {
        render: renderFn
      }

      const vnode = h(component)

      render(vnode, document.querySelector('#app'))
    </script>
  </body>
</html>

Generate 函数

generate 函数定义在 packages/compiler-core/src/codegen.ts 文件中：

ts 复制代码

export function generate(
  ast: RootNode,
  options: CodegenOptions & {
    onContextCreated?: (context: CodegenContext) => void
  } = {}
): CodegenResult {
  const context = createCodegenContext(ast, options)
  if (options.onContextCreated) options.onContextCreated(context)
  const {
    mode,
    push,
    prefixIdentifiers,
    indent,
    deindent,
    newline,
    scopeId,
    ssr
  } = context

  const hasHelpers = ast.helpers.length > 0
  const useWithBlock = !prefixIdentifiers && mode !== 'module'
  const genScopeId = !__BROWSER__ && scopeId != null && mode === 'module'
  const isSetupInlined = !__BROWSER__ && !!options.inline

  // preambles
  // in setup() inline mode, the preamble is generated in a sub context
  // and returned separately.
  const preambleContext = isSetupInlined
    ? createCodegenContext(ast, options)
    : context
  if (!__BROWSER__ && mode === 'module') {
    genModulePreamble(ast, preambleContext, genScopeId, isSetupInlined)
  } else {
    genFunctionPreamble(ast, preambleContext)
  }
  // enter render function
  const functionName = ssr ? `ssrRender` : `render`
  const args = ssr ? ['_ctx', '_push', '_parent', '_attrs'] : ['_ctx', '_cache']
  if (!__BROWSER__ && options.bindingMetadata && !options.inline) {
    // binding optimization args
    args.push('$props', '$setup', '$data', '$options')
  }
  const signature =
    !__BROWSER__ && options.isTS
      ? args.map(arg => `${arg}: any`).join(',')
      : args.join(', ')

  if (isSetupInlined) {
    push(`(${signature}) => {`)
  } else {
    push(`function ${functionName}(${signature}) {`)
  }
  indent()

  if (useWithBlock) {
    push(`with (_ctx) {`)
    indent()
    // function mode const declarations should be inside with block
    // also they should be renamed to avoid collision with user properties
    if (hasHelpers) {
      push(`const { ${ast.helpers.map(aliasHelper).join(', ')} } = _Vue`)
      push(`\n`)
      newline()
    }
  }

  // generate asset resolution statements
  if (ast.components.length) {
    genAssets(ast.components, 'component', context)
    if (ast.directives.length || ast.temps > 0) {
      newline()
    }
  }
  if (ast.directives.length) {
    genAssets(ast.directives, 'directive', context)
    if (ast.temps > 0) {
      newline()
    }
  }
  if (__COMPAT__ && ast.filters && ast.filters.length) {
    newline()
    genAssets(ast.filters, 'filter', context)
    newline()
  }

  if (ast.temps > 0) {
    push(`let `)
    for (let i = 0; i < ast.temps; i++) {
      push(`${i > 0 ? `, ` : ``}_temp${i}`)
    }
  }
  if (ast.components.length || ast.directives.length || ast.temps) {
    push(`\n`)
    newline()
  }

  // generate the VNode tree expression
  if (!ssr) {
    push(`return `)
  }
  if (ast.codegenNode) {
    genNode(ast.codegenNode, context)
  } else {
    push(`null`)
  }

  if (useWithBlock) {
    deindent()
    push(`}`)
  }

  deindent()
  push(`}`)

  return {
    ast,
    code: context.code,
    preamble: isSetupInlined ? preambleContext.code : ``,
    // SourceMapGenerator does have toJSON() method but it's not in the types
    map: context.map ? (context.map as any).toJSON() : undefined
  }
}

我们可以发现 generate 函数最终返回的是一个对象，但是 compile 函数最终会返回一个函数，这是怎么回事呢？

Vue 其实会将返回对象中 code 值创建一个函数 new Function(code)()，即 render 函数，该逻辑定义在 packages/vue-compat/src/index.ts 文件中：

ts 复制代码

function compileToFunction(
  template: string | HTMLElement,
  options?: CompilerOptions
): RenderFunction {
  // 省略

  const { code } = compile(
    template,
    extend(
      {
        hoistStatic: true,
        whitespace: 'preserve',
        onError: __DEV__ ? onError : undefined,
        onWarn: __DEV__ ? e => onError(e, true) : NOOP
      } as CompilerOptions,
      options
    )
  )

  // 省略

  // The wildcard import results in a huge object with every export
  // with keys that cannot be mangled, and can be quite heavy size-wise.
  // In the global build we know `Vue` is available globally so we can avoid
  // the wildcard object.
  const render = (
    __GLOBAL__ ? new Function(code)() : new Function('Vue', code)(runtimeDom)
  ) as RenderFunction

  // mark the function as runtime compiled
  ;(render as InternalRenderFunction)._rc = true

  return (compileCache[key] = render)
}

registerRuntimeCompiler(compileToFunction)

const Vue = createCompatVue()
Vue.compile = compileToFunction

export default Vue

我们再看下 code 属性的值：

所以，函数 本质上是一段 字符串，而函数的生成主要是对 字符串的拼接方式 以及 字符串的拼接格式 的处理。

通过观察，该函数大致分为四部分：

函数的前置代码：const _Vue = vue
函数名：function render
函数参数：_ctx, _cache
函数体：const...return...

之后通过 push 函数进行字符串拼接，而对于格式化，比如 缩进、换行 等都是定义了不同方法来处理。了解完这些，我们再回过来看下 generate 函数执行逻辑。

依旧同前面一样，通过 createCodegenContext 方法创建 generate 函数的上下文对象 context：

ts 复制代码

function createCodegenContext(
  ast: RootNode,
  {
    mode = 'function',
    prefixIdentifiers = mode === 'module',
    sourceMap = false,
    filename = `template.vue.html`,
    scopeId = null,
    optimizeImports = false,
    runtimeGlobalName = `Vue`,
    runtimeModuleName = `vue`,
    ssrRuntimeModuleName = 'vue/server-renderer',
    ssr = false,
    isTS = false,
    inSSR = false
  }: CodegenOptions
): CodegenContext {
  const context: CodegenContext = {
    mode,
    prefixIdentifiers,
    sourceMap,
    filename,
    scopeId,
    optimizeImports,
    runtimeGlobalName,
    runtimeModuleName,
    ssrRuntimeModuleName,
    ssr,
    isTS,
    inSSR,
    source: ast.loc.source,
    code: ``,
    column: 1,
    line: 1,
    offset: 0,
    indentLevel: 0,
    pure: false,
    map: undefined,
    helper(key) {
      return `_${helperNameMap[key]}`
    },
    push(code, node) {
      context.code += code
      if (!__BROWSER__ && context.map) {
        if (node) {
          let name
          if (node.type === NodeTypes.SIMPLE_EXPRESSION && !node.isStatic) {
            const content = node.content.replace(/^_ctx\./, '')
            if (content !== node.content && isSimpleIdentifier(content)) {
              name = content
            }
          }
          addMapping(node.loc.start, name)
        }
        advancePositionWithMutation(context, code)
        if (node && node.loc !== locStub) {
          addMapping(node.loc.end)
        }
      }
    },
    indent() {
      newline(++context.indentLevel)
    },
    deindent(withoutNewLine = false) {
      if (withoutNewLine) {
        --context.indentLevel
      } else {
        newline(--context.indentLevel)
      }
    },
    newline() {
      newline(context.indentLevel)
    }
  }

  function newline(n: number) {
    context.push('\n' + `  `.repeat(n))
  }

  function addMapping(loc: Position, name?: string) {
    context.map!.addMapping({
      name,
      source: context.filename,
      original: {
        line: loc.line,
        column: loc.column - 1 // source-map column is 0 based
      },
      generated: {
        line: context.line,
        column: context.column - 1
      }
    })
  }

  if (!__BROWSER__ && sourceMap) {
    // lazy require source-map implementation, only in non-browser builds
    context.map = new SourceMapGenerator()
    context.map!.setSourceContent(filename, context.source)
  }

  return context
}

该对象中包含了 push 拼接、indent 缩进、deindent 回退、newline 换行 等方法，之后执行 genFunctionPreamble 函数：

ts 复制代码

function genFunctionPreamble(ast: RootNode, context: CodegenContext) {
  const {
    ssr,
    prefixIdentifiers,
    push,
    newline,
    runtimeModuleName,
    runtimeGlobalName,
    ssrRuntimeModuleName
  } = context
  const VueBinding =
    !__BROWSER__ && ssr
      ? `require(${JSON.stringify(runtimeModuleName)})`
      : runtimeGlobalName
  // Generate const declaration for helpers
  // In prefix mode, we place the const declaration at top so it's done
  // only once; But if we not prefixing, we place the declaration inside the
  // with block so it doesn't incur the `in` check cost for every helper access.
  if (ast.helpers.length > 0) {
    if (!__BROWSER__ && prefixIdentifiers) {
      push(
        `const { ${ast.helpers.map(aliasHelper).join(', ')} } = ${VueBinding}\n`
      )
    } else {
      // "with" mode.
      // save Vue in a separate variable to avoid collision
      push(`const _Vue = ${VueBinding}\n`)
      // in "with" mode, helpers are declared inside the with block to avoid
      // has check cost, but hoists are lifted out of the function - we need
      // to provide the helper here.
      if (ast.hoists.length) {
        const staticHelpers = [
          CREATE_VNODE,
          CREATE_ELEMENT_VNODE,
          CREATE_COMMENT,
          CREATE_TEXT,
          CREATE_STATIC
        ]
          .filter(helper => ast.helpers.includes(helper))
          .map(aliasHelper)
          .join(', ')
        push(`const { ${staticHelpers} } = _Vue\n`)
      }
    }
  }
  // generate variables for ssr helpers
  if (!__BROWSER__ && ast.ssrHelpers && ast.ssrHelpers.length) {
    // ssr guarantees prefixIdentifier: true
    push(
      `const { ${ast.ssrHelpers
        .map(aliasHelper)
        .join(', ')} } = require("${ssrRuntimeModuleName}")\n`
    )
  }
  genHoists(ast.hoists, context)
  newline()
  push(`return `)
}

执行完 push 方法，函数的前置代码就拼接完成：

之后再执行 newline 换行，再拼接 return，此时 code:

跳出该函数，继续执行后续逻辑。接着处理 函数名 和 参数，拼接完，此时 code:

接着执行 indent 缩进方法，然后执行到 push(const { ${ast.helpers.map(aliasHelper).join(', ')} } = _Vue) 该逻辑，我们知道 helpers 是存放转换的方法名：

而 aliasHelper 方法会将方法名转换为字符串，即 openBlock: _openBlock：

ts 复制代码

const aliasHelper = (s: symbol) => `${helperNameMap[s]}: _${helperNameMap[s]}`

所以上述逻辑执行完，此时 code:

之后执行到 if(ast.codegenNode)，当前根节点存在 codegenNode，执行 genNode 方法：

ts 复制代码

function genNode(node: CodegenNode | symbol | string, context: CodegenContext) {
  if (isString(node)) {
    context.push(node)
    return
  }
  if (isSymbol(node)) {
    context.push(context.helper(node))
    return
  }
  switch (node.type) {
    case NodeTypes.ELEMENT:
    case NodeTypes.IF:
    case NodeTypes.FOR:
      __DEV__ &&
        assert(
          node.codegenNode != null,
          `Codegen node is missing for element/if/for node. ` +
            `Apply appropriate transforms first.`
        )
      genNode(node.codegenNode!, context)
      break
    case NodeTypes.TEXT:
      genText(node, context)
      break
    case NodeTypes.SIMPLE_EXPRESSION:
      genExpression(node, context)
      break
    case NodeTypes.INTERPOLATION:
      genInterpolation(node, context)
      break
    case NodeTypes.TEXT_CALL:
      genNode(node.codegenNode, context)
      break
    case NodeTypes.COMPOUND_EXPRESSION:
      genCompoundExpression(node, context)
      break
    case NodeTypes.COMMENT:
      genComment(node, context)
      break
    case NodeTypes.VNODE_CALL:
      genVNodeCall(node, context)
      break

    case NodeTypes.JS_CALL_EXPRESSION:
      genCallExpression(node, context)
      break
    case NodeTypes.JS_OBJECT_EXPRESSION:
      genObjectExpression(node, context)
      break
    case NodeTypes.JS_ARRAY_EXPRESSION:
      genArrayExpression(node, context)
      break
    case NodeTypes.JS_FUNCTION_EXPRESSION:
      genFunctionExpression(node, context)
      break
    case NodeTypes.JS_CONDITIONAL_EXPRESSION:
      genConditionalExpression(node, context)
      break
    case NodeTypes.JS_CACHE_EXPRESSION:
      genCacheExpression(node, context)
      break
    case NodeTypes.JS_BLOCK_STATEMENT:
      genNodeList(node.body, context, true, false)
      break

    // SSR only types
    case NodeTypes.JS_TEMPLATE_LITERAL:
      !__BROWSER__ && genTemplateLiteral(node, context)
      break
    case NodeTypes.JS_IF_STATEMENT:
      !__BROWSER__ && genIfStatement(node, context)
      break
    case NodeTypes.JS_ASSIGNMENT_EXPRESSION:
      !__BROWSER__ && genAssignmentExpression(node, context)
      break
    case NodeTypes.JS_SEQUENCE_EXPRESSION:
      !__BROWSER__ && genSequenceExpression(node, context)
      break
    case NodeTypes.JS_RETURN_STATEMENT:
      !__BROWSER__ && genReturnStatement(node, context)
      break

    /* istanbul ignore next */
    case NodeTypes.IF_BRANCH:
      // noop
      break
    default:
      if (__DEV__) {
        assert(false, `unhandled codegen node type: ${(node as any).type}`)
        // make sure we exhaust all possible types
        const exhaustiveCheck: never = node
        return exhaustiveCheck
      }
  }
}

根据逻辑，执行 genVNodeCall 方法：

ts 复制代码

function genVNodeCall(node: VNodeCall, context: CodegenContext) {
  const { push, helper, pure } = context
  const {
    tag,
    props,
    children,
    patchFlag,
    dynamicProps,
    directives,
    isBlock,
    disableTracking,
    isComponent
  } = node
  if (directives) {
    push(helper(WITH_DIRECTIVES) + `(`)
  }
  if (isBlock) {
    push(`(${helper(OPEN_BLOCK)}(${disableTracking ? `true` : ``}), `)
  }
  if (pure) {
    push(PURE_ANNOTATION)
  }
  const callHelper: symbol = isBlock
    ? getVNodeBlockHelper(context.inSSR, isComponent)
    : getVNodeHelper(context.inSSR, isComponent)
  push(helper(callHelper) + `(`, node)
  genNodeList(
    genNullableArgs([tag, props, children, patchFlag, dynamicProps]),
    context
  )
  push(`)`)
  if (isBlock) {
    push(`)`)
  }
  if (directives) {
    push(`, `)
    genNode(directives, context)
    push(`)`)
  }
}

通过 helper 获取到对应的方法名，此时拼接完 code 为：

接着再执行 genNodeList 方法：

ts 复制代码

function genNodeList(
  nodes: (string | symbol | CodegenNode | TemplateChildNode[])[],
  context: CodegenContext,
  multilines: boolean = false,
  comma: boolean = true
) {
  const { push, newline } = context
  for (let i = 0; i < nodes.length; i++) {
    const node = nodes[i]
    if (isString(node)) {
      push(node)
    } else if (isArray(node)) {
      genNodeListAsArray(node, context)
    } else {
      genNode(node, context)
    }
    if (i < nodes.length - 1) {
      if (multilines) {
        comma && push(',')
        newline()
      } else {
        comma && push(', ')
      }
    }
  }
}

第一个参数通过 genNullableArgs 方法生成参数：

ts 复制代码

function genNullableArgs(args: any[]): CallExpression['arguments'] {
  let i = args.length
  while (i--) {
    if (args[i] != null) break
  }
  return args.slice(0, i + 1).map(arg => arg || `null`)
}

该方法最终返回结果：

之后执行遍历，如果为字符串直接进行拼接否则执行 genNodeListAsArray 或 genNode 方法，所有拼接完后，最终 code 值为：

至此 generate 函数执行完毕，也代表着 compile 函数执行完成。

总结

编译器的处理过程分成了三部分：ast、transform、generate。
第一步，通过 baseParse 函数将模板 template 转换为 AST。在这一步中我们使用了有限状态自动机解析模板得到了 tokens，通过扫描 tokens 最终得到了 AST。
第二步，通过 transform 函数将 AST 转换为 Javascript AST。这一步是为了最终生成 render 函数做准备，利用了深度优先方式，进行了自下向上的逐层转化。
第三步，通过 generate 函数生成 render 函数。这一步是最后的解析环节，我们需要对 Javascript AST 处理，最终得到 render 函数。

Vue3 源码实现

vue-next-mini