Tailwind CSS 如何检测循环依赖。

在本文中，我们分析了stituteAtApply中抛出的错误。此错误与检测到的循环依赖有关。

walk(rule.nodes, (child) => {
  if (child !== node) return
  throw new Error(
   `You cannot \`@apply\` the \`${candidate}\` utility here because it creates a circular dependency.`,
   )
})

这是围绕此错误的代码的高级概述。

walk — 递归函数：

让我们从步行开始：

export function walk(
   ast: AstNode[],
   visit: (
     node: AstNode,
     utils: {
     parent: AstNode | null
     replaceWith(newNode: AstNode | AstNode[]): void
     context: Record<string, string>
   },
 ) => void | WalkAction,
 parent: AstNode | null = null,
 context: Record<string, string> = {},
) {
 for (let i = 0; i < ast.length; i++) {
   let node = ast[i]
  // We want context nodes to be transparent in walks. This means that
   // whenever we encounter one, we immediately walk through its children and
   // furthermore we also don't update the parent.
 if (node.kind === 'context') {
   walk(node.nodes, visit, parent, { …context, …node.context })
   continue
 }
let status = visit(node, {
   parent,
   replaceWith(newNode) {
   ast.splice(i, 1, …(Array.isArray(newNode) ? newNode : [newNode]))
   // We want to visit the newly replaced node(s), which start at the
   // current index (i). By decrementing the index here, the next loop
   // will process this position (containing the replaced node) again.
   i - 
 },
 context,
 }) ?? WalkAction.Continue
  // Stop the walk entirely
   if (status === WalkAction.Stop) return
  // Skip visiting the children of this node
   if (status === WalkAction.Skip) continue
  if (node.kind === 'rule') {
   walk(node.nodes, visit, node, context)
   }
 }
}

walk 是位于 ast.ts 中的递归函数。

当node.kind === ‘context’或当node.kind === ‘rule’时，它会递归调用自身，破坏条件基于状态

// Stop the walk entirely
if (status === WalkAction.Stop) return
// Skip visiting the children of this node
if (status === WalkAction.Skip) continue

现在让我们缩小一点，研究一下 apply.ts 中 walk 函数附近的代码

// Verify that we don't have any circular dependencies by verifying that
// the current node does not appear in the new nodes.
walk(newNodes, (child) => {
 if (child !== node) return
  // At this point we already know that we have a circular dependency.
  //
  // Figure out which candidate caused the circular dependency. This will
 // help to create a useful error message for the end user.
 for (let candidate of candidates) {
   let selector = `.${escape(candidate)}`
    for (let rule of candidateAst) {
     if (rule.kind !== 'rule') continue
     if (rule.selector !== selector) continue
      walk(rule.nodes, (child) => {
     if (child !== node) return
      throw new Error(
       `You cannot \`@apply\` the \`${candidate}\` utility here because it creates a circular dependency.`,
       )
     })
    }
 }
})

TailwindCSS 作者在需要时在代码库中添加了解释性注释，或者提供额外的上下文是有意义的

有评论。

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参考文献：

1. https://github.com/tailwindlabs/tailwindcss/blob/next/packages/tailwindcss/src/ast.ts#L70

2. https://github.com/tailwindlabs/tailwindcss/blob/c01b8254e822d4f328674357347ca0532f1283a0/packages/tailwindcss/src/apply.ts

3. https://stackoverflow.com/questions/71669246/need-help-using-apply-directive-in-tailwind-css

4. https://github.com/tailwindlabs/tailwindcss/issues/2807

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