refactor(docs,theme): split DocItem comp, useDoc hook (#7644)

Co-authored-by: Joshua Chen <sidachen2003@gmail.com>

packages/docusaurus-theme-common/src/contexts/doc.tsx

@@ -0,0 +1,72 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import React, {useMemo, type ReactNode, useContext} from 'react';
+import {ReactContextError} from '../utils/reactUtils';
+import type {PropDocContent} from '@docusaurus/plugin-content-docs';
+
+/**
+ * The React context value returned by the `useDoc()` hook.
+ * It contains useful data related to the currently browsed doc.
+ */
+export type DocContextValue = Pick<
+  PropDocContent,
+  'metadata' | 'frontMatter' | 'toc' | 'assets' | 'contentTitle'
+>;
+
+const Context = React.createContext<DocContextValue | null>(null);
+
+/**
+ * Note: we don't use `PropDoc` as context value on purpose. Metadata is
+ * currently stored inside the MDX component, but we may want to change that in
+ * the future. This layer is a good opportunity to decouple storage from
+ * consuming API, potentially allowing us to provide metadata as both props and
+ * route context without duplicating the chunks in the future.
+ */
+function useContextValue(content: PropDocContent): DocContextValue {
+  return useMemo(
+    () => ({
+      metadata: content.metadata,
+      frontMatter: content.frontMatter,
+      assets: content.assets,
+      contentTitle: content.contentTitle,
+      toc: content.toc,
+    }),
+    [content],
+  );
+}
+
+/**
+ * This is a very thin layer around the `content` received from the MDX loader.
+ * It provides the component to be rendered and other metadata about the doc to
+ * the children.
+ */
+export function DocProvider({
+  children,
+  content,
+}: {
+  children: ReactNode;
+  content: PropDocContent;
+}): JSX.Element {
+  const contextValue = useContextValue(content);
+  return <Context.Provider value={contextValue}>{children}</Context.Provider>;
+}
+
+/**
+ * Returns the data of the currently browsed doc. Gives access to the doc's MDX
+ * Component, front matter, metadata, TOC, etc. When swizzling a low-level
+ * component (e.g. the "Edit this page" link) and you need some extra metadata,
+ * you don't have to drill the props all the way through the component tree:
+ * simply use this hook instead.
+ */
+export function useDoc(): DocContextValue {
+  const doc = useContext(Context);
+  if (doc === null) {
+    throw new ReactContextError('DocProvider');
+  }
+  return doc;
+}