(v2) Blog only mode becomes much harder to use after alpha-62

[SamChou19815]

🐛 Bug Report

In the documentation, it mentions that preset classic can support blog-only mode seamlessly: https://v2.docusaurus.io/docs/blog#blog-only-mode

However, several changes in alpha-62 makes it much harder in two possible scenarios of blog-only mode usage:

1. alpha-62 will start complain and abort when docs folder or sidebars.json is missing. This means that if we are using preset-classic and have no docs, build cannot succeed. The workaround might be add some dummy docs.

2. alpha-62 introduces some hooks from docs plugin. If we don't use preset-classic and only use blog plugin and theme-classic, build will fail because @theme/hooks only exist in docs plugin but some components in theme-classic import it:

   docusaurus/packages/docusaurus-theme-classic/src/theme/NavbarItem/DocsVersionDropdownNavbarItem.tsx

   Lines 10 to 14 in 4f448bd

   	import {
   	useVersions,
   	useLatestVersion,
   	useActiveDocContext,
   	} from '@theme/hooks/useDocs';

To workaround the issue, I created a dummy useDocs.js in src/theme/hooks in my website:

https://github.com/SamChou19815/website/blob/e5899283386065581a3a37df9e816e9bf82e2ae8/packages/blog/src/theme/hooks/useDocs.js

but ideally blog-only mode can just work without this kind of hacks.

Have you read the Contributing Guidelines on issues?

Yes

To Reproduce

(Write your steps here:)

1. npx @docusaurus/init@next init test classic
2. Setup blog only mode according to https://v2.docusaurus.io/docs/blog#blog-only-mode
3. yarn start. It failed. See Actual Behavior section below.

Expected behavior

Blog only mode should work out-of-the-box.

Actual Behavior

Scenario 1: (with preset-classic)

module.exports = {
  // ...
  presets: [
    [
      '@docusaurus/preset-classic',
      {
        blog: {
          path: './blog',
          routeBasePath: '/', // Set this value to '/'.
        },
        theme: {
          customCss: require.resolve('./src/css/custom.css'),
        },
      },
    ],
  ],
};

➜  test yarn start
yarn run v1.22.5
$ docusaurus start
Starting the development server...
Error: The docs folder does not exist for version [current]. A docs folder is expected to be found at /Users/sam/Downloads/test/docs
    at checkVersionMetadataPaths (/Users/sam/Downloads/test/node_modules/@docusaurus/plugin-content-docs/lib/versions.js:132:15)
    at Array.forEach (<anonymous>)
    at Object.readVersionsMetadata (/Users/sam/Downloads/test/node_modules/@docusaurus/plugin-content-docs/lib/versions.js:171:22)
    at pluginContentDocs (/Users/sam/Downloads/test/node_modules/@docusaurus/plugin-content-docs/lib/index.js:27:41)
    at /Users/sam/Downloads/test/node_modules/@docusaurus/core/lib/server/plugins/init.js:72:48
    at Array.map (<anonymous>)
    at Object.initPlugins [as default] (/Users/sam/Downloads/test/node_modules/@docusaurus/core/lib/server/plugins/init.js:27:10)
    at Object.loadPlugins (/Users/sam/Downloads/test/node_modules/@docusaurus/core/lib/server/plugins/index.js:49:35)
    at Object.load (/Users/sam/Downloads/test/node_modules/@docusaurus/core/lib/server/index.js:78:74)
    at start (/Users/sam/Downloads/test/node_modules/@docusaurus/core/lib/commands/start.js:44:34)
    at /Users/sam/Downloads/test/node_modules/@docusaurus/core/bin/docusaurus.js:73:5
    at Command.<anonymous> (/Users/sam/Downloads/test/node_modules/@docusaurus/core/bin/docusaurus.js:152:23)
    at Command.listener (/Users/sam/Downloads/test/node_modules/commander/index.js:370:29)
    at Command.emit (events.js:315:20)
    at Command.parseArgs (/Users/sam/Downloads/test/node_modules/commander/index.js:892:12)
    at Command.parse (/Users/sam/Downloads/test/node_modules/commander/index.js:642:21)
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

Scenario 2: (with @docusaurus/theme-classic + @docusaurus/plugin-content-blog)

module.exports = {
  // ...
  themes: [
    [
      require.resolve('@docusaurus/theme-classic'),
      { customCss: require.resolve('./src/css/custom.css') },
    ],
  ],
  plugins: [
    [
      require.resolve('@docusaurus/plugin-content-blog'),
      {
        path: './blog',
        routeBasePath: '/', // Set this value to '/'.
      },
    ],
  ],
};

./node_modules/@docusaurus/theme-classic/lib/theme/NavbarItem/DocsVersionNavbarItem.js
Module not found: Can't resolve '@theme/hooks/useDocs' in '/Users/sam/Downloads/test/node_modules/@docusaurus/theme-classic/lib/theme/NavbarItem'

Your Environment

• Docusaurus version used: v2-alpha-62
• Environment name and version (e.g. Chrome 78.0.3904.108, Node.js 10.17.0): Node.js 12
• Operating system and version (desktop or mobile): desktop

[slorber]

Hey @SamChou19815

Here is an attempt to fix this issue: #3382