chore: add How It Workds to README [ci skip]

Author: yyx990803

README.md

@@ -42,3 +42,79 @@ There are many cool features provided by `vue-loader`:
 - State-preserving hot-reloading during development.
 
 In a nutshell, the combination of webpack and `vue-loader` gives you a modern, flexible and extremely powerful front-end workflow for authoring Vue.js applications.
+
+## How It Works
+
+> The following section is for maintainers and contributors who are interested in the internal implementation details of `vue-loader`, and is **not** required knowledge for end users.
+
+`vue-loader` is not a simple source transform loader. It handles each language blocks inside an SFC with its own dedicated loader chain (you can think of each block as a "virtual module"), and finally assembles the blocks together into the final module. Here's a brief overview of how the whole thing works:
+
+1. `vue-loader` parses the SFC source code into an *SFC Descriptor* using `@vue/compiler-sfc`. It then generates an import for each language block so the actual returned module code looks like this:
+
+    ``` js
+    // code returned from the main loader for 'source.vue'
+
+    // import the <template> block
+    import render from 'source.vue?vue&type=template'
+    // import the <script> block
+    import script from 'source.vue?vue&type=script'
+    export * from 'source.vue?vue&type=script'
+    // import <style> blocks
+    import 'source.vue?vue&type=style&index=1'
+
+    script.render = render
+    export default script
+    ```
+
+    Notice how the code is importing `source.vue` itself, but with different request queries for each block.
+
+2. We want the content in `script` block to be treated like `.js` files (and if it's `<script lang="ts">`, we want to to be treated like `.ts` files). Same for other language blocks. So we want webpack to apply any configured module rules that matches `.js` also to requests that look like `source.vue?vue&type=script`. This is what `VueLoaderPlugin` (`src/plugins.ts`) does: for each module rule in the webpack config, it creates a modified clone that targets corresponding Vue language block requests.
+
+    Suppose we have configured `babel-loader` for all `*.js` files. That rule will be cloned and applied to Vue SFC `<script>` blocks as well. Internally to webpack, a request like
+
+    ``` js
+    import script from 'source.vue?vue&type=script'
+    ```
+
+    Will expand to:
+
+    ``` js
+    import script from 'babel-loader!vue-loader!source.vue?vue&type=script'
+    ```
+
+    Notice the `vue-loader` is also matched because `vue-loader` are applied to `.vue` files.
+
+    Similarly, if you have configured `style-loader` + `css-loader` + `sass-loader` for `*.scss` files:
+
+    ``` html
+    <style scoped lang="scss">
+    ```
+
+    Will be returned by `vue-loader` as:
+
+    ``` js
+    import 'source.vue?vue&type=style&index=1&scoped&lang=scss'
+    ```
+
+    And webpack will expand it to:
+
+    ``` js
+    import 'style-loader!css-loader!sass-loader!vue-loader!source.vue?vue&type=style&index=1&scoped&lang=scss'
+    ```
+
+3. When processing the expanded requests, the main `vue-loader` will get invoked again. This time though, the loader notices that the request has queries and is targeting a specific block only. So it selects (`src/select.ts`) the inner content of the target block and passes it on to the loaders matched after it.
+
+4. For the `<script>` block, this is pretty much it. For `<template>` and `<style>` blocks though, a few extra tasks need to be performed:
+
+    - We need to compile the template using the Vue template compiler;
+    - We need to post-process the CSS in `<style scoped>` blocks, **after** `css-loader` but **before** `style-loader`.
+
+    Technically, these are additional loaders (`src/templateLoader.ts` and `src/stylePostLoader.ts`) that need to be injected into the expanded loader chain. It would be very complicated if the end users have to configure this themselves, so `VueLoaderPlugin` also injects a global [Pitching Loader](https://webpack.js.org/api/loaders/#pitching-loader) (`src/pitcher.ts`) that intercepts Vue `<template>` and `<style>` requests and injects the necessary loaders. The final requests look like the following:
+
+    ``` js
+    // <template lang="pug">
+    import 'vue-loader/template-loader!pug-loader!source.vue?vue&type=template'
+
+    // <style scoped lang="scss">
+    import 'style-loader!vue-loader/style-post-loader!css-loader!sass-loader!vue-loader!source.vue?vue&type=style&index=1&scoped&lang=scss'
+    ```