Allow preprocessing of value declarations (#30)

* Added support for css-loader ~ syntax of module imports

* Add config, remove plugins after current.

* Update readme

* Revert breaking url resolution change

* Add importsAsModuleRequests option

Co-authored-by: Shipov Mikhail <mshipov@yandex.ru>

README.md

@@ -9,6 +9,7 @@
 [postcss-calc]: https://github.com/postcss/postcss-calc
 [postcss-cssnext]: https://github.com/MoOx/postcss-cssnext
 [postcss-color-function]: https://github.com/postcss/postcss-color-function
+[postcss-modules-tilda]: https://github.com/princed/postcss-modules-tilda
 [postcss-modules-values]: https://github.com/css-modules/postcss-modules-values
 [modules-values-extract]: https://github.com/alexhisen/modules-values-extract
 [enhanced-resolve]: https://github.com/webpack/enhanced-resolve/#contributing
@@ -109,6 +110,15 @@ body { background: myBrandColor }
 body { background: blue }
 ```
 
+#### preprocessValues `boolean`
+
+When enabled, permit plugins defined earlier in the PostCSS pipeline to modify `@value` declarations before they are recorded by this plugin.
+
+#### importsAsModuleRequests `boolean`
+
+When enabled, value imports will be resolved as module requests, in line with `css-loader`'s resolution logic [as of 2.0.0](https://github.com/webpack-contrib/css-loader/blob/master/CHANGELOG.md#200-2018-12-07).
+If your code is written with pre-2.0 import syntax, and utilises [postcss-modules-tilda] for compatibility, this option is not required.
+
 ### calc() and @value
 
 To enable calculations *inside* **@value**, enable media queries support in [postcss-calc]:

index.js

@@ -3,6 +3,7 @@ const path = require('path');
 const promisify = require('es6-promisify');
 const { CachedInputFileSystem, NodeJsInputFileSystem, ResolverFactory } = require('enhanced-resolve');
 const valuesParser = require('postcss-values-parser');
+const { urlToRequest } = require('loader-utils');
 
 const matchImports = /^(.+?|\([\s\S]+?\))\s+from\s+("[^"]*"|'[^']*'|[\w-]+)$/;
 const matchValueDefinition = /(?:\s+|^)([\w-]+)(:?\s+)(.+?)(\s*)$/g;
@@ -136,6 +137,8 @@ const factory = ({
   fs = nodeFs,
   noEmitExports = false,
   resolve: resolveOptions = {},
+  preprocessValues = false,
+  importsAsModuleRequests = false,
 } = {}) => async (root, rootResult) => {
   const resolver = ResolverFactory.createResolver(Object.assign(
     { fileSystem: fs },
@@ -144,10 +147,23 @@ const factory = ({
   const resolve = promisify(resolver.resolve, resolver);
   const readFile = promisify(fs.readFile, fs);
 
+  let preprocessPlugins = [];
+  if (preprocessValues) {
+    const rootPlugins = rootResult.processor.plugins;
+    const oursPluginIndex = rootPlugins
+      .findIndex(plugin => plugin.postcssPlugin === PLUGIN);
+    preprocessPlugins = rootPlugins.slice(0, oursPluginIndex);
+  }
+
   async function walkFile(from, dir, requiredDefinitions) {
-    const resolvedFrom = await resolve(concordContext, dir, from);
+    const request = importsAsModuleRequests ? urlToRequest(from) : from;
+    const resolvedFrom = await resolve(concordContext, dir, request);
     const content = await readFile(resolvedFrom);
-    const result = await postcss([walkerPlugin(walk, requiredDefinitions, walkFile)])
+    const plugins = [
+      ...preprocessPlugins,
+      walkerPlugin(walk, requiredDefinitions, walkFile),
+    ];
+    const result = await postcss(plugins)
       .process(content, { from: resolvedFrom });
 
     return result.messages[0].value;

package.json

@@ -21,7 +21,8 @@
   "dependencies": {
     "enhanced-resolve": "^3.1.0",
     "es6-promisify": "^5.0.0",
-    "postcss": "^6.0.1",
+    "loader-utils": "^2.0.0",
+    "postcss": "^7.0.0",
     "postcss-values-parser": "^1.3.1"
   },
   "devDependencies": {

test.js

@@ -11,13 +11,26 @@ const parserOpts = {
   to: resolve(__dirname, 'fixtures/to.css'),
 };
 
-function run(t, input, output, opts = {}) {
-  return postcss([plugin(opts)]).process(input, parserOpts).then((result) => {
+function run(t, input, output, opts = {}, extraPlugins = []) {
+  return postcss([
+    ...extraPlugins,
+    plugin(opts),
+  ]).process(input, parserOpts).then((result) => {
     t.is(result.css, output);
     t.is(result.warnings().length, 0);
   });
 }
 
+// Simple plugin that replaces "black" with "purple" within @value declarations,
+// used to test preprocess value transformation
+const blackToPurplePlugin = postcss.plugin('testBlackToPurple', () => (root) => {
+  root.walkAtRules('value', (atRule) => {
+    atRule.replaceWith(atRule.clone({
+      params: atRule.params.replace('black', 'purple'),
+    }));
+  });
+});
+
 test('should pass through an empty string', async (t) => {
   await run(t, '', '');
 });
@@ -373,6 +386,16 @@ test('should replace an import from modules', async (t) => {
   );
 });
 
+test('should apply extra plugins to inner processing', async (t) => {
+  await run(
+    t,
+    '@value module from "module/module.css";\n.a { color: module; }',
+    '@value module from "module/module.css";\n.a { color: purple; }',
+    { preprocessValues: true },
+    [blackToPurplePlugin()],
+  );
+});
+
 test('should replace an import from main file of module', async (t) => {
   await run(
     t,
@@ -389,6 +412,15 @@ test('should replace an import from scoped modules', async (t) => {
   );
 });
 
+test('should resolve imports as module requests', async (t) => {
+  await run(
+    t,
+    '@value scoped-module from "~@scope/module/module.css";\n@value base from "level1.css";\n.a { color: scoped-module; width: base; }',
+    '@value scoped-module from "~@scope/module/module.css";\n@value base from "level1.css";\n.a { color: purple; width: 10px; }',
+    { importsAsModuleRequests: true },
+  );
+});
+
 test('variables are also present in messages', async (t) => {
   const input = '@value myColor: blue; @value myColor2: myColor';
   const processor = postcss([plugin]);