publicPath can now be a function

README.md

@@ -38,6 +38,13 @@ npm install --save-dev mini-css-extract-plugin
 
 ### Configuration
 
+#### `publicPath`
+
+Type: `String|Function`
+Default: the `publicPath` in `webpackOptions.output`
+
+Specifies a custom public path for the target file(s).
+
 #### Minimal example
 
 **webpack.config.js**
@@ -74,6 +81,45 @@ module.exports = {
 }
 ```
 
+#### `publicPath` function example
+
+**webpack.config.js**
+
+```js
+const MiniCssExtractPlugin = require("mini-css-extract-plugin");
+module.exports = {
+  plugins: [
+    new MiniCssExtractPlugin({
+      // Options similar to the same options in webpackOptions.output
+      // both options are optional
+      filename: "[name].css",
+      chunkFilename: "[id].css"
+    })
+  ],
+  module: {
+    rules: [
+      {
+        test: /\.css$/,
+        use: [
+          {
+            loader: MiniCssExtractPlugin.loader,
+            options: {
+              publicPath: (resourcePath, context) => {
+                // publicPath is the relative path of the resource to the context
+                // e.g. for ./css/admin/main.css the publicPath will be ../../
+                // while for ./css/main.css the publicPath will be ../
+                return path.relative(path.dirname(resourcePath), context) + '/'
+              },
+            }
+          },
+          "css-loader"
+        ]
+      }
+    ]
+  }
+}
+```
+
 #### Advanced configuration example
 
 This plugin should be used only on `production` builds without `style-loader` in the loaders chain, especially if you want to have HMR in `development`.

src/loader.js

@@ -6,6 +6,9 @@ import NodeTargetPlugin from 'webpack/lib/node/NodeTargetPlugin';
 import LibraryTemplatePlugin from 'webpack/lib/LibraryTemplatePlugin';
 import SingleEntryPlugin from 'webpack/lib/SingleEntryPlugin';
 import LimitChunkCountPlugin from 'webpack/lib/optimize/LimitChunkCountPlugin';
+import validateOptions from 'schema-utils';
+
+import schema from './options.json';
 
 const MODULE_TYPE = 'css/mini-extract';
 const pluginName = 'mini-css-extract-plugin';
@@ -29,12 +32,19 @@ const findModuleById = (modules, id) => {
 
 export function pitch(request) {
   const query = loaderUtils.getOptions(this) || {};
+
+  validateOptions(schema, query, 'Mini CSS Extract Plugin Loader');
+
   const loaders = this.loaders.slice(this.loaderIndex + 1);
   this.addDependency(this.resourcePath);
   const childFilename = '*'; // eslint-disable-line no-path-concat
   const publicPath =
     typeof query.publicPath === 'string'
-      ? query.publicPath
+      ? query.publicPath.endsWith('/')
+        ? query.publicPath
+        : `${query.publicPath}/`
+      : typeof query.publicPath === 'function'
+      ? query.publicPath(this.resourcePath, this.rootContext)
       : this._compilation.outputOptions.publicPath;
   const outputOptions = {
     filename: childFilename,

src/options.json

@@ -0,0 +1,19 @@
+{
+	"additionalProperties": true,
+	"properties": {
+	  "publicPath": {
+		"anyOf": [
+		  {
+			"type": "string"
+		  },
+		  {
+			"instanceof": "Function"
+		  }
+		]
+	  }
+	},
+	"errorMessages": {
+	  "publicPath": "should be {String} or {Function} (https://github.com/webpack-contrib/mini-css-extract-plugin#publicpath)"
+	},
+	"type": "object"
+  }

test/TestCases.test.js

@@ -59,18 +59,33 @@ describe('TestCases', () => {
             );
             return;
           }
-          const expectedDirectory = path.resolve(directoryForCase, 'expected');
-          for (const file of fs.readdirSync(expectedDirectory)) {
-            const content = fs.readFileSync(
-              path.resolve(expectedDirectory, file),
-              'utf-8'
-            );
-            const actualContent = fs.readFileSync(
-              path.resolve(outputDirectoryForCase, file),
-              'utf-8'
-            );
-            expect(actualContent).toEqual(content);
+
+          function compareDirectory(actual, expected) {
+            for (const file of fs.readdirSync(expected, {
+              withFileTypes: true,
+            })) {
+              if (file.isFile()) {
+                const content = fs.readFileSync(
+                  path.resolve(expected, file.name),
+                  'utf-8'
+                );
+                const actualContent = fs.readFileSync(
+                  path.resolve(actual, file.name),
+                  'utf-8'
+                );
+                expect(actualContent).toEqual(content);
+              } else if (file.isDirectory()) {
+                compareDirectory(
+                  path.resolve(actual, file.name),
+                  path.resolve(expected, file.name)
+                );
+              }
+            }
           }
+
+          const expectedDirectory = path.resolve(directoryForCase, 'expected');
+          compareDirectory(outputDirectoryForCase, expectedDirectory);
+
           done();
         });
       }, 10000);

test/cases/publicpath-function/expected/nested/again/style.css

@@ -0,0 +1,2 @@
+body { background: green; background-image: url(../../cd0bb358c45b584743d8ce4991777c42.svg); }
+

test/cases/publicpath-function/expected/nested/style.css

@@ -0,0 +1,2 @@
+body { background: red; background-image: url(../cd0bb358c45b584743d8ce4991777c42.svg); }
+

test/cases/publicpath-function/nested/again/style.css

@@ -0,0 +1 @@
+body { background: green; background-image: url(../../react.svg); }

test/cases/publicpath-function/nested/style.css

@@ -0,0 +1 @@
+body { background: red; background-image: url(../react.svg); }

test/cases/publicpath-function/webpack.config.js

@@ -0,0 +1,41 @@
+const Self = require('../../../');
+const path = require('path')
+
+module.exports = {
+  entry: {
+    // Specific CSS entry point, with output to a nested folder
+    'nested/style': './nested/style.css',
+    // Note that relative nesting of output is the same as that of the input
+    'nested/again/style': './nested/again/style.css',
+  },
+  module: {
+    rules: [
+      {
+        test: /\.css$/,
+        use: [
+          {
+            loader: Self.loader,
+            options: {
+              // Compute publicPath relative to the CSS output
+              publicPath: (resourcePath, context) => path.relative(path.dirname(resourcePath), context) + '/',
+            }
+          },
+          'css-loader',
+        ],
+      }, {
+        test: /\.(svg|png)$/,
+        use: [{
+          loader: 'file-loader',
+          options: {
+            filename: '[name].[ext]'
+          }
+        }]
+      }
+    ],
+  },
+  plugins: [
+    new Self({
+      filename: '[name].css',
+    }),
+  ],
+};

test/cases/publicpath-trailing-slash/expected/main.css

@@ -0,0 +1,2 @@
+body { background: red; background-image: url(/static/img/cd0bb358c45b584743d8ce4991777c42.svg); }
+

test/cases/publicpath-trailing-slash/index.js

@@ -0,0 +1 @@
+import './style.css';

test/cases/publicpath-trailing-slash/style.css

@@ -0,0 +1 @@
+body { background: red; background-image: url(./react.svg); }

test/cases/publicpath-trailing-slash/webpack.config.js

@@ -0,0 +1,34 @@
+const Self = require('../../../');
+
+module.exports = {
+  entry: './index.js',
+  module: {
+    rules: [
+      {
+        test: /\.css$/,
+        use: [
+          {
+            loader: Self.loader,
+            options: {
+              publicPath: '/static/img'
+            }
+          },
+          'css-loader',
+        ],
+      }, {
+        test: /\.(svg|png)$/,
+        use: [{
+          loader: 'file-loader',
+          options: {
+            filename: '[name].[ext]'
+          }
+        }]
+      }
+    ],
+  },
+  plugins: [
+    new Self({
+      filename: '[name].css',
+    }),
+  ],
+};