Write plugin code

scripts/build-plugins/rollup-plugin-build-themes.js

@@ -0,0 +1,64 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+async function readCSSSource(location) {
+    const fs = require("fs").promises;
+    const path = require("path");
+    const resolvedLocation = path.resolve(__dirname, "../../",  `${location}/theme.css`);
+    const data = await fs.readFile(resolvedLocation);
+    return data;
+}
+
+async function appendVariablesToCSS(variables, cssSource) {
+    return cssSource + `:root{\n${Object.entries(variables).reduce((acc, [key, value]) => acc + `--${key}: ${value};\n`, "")} }\n\n`;
+}
+
+module.exports = function buildThemes(options) {
+    let manifest, variants;
+
+    return {
+        name: "build-themes",
+        enforce: "pre",
+
+        async buildStart() {
+            const { manifestLocations } = options;
+            for (const location of manifestLocations) {
+                manifest = require(`${location}/manifest.json`);
+                variants = manifest.values.variants;
+                for (const [variant] of Object.entries(variants)) {
+                    const themeName = manifest.name;
+                    // emit the css as built theme bundle
+                    this.emitFile({
+                        type: "chunk",
+                        id: `${location}/theme.css?variant=${variant}`,
+                        fileName: `theme-${themeName}-${variant}.css`,
+                    });
+                }
+            }
+        },
+
+        async load(id) {
+            const result = id.match(/(.+)\/theme.css\?variant=(.+)/);
+            if (result) {
+                const [, location, variant] = result;
+                const cssSource = await readCSSSource(location);
+                const config = variants[variant];
+                return await appendVariablesToCSS(config.variables, cssSource);
+            }
+            return null;
+        },
+    }
+}