Support theming in dev server

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

@@ -13,6 +13,7 @@ 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.
 */
+const path = require('path');
 
 async function readCSSSource(location) {
     const fs = require("fs").promises;
@@ -22,8 +23,35 @@ async function readCSSSource(location) {
     return data;
 }
 
-async function appendVariablesToCSS(variables, cssSource) {
-    return cssSource + `:root{\n${Object.entries(variables).reduce((acc, [key, value]) => acc + `--${key}: ${value};\n`, "")} }\n\n`;
+function getRootSectionWithVariables(variables) {
+    return `:root{\n${Object.entries(variables).reduce((acc, [key, value]) => acc + `--${key}: ${value};\n`, "")} }\n\n`;
+}
+
+function appendVariablesToCSS(variables, cssSource) {
+    return cssSource + getRootSectionWithVariables(variables);
+}
+
+function findLocationFromThemeName(name, locations) {
+    const themeLocation = locations.find(location => {
+        const manifest = require(`${location}/manifest.json`);
+        if (manifest.name === name) {
+            return true;
+        }
+    });
+    if (!themeLocation) {
+        throw new Error(`Cannot find location from theme name "${name}"`);
+    }
+    return themeLocation;
+}
+
+function findManifestFromThemeName(name, locations) {
+    for (const location of locations) {
+        const manifest = require(`${location}/manifest.json`);
+        if (manifest.name === name) {
+            return manifest;
+        }
+    }
+    throw new Error(`Cannot find manifest from theme name "${name}"`);
 }
 
 function parseBundle(bundle) {
@@ -68,11 +96,20 @@ function parseBundle(bundle) {
 
 module.exports = function buildThemes(options) {
     let manifest, variants, defaultDark, defaultLight;
+    let isDevelopment = false;
+    const virtualModuleId = '@theme/'
+    const resolvedVirtualModuleId = '\0' + virtualModuleId;
 
     return {
         name: "build-themes",
         enforce: "pre",
 
+        configResolved(config) {
+            if (config.command === "serve") {
+                isDevelopment = true;
+            }
+        },
+
         async buildStart() {
             const { manifestLocations } = options;
             for (const location of manifestLocations) {
@@ -106,7 +143,41 @@ module.exports = function buildThemes(options) {
             }
         },
 
+        resolveId(id) {
+            if (id.startsWith(virtualModuleId)) {
+                return isDevelopment? '\0' + id: false;
+            }
+        },
+
         async load(id) {
+            if (isDevelopment) {
+                if (id.startsWith(resolvedVirtualModuleId)) {
+                    let [theme, variant, file] = id.substr(resolvedVirtualModuleId.length).split("/");
+                    if (theme === "default") {
+                        theme = "Element";
+                    }
+                    if (!variant || variant === "default") {
+                        variant = "light";
+                    }
+                    if (!file) {
+                        file = "index.js";
+                    }
+                    switch (file) {
+                        case "index.js": {
+                            const location = findLocationFromThemeName(theme, options.manifestLocations);
+                            return `import "${path.resolve(`${location}/theme.css`)}";` +
+                                `import "@theme/${theme}/${variant}/variables.css"`;
+                        }
+                        case "variables.css": { 
+                            const manifest = findManifestFromThemeName(theme, options.manifestLocations);
+                            const variables = manifest.values.variants[variant].variables;
+                            const css =  getRootSectionWithVariables(variables);
+                            return css;
+                        }
+                    }
+                }
+            }
+            else {
                 const result = id.match(/(.+)\/theme.css\?variant=(.+)/);
                 if (result) {
                     const [, location, variant] = result;
@@ -115,9 +186,14 @@ module.exports = function buildThemes(options) {
                     return await appendVariablesToCSS(config.variables, cssSource);
                 }
                 return null;
+            }
 },
 
     transformIndexHtml(_, ctx) {
+        if (isDevelopment) {
+            // Don't add default stylesheets to index.html on dev
+            return;
+        } 
         let darkThemeLocation, lightThemeLocation;
         for (const [, bundle] of Object.entries(ctx.bundle)) {
             if (bundle.name === defaultDark) {