Support remote and local assets in custom CSS

(This includes images and fonts using url().)

Using onResolve (as suggested in #786 (comment))

closes #786
supersedes #788

Author: Fil

src/build.ts

@@ -92,6 +92,7 @@ export async function build(
 
   // For cache-breaking we rename most assets to include content hashes.
   const aliases = new Map<string, string>();
+  const plainaliases = new Map<string, string>();
 
   // Add the search bundle and data, if needed.
   if (config.search) {
@@ -106,6 +107,7 @@ export async function build(
 
   // Generate the client bundles (JavaScript and styles). TODO Use a content
   // hash, or perhaps the Framework version number for built-in modules.
+  const delayedStylesheets = new Set<string>();
   if (addPublic) {
     for (const path of globalImports) {
       if (path.startsWith("/_observablehq/") && path.endsWith(".js")) {
@@ -136,14 +138,9 @@ export async function build(
         const sourcePath = await populateNpmCache(root, path); // TODO effects
         await effects.copyFile(sourcePath, path);
       } else if (!/^\w+:/.test(specifier)) {
-        const sourcePath = join(root, specifier);
-        effects.output.write(`${faint("build")} ${sourcePath} ${faint("→")} `);
-        const contents = await bundleStyles({path: sourcePath, minify: true});
-        const hash = createHash("sha256").update(contents).digest("hex").slice(0, 8);
-        const ext = extname(specifier);
-        const alias = `/${join("_import", dirname(specifier), `${basename(specifier, ext)}.${hash}${ext}`)}`;
-        aliases.set(resolveStylesheetPath(root, specifier), alias);
-        await effects.writeFile(alias, contents);
+        // Uses a side effect to register file assets on custom stylesheets
+        delayedStylesheets.add(specifier);
+        await bundleStyles({path: join(root, specifier), files});
       }
     }
   }
@@ -170,9 +167,24 @@ export async function build(
     const ext = extname(file);
     const alias = `/${join("_file", dirname(file), `${basename(file, ext)}.${hash}${ext}`)}`;
     aliases.set(loaders.resolveFilePath(file), alias);
+    plainaliases.set(file, alias);
     await effects.writeFile(alias, contents);
   }
 
+  // Write delayed stylesheets
+  if (addPublic) {
+    for (const specifier of delayedStylesheets) {
+      const sourcePath = join(root, specifier);
+      effects.output.write(`${faint("build")} ${sourcePath} ${faint("→")} `);
+      const contents = await bundleStyles({path: sourcePath, minify: true, aliases: plainaliases});
+      const hash = createHash("sha256").update(contents).digest("hex").slice(0, 8);
+      const ext = extname(specifier);
+      const alias = `/${join("_import", dirname(specifier), `${basename(specifier, ext)}.${hash}${ext}`)}`;
+      aliases.set(resolveStylesheetPath(root, specifier), alias);
+      await effects.writeFile(alias, contents);
+    }
+  }
+
   // Download npm imports. TODO It might be nice to use content hashes for
   // these, too, but it would involve rewriting the files since populateNpmCache
   // doesn’t let you pass in a resolver.

src/rollup.ts

@@ -1,7 +1,8 @@
-import {extname} from "node:path/posix";
+import {extname, join} from "node:path/posix";
 import {nodeResolve} from "@rollup/plugin-node-resolve";
 import type {CallExpression} from "acorn";
 import {simple} from "acorn-walk";
+import type {PluginBuild} from "esbuild";
 import {build} from "esbuild";
 import type {AstNode, OutputChunk, Plugin, ResolveIdResult} from "rollup";
 import {rollup} from "rollup";
@@ -36,16 +37,33 @@ function rewriteInputsNamespace(code: string) {
 export async function bundleStyles({
   minify = false,
   path,
-  theme
+  theme,
+  files,
+  aliases
 }: {
   minify?: boolean;
   path?: string;
   theme?: string[];
+  files?: Set<string>;
+  aliases?: Map<string, string>;
 }): Promise<string> {
+  const assets = {
+    name: "resolve CSS assets",
+    setup(build: PluginBuild) {
+      build.onResolve({filter: /^\w+:\/\//}, (args) => ({path: args.path, external: true}));
+      build.onResolve({filter: /./}, (args) => {
+        if (args.path.endsWith(".css") || args.path.match(/^[#.]/)) return;
+        if (files) files.add(args.path); // /!\ modifies files as a side effect
+        const path = join("..", aliases?.get(args.path) ?? join("_file", args.path));
+        return {path, external: true};
+      });
+    }
+  };
   const result = await build({
     bundle: true,
     ...(path ? {entryPoints: [path]} : {stdin: {contents: renderTheme(theme!), loader: "css"}}),
     write: false,
+    plugins: [assets],
     minify,
     alias: STYLE_MODULES
   });

test/input/build/css-public/horse.jpg

@@ -0,0 +1,12 @@
+---
+style: style.css
+---
+
+# CSS assets
+
+Atkinson Hyperlegible font is named after Braille Institute founder, J. Robert Atkinson. What makes it different from traditional typography design is that it focuses on letterform distinction to increase character recognition, ultimately improving readability. [We are making it free for anyone to use!](https://brailleinstitute.org/freefont)
+
+<figure>
+<div class="bg" style="height: 518px;"></div>
+<figcaption>This image is set with CSS.</figcaption>
+</figure>

test/input/build/css-public/index.md

@@ -0,0 +1,20 @@
+@import url("observablehq:default.css");
+@import url("observablehq:theme-air.css");
+
+:root {
+  --serif: "Atkinson Hyperlegible";
+}
+
+div.bg {
+  background-image: url("horse.jpg");
+}
+
+div.dont-break-hashes {
+  offset-path: url(#path);
+}
+
+@font-face {
+  font-family: "Atkinson Hyperlegible";
+  src: url(https://fonts.gstatic.com/s/atkinsonhyperlegible/v11/9Bt23C1KxNDXMspQ1lPyU89-1h6ONRlW45G04pIoWQeCbA.woff2)
+    format("woff2");
+}