Adds inlineStyle() function.

Refs #38.

The `inlineStyle()` function simply loads the given input from runfiles and returns it in a `<style />` element. This is useful for inlining styles in specific parts of the document, such as inside a declarative shadow DOM template.

packages/rules_prerender/BUILD.bazel

@@ -168,13 +168,18 @@ jasmine_node_test(
 ts_library(
     name = "styles",
     srcs = ["styles.ts"],
-    deps = ["//common/models:prerender_annotation"],
+    deps = [
+        "//common:fs",
+        "//common:runfiles",
+        "//common/models:prerender_annotation",
+    ],
 )
 
 ts_library(
     name = "styles_test_lib",
     srcs = ["styles_test.ts"],
     testonly = True,
+    data = ["//packages/rules_prerender/testdata:styles.css"],
     deps = [
         ":styles",
         "@npm//@types/jasmine",

packages/rules_prerender/index.ts

@@ -2,4 +2,4 @@
 
 export { PrerenderResource } from 'rules_prerender/common/models/prerender_resource';
 export { includeScript } from 'rules_prerender/packages/rules_prerender/scripts';
-export { includeStyle } from 'rules_prerender/packages/rules_prerender/styles';
+export { includeStyle, inlineStyle } from 'rules_prerender/packages/rules_prerender/styles';

packages/rules_prerender/styles.ts

@@ -1,4 +1,6 @@
+import * as fs from 'rules_prerender/common/fs';
 import { createAnnotation } from 'rules_prerender/common/models/prerender_annotation';
+import { resolveRunfile } from 'rules_prerender/common/runfiles';
 
 /**
  * Returns a prerender annotation as a string to be included in prerendered
@@ -12,3 +14,16 @@ export function includeStyle(path: string): string {
     });
     return `<!-- ${annotation} -->`;
 }
+
+/**
+ * Reads the given CSS file at the provided runfiles path and returns it in a
+ * `<style />` tag to be inlined in the document.
+ */
+export async function inlineStyle(path: string): Promise<string> {
+    const styles = await fs.readFile(resolveRunfile(path), 'utf-8');
+    return `
+<style>
+${styles}
+</style>
+    `.trim();
+}

packages/rules_prerender/styles_test.ts

@@ -1,14 +1,32 @@
 import 'jasmine';
 
-import { includeStyle } from 'rules_prerender/packages/rules_prerender/styles';
+import { includeStyle, inlineStyle } from 'rules_prerender/packages/rules_prerender/styles';
 
 describe('styles', () => {
     describe('includeStyle()', () => {
         it('returns a style annotation in an HTML comment', () => {
             const annotation = includeStyle('foo/bar/baz.css');
 
-            expect(annotation)
-                .toBe('<!-- bazel:rules_prerender:PRIVATE_DO_NOT_DEPEND_OR_ELSE - {"type":"style","path":"foo/bar/baz.css"} -->');
+            expect(annotation).toBe(
+                '<!-- bazel:rules_prerender:PRIVATE_DO_NOT_DEPEND_OR_ELSE - {"type":"style","path":"foo/bar/baz.css"} -->');
+        });
+    });
+
+    describe('inlineStyle()', () => {
+        it('resolves to a `<style />` element with the runfiles content of the given file path', async () => {
+            const styles = await inlineStyle(
+                'rules_prerender/packages/rules_prerender/testdata/styles.css');
+            expect(styles).toBe(`
+<style>
+.foo { color: red; }
+
+</style>
+            `.trim());
+        });
+
+        it('rejects when the given file path is not found in runfiles', async () => {
+            await expectAsync(inlineStyle('rules_prerender/does/not/exist.css'))
+                .toBeRejected();
         });
     });
 });

packages/rules_prerender/testdata/BUILD.bazel

@@ -0,0 +1,3 @@
+package(default_visibility = ["//packages/rules_prerender:__pkg__"])
+
+exports_files(["styles.css"])

packages/rules_prerender/testdata/styles.css

@@ -0,0 +1 @@
+.foo { color: red; }