Adds support for publicPath to enable serving assets from different locations. (#299)

Summary:
<!-- Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. The two fields below are mandatory. -->

**Summary**

<!-- Explain the **motivation** for making this change. What existing problem does the pull request solve? -->

This is an updated followup from #280, which we would still need to address the following assumptions about `/assets/`:

https://github.com/facebook/metro/blob/e7deea19001d903a47868bb1fe6456bdc4b585e7/packages/metro/src/Server.js#L332

https://github.com/facebook/metro/blob/e7deea19001d903a47868bb1fe6456bdc4b585e7/packages/metro/src/Server.js#L379

https://github.com/facebook/metro/blob/e7deea19001d903a47868bb1fe6456bdc4b585e7/packages/metro/src/Server.js#L385

As pointed out by gdborton, there isn't currently a way to make this a configurable option. I am not certain how to find a proper workaround for `processRequest`.

We found a temporary solution to have our express app pick up serving the assets from a publicPath as a middleware. But the change in this PR is still necessary to get it working fully.

**Test plan**

<!-- Demonstrate the code is solid. Example: The exact commands you ran and their output, screenshots / videos if the pull request changes UI. -->

- Will add tests once we figure out a comprehensive solution
Pull Request resolved: #299

Reviewed By: rafeca

Differential Revision: D12939229

Pulled By: mjesun

fbshipit-source-id: 769c23468c5ac434f8319e5e7caaf46dd6453f2d

Author: gdborton

packages/metro-config/src/__tests__/__snapshots__/loadConfig-test.js.snap

@@ -109,6 +109,7 @@ Object {
     "minifierPath": "metro-minify-uglify",
     "optimizationSizeLimit": 153600,
     "postMinifyProcess": [Function],
+    "publicPath": "/assets",
     "transformVariants": Object {
       "default": Object {},
     },
@@ -230,6 +231,7 @@ Object {
     "minifierPath": "metro-minify-uglify",
     "optimizationSizeLimit": 153600,
     "postMinifyProcess": [Function],
+    "publicPath": "/assets",
     "transformVariants": Object {
       "default": Object {},
     },

packages/metro-config/src/configTypes.flow.js

@@ -137,6 +137,7 @@ type TransformerConfigT = {|
   postMinifyProcess: PostMinifyProcess,
   transformVariants: TransformVariants,
   workerPath: string,
+  publicPath: string,
 |};
 
 type MetalConfigT = {|

packages/metro-config/src/convertConfig.js

@@ -146,6 +146,7 @@ async function convertOldToNew({
         ? transformVariants()
         : defaultConfig.transformer.transformVariants,
       workerPath: getWorkerPath(),
+      publicPath: '/assets',
     },
 
     reporter,

packages/metro-config/src/defaults/index.js

@@ -99,6 +99,7 @@ const getDefaultValues = (projectRoot: ?string): ConfigT => ({
     postMinifyProcess: x => x,
     transformVariants: {default: {}},
     workerPath: 'metro/src/DeltaBundler/Worker',
+    publicPath: '/assets',
   },
   cacheStores: [
     new FileStore({

packages/metro/src/Assets.js

@@ -177,8 +177,9 @@ async function getAssetData(
   localPath: string,
   assetDataPlugins: $ReadOnlyArray<string>,
   platform: ?string = null,
+  publicPath: string,
 ): Promise<AssetData> {
-  let assetUrlPath = path.join('/assets', path.dirname(localPath));
+  let assetUrlPath = path.join(publicPath, path.dirname(localPath));
 
   // On Windows, change backslashes to slashes to get proper URL path from file path.
   if (path.sep === '\\') {

packages/metro/src/DeltaBundler/Serializers/getAssets.js

@@ -23,6 +23,7 @@ type Options = {|
   assetPlugins: $ReadOnlyArray<string>,
   platform: ?string,
   projectRoot: string,
+  publicPath: string,
 |};
 
 async function getAssets(
@@ -44,6 +45,7 @@ async function getAssets(
           path.relative(options.projectRoot, module.path),
           options.assetPlugins,
           options.platform,
+          options.publicPath,
         ),
       );
     }

packages/metro/src/JSTransformer/worker.js

@@ -51,6 +51,7 @@ type BabelTransformerOptions = $ReadOnly<{
   minify: boolean,
   platform: ?string,
   projectRoot: string,
+  publicPath: string,
 }>;
 
 export type BabelTransformerArgs = $ReadOnly<{|
@@ -88,6 +89,7 @@ export type JsTransformerConfig = $ReadOnly<{|
   minifierConfig: MinifierConfig,
   minifierPath: string,
   optimizationSizeLimit: number,
+  publicPath: string,
 |}>;
 
 export type CustomTransformOptions = {[string]: mixed, __proto__: null};
@@ -184,6 +186,7 @@ class JsTransformer {
         // is used by other tooling, and this would affect it.
         inlineRequires: false,
         projectRoot: this._projectRoot,
+        publicPath: this._config.publicPath,
       },
       plugins: [],
       src: sourceCode,

packages/metro/src/Server.js

@@ -237,6 +237,7 @@ class Server {
       assetPlugins: this._config.transformer.assetPlugins,
       platform: transformOptions.platform,
       projectRoot: this._config.projectRoot,
+      publicPath: this._config.transformer.publicPath,
     });
   }
 
@@ -802,6 +803,7 @@ class Server {
         processModuleFilter: this._config.serializer.processModuleFilter,
         assetPlugins: this._config.transformer.assetPlugins,
         platform: transformOptions.platform,
+        publicPath: this._config.transformer.publicPath,
         projectRoot: this._config.projectRoot,
       });
     },

packages/metro/src/__tests__/Assets-test.js

@@ -125,7 +125,13 @@ describe('getAssetData', () => {
       'b@4.5x.png': 'b4.5 image',
     });
 
-    return getAssetData('/root/imgs/b.png', 'imgs/b.png', []).then(data => {
+    return getAssetData(
+      '/root/imgs/b.png',
+      'imgs/b.png',
+      [],
+      null,
+      '/assets',
+    ).then(data => {
       expect(data).toEqual(
         expect.objectContaining({
           __packager_asset: true,
@@ -153,7 +159,13 @@ describe('getAssetData', () => {
       'b@4.5x.jpg': 'b4.5 image',
     });
 
-    const data = await getAssetData('/root/imgs/b.jpg', 'imgs/b.jpg', []);
+    const data = await getAssetData(
+      '/root/imgs/b.jpg',
+      'imgs/b.jpg',
+      [],
+      null,
+      '/assets',
+    );
 
     expect(data).toEqual(
       expect.objectContaining({
@@ -173,6 +185,40 @@ describe('getAssetData', () => {
     );
   });
 
+  it('respects `options.publicPath` for output httpServerLocation', async () => {
+    writeImages({
+      'b@1x.jpg': 'b1 image',
+      'b@2x.jpg': 'b2 image',
+      'b@4x.jpg': 'b4 image',
+      'b@4.5x.jpg': 'b4.5 image',
+    });
+
+    const data = await getAssetData(
+      '/root/imgs/b.jpg',
+      'imgs/b.jpg',
+      [],
+      null,
+      '/public_paths/foo-boar/',
+    );
+
+    expect(data).toEqual(
+      expect.objectContaining({
+        __packager_asset: true,
+        type: 'jpg',
+        name: 'b',
+        scales: [1, 2, 4, 4.5],
+        fileSystemLocation: '/root/imgs',
+        httpServerLocation: '/public_paths/foo-boar/imgs',
+        files: [
+          '/root/imgs/b@1x.jpg',
+          '/root/imgs/b@2x.jpg',
+          '/root/imgs/b@4x.jpg',
+          '/root/imgs/b@4.5x.jpg',
+        ],
+      }),
+    );
+  });
+
   it('loads and runs asset plugins', async () => {
     jest.mock(
       'mockPlugin1',
@@ -206,10 +252,13 @@ describe('getAssetData', () => {
       'b@3x.png': 'b3 image',
     });
 
-    const data = await getAssetData('/root/imgs/b.png', 'imgs/b.png', [
-      'mockPlugin1',
-      'asyncMockPlugin2',
-    ]);
+    const data = await getAssetData(
+      '/root/imgs/b.png',
+      'imgs/b.png',
+      ['mockPlugin1', 'asyncMockPlugin2'],
+      null,
+      '/assets',
+    );
 
     expect(data).toEqual(
       expect.objectContaining({
@@ -247,21 +296,35 @@ describe('getAssetData', () => {
         hash.update(fs.readFileSync(path.join('/root/imgs', name), 'utf8'));
       }
 
-      expect(await getAssetData('/root/imgs/b.jpg', 'imgs/b.jpg', [])).toEqual(
-        expect.objectContaining({hash: hash.digest('hex')}),
-      );
+      expect(
+        await getAssetData(
+          '/root/imgs/b.jpg',
+          'imgs/b.jpg',
+          [],
+          null,
+          '/assets',
+        ),
+      ).toEqual(expect.objectContaining({hash: hash.digest('hex')}));
     });
 
     it('changes the hash when the passed-in file watcher emits an `all` event', async () => {
       const initialData = await getAssetData(
         '/root/imgs/b.jpg',
         'imgs/b.jpg',
         [],
+        null,
+        '/assets',
       );
 
       fs.writeFileSync('/root/imgs/b@4x.jpg', 'updated data');
 
-      const data = await getAssetData('/root/imgs/b.jpg', 'imgs/b.jpg', []);
+      const data = await getAssetData(
+        '/root/imgs/b.jpg',
+        'imgs/b.jpg',
+        [],
+        null,
+        '/assets',
+      );
       expect(data.hash).not.toEqual(initialData.hash);
     });
   });

packages/metro/src/assetTransformer.js

@@ -36,6 +36,7 @@ async function transform(
     filename,
     assetDataPlugins,
     options.platform,
+    options.publicPath,
   );
 
   return {