Use asset catalog for ios images

packages/cli-plugin-metro/src/commands/bundle/assetCatalogIOS.ts

@@ -0,0 +1,70 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ */
+
+import path from 'path';
+import fs from 'fs-extra';
+import assetPathUtils from './assetPathUtils';
+import {AssetData} from './buildBundle';
+
+export function cleanAssetCatalog(catalogDir: string): void {
+  const files = fs
+    .readdirSync(catalogDir)
+    .filter((file) => file.endsWith('.imageset'));
+  for (const file of files) {
+    fs.removeSync(path.join(catalogDir, file));
+  }
+}
+
+type ImageSet = {
+  basePath: string;
+  files: {name: string; src: string; scale: number}[];
+};
+
+export function getImageSet(
+  catalogDir: string,
+  asset: AssetData,
+  scales: readonly number[],
+): ImageSet {
+  const fileName = assetPathUtils.getResourceIdentifier(asset);
+  return {
+    basePath: path.join(catalogDir, `${fileName}.imageset`),
+    files: scales.map((scale, idx) => {
+      const suffix = scale === 1 ? '' : `@${scale}x`;
+      return {
+        name: `${fileName + suffix}.${asset.type}`,
+        scale,
+        src: asset.files[idx],
+      };
+    }),
+  };
+}
+
+export function isCatalogAsset(asset: AssetData): boolean {
+  return asset.type === 'png' || asset.type === 'jpg' || asset.type === 'jpeg';
+}
+
+export function writeImageSet(imageSet: ImageSet): void {
+  fs.mkdirsSync(imageSet.basePath);
+
+  for (const file of imageSet.files) {
+    const dest = path.join(imageSet.basePath, file.name);
+    fs.copyFileSync(file.src, dest);
+  }
+
+  fs.writeJSONSync(path.join(imageSet.basePath, 'Contents.json'), {
+    images: imageSet.files.map((file) => ({
+      filename: file.name,
+      idiom: 'universal',
+      scale: `${file.scale}x`,
+    })),
+    info: {
+      author: 'xcode',
+      version: 1,
+    },
+  });
+}

packages/cli-plugin-metro/src/commands/bundle/assetPathUtils.ts

@@ -64,7 +64,7 @@ function getAndroidResourceFolderName(
   return androidFolder;
 }
 
-function getAndroidResourceIdentifier(asset: PackagerAsset): string {
+function getResourceIdentifier(asset: PackagerAsset): string {
   const folderPath = getBasePath(asset);
   return `${folderPath}/${asset.name}`
     .toLowerCase()
@@ -84,6 +84,6 @@ function getBasePath(asset: PackagerAsset): string {
 export default {
   getAndroidAssetSuffix,
   getAndroidResourceFolderName,
-  getAndroidResourceIdentifier,
+  getResourceIdentifier,
   getBasePath,
 };

packages/cli-plugin-metro/src/commands/bundle/buildBundle.ts

@@ -119,7 +119,12 @@ export async function buildBundleWithConfig(
     });
 
     // When we're done saving bundle output and the assets, we're done.
-    return await saveAssets(outputAssets, args.platform, args.assetsDest);
+    return await saveAssets(
+      outputAssets,
+      args.platform,
+      args.assetsDest,
+      args.assetCatalogDest,
+    );
   } finally {
     server.end();
   }

packages/cli-plugin-metro/src/commands/bundle/bundleCommandLineArgs.ts

@@ -10,6 +10,7 @@ import path from 'path';
 
 export interface CommandLineArgs {
   assetsDest?: string;
+  assetCatalogDest?: string;
   entryFile: string;
   resetCache: boolean;
   resetGlobalCache: boolean;
@@ -102,6 +103,10 @@ export default [
     description:
       'Experimental, transform JS for a specific JS engine. Currently supported: hermes, hermes-canary, default',
   },
+  {
+    name: '--asset-catalog-dest [string]',
+    description: 'Path where to create an iOS Asset Catalog for images',
+  },
   {
     name: '--reset-cache',
     description: 'Removes cached files',

packages/cli-plugin-metro/src/commands/bundle/getAssetDestPathAndroid.ts

@@ -14,7 +14,7 @@ function getAssetDestPathAndroid(asset: PackagerAsset, scale: number): string {
     asset,
     scale,
   );
-  const fileName = assetPathUtils.getAndroidResourceIdentifier(asset);
+  const fileName = assetPathUtils.getResourceIdentifier(asset);
   return path.join(androidFolder, `${fileName}.${asset.type}`);
 }
 

packages/cli-plugin-metro/src/commands/bundle/saveAssets.ts

@@ -6,14 +6,19 @@
  *
  */
 
-import path from 'path';
+import {logger} from '@react-native-community/cli-tools';
 import fs from 'fs';
-
+import path from 'path';
+import {
+  cleanAssetCatalog,
+  getImageSet,
+  isCatalogAsset,
+  writeImageSet,
+} from './assetCatalogIOS';
+import {AssetData} from './buildBundle';
 import filterPlatformAssetScales from './filterPlatformAssetScales';
 import getAssetDestPathAndroid from './getAssetDestPathAndroid';
 import getAssetDestPathIOS from './getAssetDestPathIOS';
-import {logger} from '@react-native-community/cli-tools';
-import type {AssetData} from './buildBundle';
 
 interface CopiedFiles {
   [src: string]: string;
@@ -23,20 +28,23 @@ function saveAssets(
   assets: AssetData[],
   platform: string,
   assetsDest: string | undefined,
+  assetCatalogDest: string | undefined,
 ) {
   if (!assetsDest) {
     logger.warn('Assets destination folder is not set, skipping...');
-    return Promise.resolve();
+    return;
   }
 
+  const filesToCopy: CopiedFiles = Object.create(null); // Map src -> dest
+
   const getAssetDestPath =
     platform === 'android' ? getAssetDestPathAndroid : getAssetDestPathIOS;
 
-  const filesToCopy: CopiedFiles = Object.create(null); // Map src -> dest
-  assets.forEach((asset) => {
+  const addAssetToCopy = (asset: AssetData) => {
     const validScales = new Set(
       filterPlatformAssetScales(platform, asset.scales),
     );
+
     asset.scales.forEach((scale, idx) => {
       if (!validScales.has(scale)) {
         return;
@@ -45,7 +53,37 @@ function saveAssets(
       const dest = path.join(assetsDest, getAssetDestPath(asset, scale));
       filesToCopy[src] = dest;
     });
-  });
+  };
+
+  if (platform === 'ios' && assetCatalogDest != null) {
+    // Use iOS Asset Catalog for images. This will allow Apple app thinning to
+    // remove unused scales from the optimized bundle.
+    const catalogDir = path.join(assetCatalogDest, 'RNAssets.xcassets');
+    if (!fs.existsSync(catalogDir)) {
+      logger.error(
+        `Could not find asset catalog 'RNAssets.xcassets' in ${assetCatalogDest}. Make sure to create it if it does not exist.`,
+      );
+      return;
+    }
+
+    logger.info('Adding images to asset catalog', catalogDir);
+    cleanAssetCatalog(catalogDir);
+    for (const asset of assets) {
+      if (isCatalogAsset(asset)) {
+        const imageSet = getImageSet(
+          catalogDir,
+          asset,
+          filterPlatformAssetScales(platform, asset.scales),
+        );
+        writeImageSet(imageSet);
+      } else {
+        addAssetToCopy(asset);
+      }
+    }
+    logger.info('Done adding images to asset catalog');
+  } else {
+    assets.forEach(addAssetToCopy);
+  }
 
   return copyAll(filesToCopy);
 }
@@ -57,7 +95,7 @@ function copyAll(filesToCopy: CopiedFiles) {
   }
 
   logger.info(`Copying ${queue.length} asset files`);
-  return new Promise((resolve, reject) => {
+  return new Promise<void>((resolve, reject) => {
     const copyNext = (error?: NodeJS.ErrnoException) => {
       if (error) {
         reject(error);