react-native: BundleSegments

Reviewed By: davidaurelio

Differential Revision: D6231309

fbshipit-source-id: 565cbadedc5fd8ab25025b5846c098f24fb15a82

Libraries/Utilities/BundleSegments.js

@@ -0,0 +1,54 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
+ * @format
+ * @providesModule BundleSegments
+ */
+
+'use strict';
+
+let segmentLoaders = new Map();
+
+/**
+ * Ensure that a bundle segment is ready for use, for example requiring some of
+ * its module. We cache load promises so as to avoid calling `fetchBundle` twice
+ * for the same bundle. We assume that once a segment is fetched/loaded, it is
+ * never gettting removed during this instance of the JavaScript VM.
+ */
+async function loadForModule(moduleID: number): Promise<void> {
+  const {segmentId} = (require: $FlowFixMe).unpackModuleId(moduleID);
+  if (segmentId === 0) {
+    return;
+  }
+  let segmentLoader = segmentLoaders.get(segmentId);
+  if (segmentLoader != null) {
+    return await segmentLoader;
+  }
+  // FIXME: `fetchBundle` should be renamed `fetchSegment`.
+  const {fetchBundle} = global;
+  if (fetchBundle == null) {
+    throw new Error(
+      'When bundle splitting is enabled, the `global.fetchBundle` function ' +
+        'must be provided to be able to load particular bundle segments.',
+    );
+  }
+  segmentLoader = new Promise((resolve, reject) => {
+    fetchBundle(segmentId, error => {
+      if (error != null) {
+        reject(error);
+        return;
+      }
+      resolve();
+    });
+  });
+  segmentLoaders.set(segmentId, segmentLoader);
+  return await segmentLoader;
+}
+
+module.exports = {loadForModule};