metro: introduce asyncRequire function

Reviewed By: davidaurelio

Differential Revision: D6498107

fbshipit-source-id: a9c4ab634e60f19b7058205eddcd248f57f63500

Libraries/Utilities/asyncRequire.js

@@ -0,0 +1,79 @@
+/**
+ * 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 asyncRequire
+ */
+
+'use strict';
+
+/**
+ * The bundler must register the dependency properly when generating a call to
+ * `asyncRequire`, that allows us to call `require` dynamically with confidence
+ * the module ID is indeed valid and available.
+ */
+function asyncRequire(moduleID: number): Promise<mixed> {
+  return Promise.resolve()
+    .then(() => {
+      const {segmentId} = (require: $FlowFixMe).unpackModuleId(moduleID);
+      return loadSegment(segmentId);
+    })
+    .then(() => require.call(null, (moduleID: $FlowFixMe)));
+}
+
+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 `fetchSegment`
+ * 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.
+ *
+ * Segment #0 is the main segment, that is always available by definition, so
+ * we never try to load anything.
+ *
+ * We don't use async/await syntax to avoid depending on `regeneratorRuntime`.
+ */
+function loadSegment(segmentId: number): Promise<void> {
+  return Promise.resolve().then(() => {
+    if (segmentId === 0) {
+      return;
+    }
+    let segmentLoader = segmentLoaders.get(segmentId);
+    if (segmentLoader != null) {
+      return segmentLoader;
+    }
+    const {fetchSegment} = global;
+    if (fetchSegment == null) {
+      throw new FetchSegmentNotAvailableError();
+    }
+    segmentLoader = new Promise((resolve, reject) => {
+      fetchSegment(segmentId, error => {
+        if (error != null) {
+          reject(error);
+          return;
+        }
+        resolve();
+      });
+    });
+    segmentLoaders.set(segmentId, segmentLoader);
+    return segmentLoader;
+  });
+}
+
+class FetchSegmentNotAvailableError extends Error {
+  constructor() {
+    super(
+      'When bundle splitting is enabled, the `global.fetchSegment` function ' +
+        'must be provided to be able to load particular bundle segments.',
+    );
+  }
+}
+
+module.exports = asyncRequire;