Extract getDevServerURL into reusable module

Summary:
Many RN devtools (inspector, source maps, etc.) rely on packager.
This refactors individual SourceCode.scriptURL parsing into one function
that will be easier to change in the future.

Reviewed By: javache

Differential Revision: D3348465

fbshipit-source-id: 5a55939ea59e1517cb63bcbe4963f57f02ab15f3

Libraries/JavaScriptAppEngine/Initialization/getDevServer.js

@@ -0,0 +1,40 @@
+/**
+ * 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.
+ *
+ * @providesModule getDevServer
+ * @flow
+ */
+'use strict';
+
+const {SourceCode} = require('NativeModules');
+
+let _cachedDevServerURL: ?string;
+const FALLBACK = 'http://localhost:8081/';
+
+type DevServerInfo = {
+  url: string;
+  bundleLoadedFromServer: boolean;
+};
+
+/**
+ * Many RN development tools rely on the development server (packager) running
+ * @return URL to packager with trailing slash
+ */
+function getDevServer(): DevServerInfo {
+  if (_cachedDevServerURL === undefined) {
+    const match = SourceCode.scriptURL && SourceCode.scriptURL.match(/^https?:\/\/.*?\//);
+    _cachedDevServerURL = match ? match[0] : null;
+  }
+
+  return {
+    url: _cachedDevServerURL || FALLBACK,
+    bundleLoadedFromServer: _cachedDevServerURL !== null,
+  };
+}
+
+module.exports = getDevServer;