Introduce nativeImageSource API

Summary: The goal is to replace `require('image!...')` with an API that communicates better of what's going on under the hood.

Reviewed By: yungsters, fkgozali

Differential Revision: D4186241

fbshipit-source-id: b764588dbbd9494dd6905b2346e3274b575a9644

Libraries/Image/nativeImageSource.js

@@ -0,0 +1,53 @@
+/**
+ * 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 nativeImageSource
+ * @flow
+ */
+'use strict';
+
+const Platform = require('Platform');
+
+type SourceSpec = {
+  ios?: string,
+  android?: string,
+
+  // For more details on width and height, see
+  // http://facebook.github.io/react-native/docs/images.html#why-not-automatically-size-everything
+  width: number,
+  height: number,
+}
+
+/**
+ * In hybrid apps, use `nativeImageSource` to access images that are already available
+ * on the native side, for example in Xcode Asset Catalogs or Android's drawable folder.
+ *
+ * However, keep in mind that React Native Packager does not guarantee that the image exists. If
+ * the image is missing you'll get an empty box. When adding new images your app needs to be
+ * recompiled.
+ *
+ * Prefer Static Image Resources system which provides more guarantees, automates measurements and
+ * allows adding new images without rebuilding the native app. For more details visit:
+ *
+ *   http://facebook.github.io/react-native/docs/images.html
+ *
+ */
+function nativeImageSource(spec: SourceSpec): Object {
+  const uri = Platform.select(spec);
+  if (!uri) {
+    console.warn(`No image name given for ${Platform.OS}: ${JSON.stringify(spec)}`);
+  }
+
+  return {
+    uri,
+    width: spec.width,
+    height: spec.height,
+  };
+}
+
+module.exports = nativeImageSource;