Support multiple sources

Summary:
The nodes version of D3364550. The only difference is that here we
don't get `onSizeChanged` but `onBoundsChanged`, and we need to compute the
height/width of the target image from those bounds. ahmedre please let me know
if any of these assumptions are in any way incorrect.

Reviewed By: ahmedre

Differential Revision: D3424843

ReactAndroid/src/main/java/com/facebook/react/flat/MultiSourceImageHelper.java

@@ -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.
+ */
+
+package com.facebook.react.flat;
+
+import javax.annotation.Nullable;
+
+import java.util.Map;
+
+/**
+ * Helper class for computing the source to be used for an Image.
+ */
+/* package */ class MultiSourceImageHelper {
+
+  /**
+   * Chooses the image source with the size closest to the target image size. Must be called only
+   * after the layout pass when the sizes of the target image have been computed, and when there
+   * are at least two sources to choose from.
+   */
+  public static @Nullable String getImageSourceFromMultipleSources(
+      double targetImageSize,
+      Map<String, Double> sources) {
+    double bestPrecision = Double.MAX_VALUE;
+    String imageSource = null;
+    for (Map.Entry<String, Double> source : sources.entrySet()) {
+      final double precision = Math.abs(1.0 - (source.getValue()) / targetImageSize);
+      if (precision < bestPrecision) {
+        bestPrecision = precision;
+        imageSource = source.getKey();
+      }
+    }
+    return imageSource;
+  }
+}