Unify XCAsset handling logic

Summary: @​public

This diff unifies the logic for detecting when images refer to XCAsset files into a single function (RCTXCAssetNameForURL) and uses it for both +[RCTConvert UIImage:] and RCTImageLoader.

I've also tightened the definition of XCAssets so that it only applies to images inside .car files, not any image inside the main bundle. This avoids using the +[UIImage imageNamed:] when not strictly necessary, which is desirable since that method is not thread-safe, and has undocumented caching behavior that is difficult to reason about.

Reviewed By: @javache

Differential Revision: D2526400

fb-gh-sync-id: 7199c2a44f1d55ff236d2c38a0a9368739b993d5

Libraries/Image/RCTXCAssetImageLoader.m

@@ -0,0 +1,59 @@
+/**
+ * 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.
+ */
+
+#import "RCTXCAssetImageLoader.h"
+
+#import "RCTUtils.h"
+
+@implementation RCTXCAssetImageLoader
+
+RCT_EXPORT_MODULE()
+
+- (BOOL)canLoadImageURL:(NSURL *)requestURL
+{
+  return RCTIsXCAssetURL(requestURL);
+}
+
+- (float)imageLoaderPriority
+{
+  return 100; // higher priority than any ordinary file loader
+}
+
+ - (RCTImageLoaderCancellationBlock)loadImageForURL:(NSURL *)imageURL size:(CGSize)size scale:(CGFloat)scale resizeMode:(UIViewContentMode)resizeMode progressHandler:(RCTImageLoaderProgressBlock)progressHandler completionHandler:(RCTImageLoaderCompletionBlock)completionHandler
+{
+  __block BOOL cancelled = NO;
+  dispatch_async(dispatch_get_main_queue(), ^{
+    if (cancelled) {
+      return;
+    }
+
+    NSString *imageName = RCTBundlePathForURL(imageURL);
+    UIImage *image = [UIImage imageNamed:imageName];
+    if (image) {
+      if (progressHandler) {
+        progressHandler(1, 1);
+      }
+
+      if (completionHandler) {
+        completionHandler(nil, image);
+      }
+    } else {
+      if (completionHandler) {
+        NSString *message = [NSString stringWithFormat:@"Could not find image named %@", imageName];
+        completionHandler(RCTErrorWithMessage(message), nil);
+      }
+    }
+  });
+
+  return ^{
+    cancelled = YES;
+  };
+}
+
+@end