Use CGImageSourceCreateImageAtIndex to decode full-sized images

Author: tsapeta

packages/react-native/Libraries/Image/RCTImageUtils.mm

@@ -293,18 +293,28 @@ BOOL RCTUpscalingRequired(
   // Calculate target size
   CGSize targetSize = RCTTargetSize(sourceSize, 1, destSize, destScale, resizeMode, NO);
   CGSize targetPixelSize = RCTSizeInPixels(targetSize, destScale);
-  CGFloat maxPixelSize =
-      fmax(fmin(sourceSize.width, targetPixelSize.width), fmin(sourceSize.height, targetPixelSize.height));
-
-  NSDictionary<NSString *, NSNumber *> *options = @{
-    (id)kCGImageSourceShouldAllowFloat : @YES,
-    (id)kCGImageSourceCreateThumbnailWithTransform : @YES,
-    (id)kCGImageSourceCreateThumbnailFromImageAlways : @YES,
-    (id)kCGImageSourceThumbnailMaxPixelSize : @(maxPixelSize),
-  };
-
-  // Get thumbnail
-  CGImageRef imageRef = CGImageSourceCreateThumbnailAtIndex(sourceRef, 0, (__bridge CFDictionaryRef)options);
+  CGImageRef imageRef;
+  BOOL createFullImage = targetPixelSize.width == 0 || targetPixelSize.height == 0 || (sourceSize.width <= targetPixelSize.width && sourceSize.height <= targetPixelSize.height);
+
+  if (createFullImage) {
+    // Get an image in full size. This is faster than `CGImageSourceCreateThumbnailAtIndex`
+    // and consumes less memory if only the target size doesn't require downscaling.
+    imageRef = CGImageSourceCreateImageAtIndex(sourceRef, 0, (__bridge CFDictionaryRef)@{
+      (id)kCGImageSourceShouldAllowFloat : @YES,
+    });
+  } else {
+    CGFloat maxPixelSize = fmax(targetPixelSize.width, targetPixelSize.height);
+
+    // Get a thumbnail of the source image. This is usually slower than creating a full-sized image,
+    // but takes up less memory once it's done.
+    imageRef = CGImageSourceCreateThumbnailAtIndex(sourceRef, 0, (__bridge CFDictionaryRef)@{
+      (id)kCGImageSourceShouldAllowFloat : @YES,
+      (id)kCGImageSourceCreateThumbnailWithTransform : @YES,
+      (id)kCGImageSourceCreateThumbnailFromImageAlways : @YES,
+      (id)kCGImageSourceThumbnailMaxPixelSize : @(maxPixelSize),
+    });
+  }
+
   CFRelease(sourceRef);
   if (!imageRef) {
     return nil;