Added the helper convenient API `sd_isThumbnail`

Fix the metadata does not saved

SDWebImage/Core/SDWebImageManager.m

@@ -489,19 +489,10 @@ static id<SDImageLoader> _defaultImageLoader;
     shouldTransformImage = shouldTransformImage && (!originalImage.sd_isAnimated || (options & SDWebImageTransformAnimatedImage));
     shouldTransformImage = shouldTransformImage && (!originalImage.sd_isVector || (options & SDWebImageTransformVectorImage));
     // thumbnail check
-    CGSize thumbnailSize = CGSizeZero;
-    NSValue *thumbnailSizeValue = originalImage.sd_decodeOptions[SDImageCoderDecodeThumbnailPixelSize];
-    if (thumbnailSizeValue != nil) {
-#if SD_MAC
-        thumbnailSize = thumbnailSizeValue.sizeValue;
-#else
-        thumbnailSize = thumbnailSizeValue.CGSizeValue;
-#endif
-    }
-    BOOL shouldEncodeThumbnail = thumbnailSize.width > 0 && thumbnailSize.height > 0;
+    BOOL isThumbnail = originalImage.sd_isThumbnail;
     NSData *cacheData = originalData;
     UIImage *cacheImage = originalImage;
-    if (shouldEncodeThumbnail) {
+    if (isThumbnail) {
         cacheData = nil; // thumbnail don't store full size data
         originalImage = nil; // thumbnail don't have full size image
     }

SDWebImage/Core/UIImage+Metadata.h

@@ -71,6 +71,12 @@
  */
 @property (nonatomic, assign) BOOL sd_isTransformed;
 
+/**
+ A bool value indicating that the image is using thumbnail decode with smaller size, so the image data may not always match original download one.
+ @note This just check `sd_decodeOptions[.decodeThumbnailPixelSize] > CGSize.zero`
+ */
+@property (nonatomic, assign, readonly) BOOL sd_isThumbnail;
+
 /**
  A dictionary value contains the decode options when decoded from SDWebImage loading system (say, `SDImageCacheDecodeImageData/SDImageLoaderDecode[Progressive]ImageData`)
  It may not always available and only image decoding related options will be saved. (including [.decodeScaleFactor, .decodeThumbnailPixelSize, .decodePreserveAspectRatio, .decodeFirstFrameOnly])

SDWebImage/Core/UIImage+Metadata.m

@@ -197,6 +197,19 @@
     objc_setAssociatedObject(self, @selector(sd_decodeOptions), sd_decodeOptions, OBJC_ASSOCIATION_COPY_NONATOMIC);
 }
 
+-(BOOL)sd_isThumbnail {
+    CGSize thumbnailSize = CGSizeZero;
+    NSValue *thumbnailSizeValue = self.sd_decodeOptions[SDImageCoderDecodeThumbnailPixelSize];
+    if (thumbnailSizeValue != nil) {
+    #if SD_MAC
+        thumbnailSize = thumbnailSizeValue.sizeValue;
+    #else
+        thumbnailSize = thumbnailSizeValue.CGSizeValue;
+    #endif
+    }
+    return thumbnailSize.width > 0 && thumbnailSize.height > 0;
+}
+
 - (SDImageCoderOptions *)sd_decodeOptions {
     SDImageCoderOptions *value = objc_getAssociatedObject(self, @selector(sd_decodeOptions));
     if ([value isKindOfClass:NSDictionary.class]) {

SDWebImage/Private/SDAssociatedObject.m

@@ -18,6 +18,7 @@ void SDImageCopyAssociatedObject(UIImage * _Nullable source, UIImage * _Nullable
     }
     // Image Metadata
     target.sd_isIncremental = source.sd_isIncremental;
+    target.sd_isTransformed = source.sd_isTransformed;
     target.sd_decodeOptions = source.sd_decodeOptions;
     target.sd_imageLoopCount = source.sd_imageLoopCount;
     target.sd_imageFormat = source.sd_imageFormat;