Refactory the current thumbnail && transformer about cache key. Developer should have the API to calcualte the cache key from thumbnail or transformer, not hard-coded.

SDWebImage/Core/SDImageCache.h

@@ -247,7 +247,7 @@ typedef NS_OPTIONS(NSUInteger, SDImageCacheOptions) {
 /**
  * Operation that queries the cache asynchronously and call the completion when done.
  *
- * @param key       The unique key used to store the wanted image
+ * @param key       The unique key used to store the wanted image. If you need transformer's image, calculate the key with `SDTransformedKeyForKey` or generate the cache key from url with `cacheKeyForURL:context:`.
  * @param doneBlock The completion block. Will not get called if the operation is cancelled
  *
  * @return a NSOperation instance containing the cache op
@@ -257,7 +257,7 @@ typedef NS_OPTIONS(NSUInteger, SDImageCacheOptions) {
 /**
  * Asynchronously queries the cache with operation and call the completion when done.
  *
- * @param key       The unique key used to store the wanted image
+ * @param key       The unique key used to store the wanted image. If you need transformer's image, calculate the key with `SDTransformedKeyForKey` or generate the cache key from url with `cacheKeyForURL:context:`.
  * @param options   A mask to specify options to use for this cache query
  * @param doneBlock The completion block. Will not get called if the operation is cancelled
  *
@@ -268,7 +268,7 @@ typedef NS_OPTIONS(NSUInteger, SDImageCacheOptions) {
 /**
  * Asynchronously queries the cache with operation and call the completion when done.
  *
- * @param key       The unique key used to store the wanted image
+ * @param key       The unique key used to store the wanted image. If you need transformer's image, calculate the key with `SDTransformedKeyForKey` or generate the cache key from url with `cacheKeyForURL:context:`.
  * @param options   A mask to specify options to use for this cache query
  * @param context   A context contains different options to perform specify changes or processes, see `SDWebImageContextOption`. This hold the extra objects which `options` enum can not hold.
  * @param doneBlock The completion block. Will not get called if the operation is cancelled

SDWebImage/Core/SDImageCache.m

@@ -9,7 +9,6 @@
 #import "SDImageCache.h"
 #import "NSImage+Compatibility.h"
 #import "SDImageCodersManager.h"
-#import "SDImageTransformer.h"
 #import "SDImageCoderHelper.h"
 #import "SDAnimatedImage.h"
 #import "UIImage+MemoryCacheCost.h"
@@ -455,13 +454,6 @@
         return nil;
     }
     
-    id<SDImageTransformer> transformer = context[SDWebImageContextImageTransformer];
-    if (transformer) {
-        // grab the transformed disk image if transformer provided
-        NSString *transformerKey = [transformer transformerKey];
-        key = SDTransformedKeyForKey(key, transformerKey);
-    }
-    
     // First check the in-memory cache...
     UIImage *image;
     if (queryCacheType != SDImageCacheTypeDisk) {

SDWebImage/Core/SDImageCoder.h

@@ -61,7 +61,7 @@ FOUNDATION_EXPORT SDImageCoderOption _Nonnull const SDImageCoderEncodeCompressio
  But this may be useful for some custom coders, because some business logic may dependent on things other than image or image data inforamtion only.
  See `SDWebImageContext` for more detailed information.
  */
-FOUNDATION_EXPORT SDImageCoderOption _Nonnull const SDImageCoderWebImageContext;
+FOUNDATION_EXPORT SDImageCoderOption _Nonnull const SDImageCoderWebImageContext API_DEPRECATED("The coder component will be seperated from Core subspec in the future. Update your code to not rely on this context option.", macos(10.10, API_TO_BE_DEPRECATED), ios(8.0, API_TO_BE_DEPRECATED), tvos(9.0, API_TO_BE_DEPRECATED), watchos(2.0, API_TO_BE_DEPRECATED));;
 
 #pragma mark - Coder
 /**

SDWebImage/Core/SDImageTransformer.h

@@ -18,6 +18,15 @@
  */
 FOUNDATION_EXPORT NSString * _Nullable SDTransformedKeyForKey(NSString * _Nullable key, NSString * _Nonnull transformerKey);
 
+/**
+ Return the thumbnailed cache key which applied with specify thumbnailSize and preserveAspectRatio control.
+ @param key The original cache key
+ @param thumbnailPixelSize The thumbnail pixel size
+ @param preserveAspectRatio The preserve aspect ratio option
+ @return The thumbnailed cache key
+ */
+FOUNDATION_EXPORT NSString * _Nullable SDThumbnailedKeyForKey(NSString * _Nullable key, CGSize thumbnailPixelSize, BOOL preserveAspectRatio);
+
 /**
  A transformer protocol to transform the image load from cache or from download.
  You can provide transformer to cache and manager (Through the `transformer` property or context option `SDWebImageContextImageTransformer`).
@@ -38,10 +47,10 @@ FOUNDATION_EXPORT NSString * _Nullable SDTransformedKeyForKey(NSString * _Nullab
  Transform the image to another image.
 
  @param image The image to be transformed
- @param key The cache key associated to the image
+ @param key The cache key associated to the image. This arg is a hint for image source, not always useful and should be nullable. In the future we will remove this arg.
  @return The transformed image, or nil if transform failed
  */
-- (nullable UIImage *)transformedImageWithImage:(nonnull UIImage *)image forKey:(nonnull NSString *)key;
+- (nullable UIImage *)transformedImageWithImage:(nonnull UIImage *)image forKey:(nonnull NSString *)key API_DEPRECATED("The key arg will be removed in the future. Update your code and don't rely on that.", macos(10.10, API_TO_BE_DEPRECATED), ios(8.0, API_TO_BE_DEPRECATED), tvos(9.0, API_TO_BE_DEPRECATED), watchos(2.0, API_TO_BE_DEPRECATED));
 
 @end
 

SDWebImage/Core/SDImageTransformer.m

@@ -38,6 +38,11 @@ NSString * _Nullable SDTransformedKeyForKey(NSString * _Nullable key, NSString *
     }
 }
 
+NSString * _Nullable SDThumbnailedKeyForKey(NSString * _Nullable key, CGSize thumbnailPixelSize, BOOL preserveAspectRatio) {
+    NSString *thumbnailKey = [NSString stringWithFormat:@"Thumbnail({%f,%f},%d)", thumbnailPixelSize.width, thumbnailPixelSize.height, preserveAspectRatio];
+    return SDTransformedKeyForKey(key, thumbnailKey);
+}
+
 @interface SDImagePipelineTransformer ()
 
 @property (nonatomic, copy, readwrite, nonnull) NSArray<id<SDImageTransformer>> *transformers;

SDWebImage/Core/SDWebImageManager.h

@@ -266,4 +266,10 @@ SDWebImageManager *manager = [SDWebImageManager sharedManager];
  */
 - (nullable NSString *)cacheKeyForURL:(nullable NSURL *)url;
 
+/**
+ * Return the cache key for a given URL and context option.
+ * Some option like `.thumbnailPixelSize` and `imageTransformer` will effect the generated cache key, using this if you have those context associated.
+*/
+- (nullable NSString *)cacheKeyForURL:(nullable NSURL *)url context:(nullable SDWebImageContext *)context;
+
 @end

SDWebImage/Core/SDWebImageManager.m

@@ -114,6 +114,7 @@ static id<SDImageLoader> _defaultImageLoader;
     } else {
         key = url.absoluteString;
     }
+    
     // Thumbnail Key Appending
     NSValue *thumbnailSizeValue = context[SDWebImageContextImageThumbnailPixelSize];
     if (thumbnailSizeValue != nil) {
@@ -123,14 +124,21 @@ static id<SDImageLoader> _defaultImageLoader;
 #else
         thumbnailSize = thumbnailSizeValue.CGSizeValue;
 #endif
-        
         BOOL preserveAspectRatio = YES;
         NSNumber *preserveAspectRatioValue = context[SDWebImageContextImagePreserveAspectRatio];
         if (preserveAspectRatioValue != nil) {
             preserveAspectRatio = preserveAspectRatioValue.boolValue;
         }
-        NSString *transformerKey = [NSString stringWithFormat:@"Thumbnail({%f,%f},%d)", thumbnailSize.width, thumbnailSize.height, preserveAspectRatio];
-        key = SDTransformedKeyForKey(key, transformerKey);
+        key = SDThumbnailedKeyForKey(key, thumbnailSize, preserveAspectRatio);
+    }
+    
+    // Transformer Key Appending
+    id<SDImageTransformer> transformer = self.transformer;
+    if (context[SDWebImageContextImageTransformer]) {
+        transformer = context[SDWebImageContextImageTransformer];
+    }
+    if (transformer) {
+        key = SDTransformedKeyForKey(key, transformer.transformerKey);
     }
     
     return key;
@@ -409,8 +417,6 @@ static id<SDImageLoader> _defaultImageLoader;
             @autoreleasepool {
                 UIImage *transformedImage = [transformer transformedImageWithImage:originalImage forKey:key];
                 if (transformedImage && finished) {
-                    NSString *transformerKey = [transformer transformerKey];
-                    NSString *cacheKey = SDTransformedKeyForKey(key, transformerKey);
                     BOOL imageWasTransformed = ![transformedImage isEqual:originalImage];
                     NSData *cacheData;
                     // pass nil if the image was transformed, so we can recalculate the data from the image
@@ -421,7 +427,7 @@ static id<SDImageLoader> _defaultImageLoader;
                     }
                     // keep the original image format and extended data
                     SDImageCopyAssociatedObject(originalImage, transformedImage);
-                    [self storeImage:transformedImage imageData:cacheData forKey:cacheKey cacheType:storeCacheType options:options context:context completion:^{
+                    [self storeImage:transformedImage imageData:cacheData forKey:key cacheType:storeCacheType options:options context:context completion:^{
                         [self callCompletionBlockForOperation:operation completion:completedBlock image:transformedImage data:originalData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                     }];
                 } else {