Add transformer property in manager level to allow a central control of image transform(optional)

SDWebImage/SDWebImageDefine.h

@@ -22,6 +22,6 @@ FOUNDATION_EXPORT SDWebImageContextOption _Nonnull const SDWebImageContextSetIma
 FOUNDATION_EXPORT SDWebImageContextOption _Nonnull const SDWebImageContextCustomManager;
 
 /**
- A id<SDWebImageTransformer> instance which conforms SDWebImageTransformer protocol. It's used for image transform after the image load finished and store the transformed image to cache. (id<SDWebImageTransformer>)
+ A id<SDWebImageTransformer> instance which conforms SDWebImageTransformer protocol. It's used for image transform after the image load finished and store the transformed image to cache. If you provide one, it will ignore the `transformer` in manager and use provided one instead. (id<SDWebImageTransformer>)
  */
 FOUNDATION_EXPORT SDWebImageContextOption _Nonnull const SDWebImageContextCustomTransformer;

SDWebImage/SDWebImageManager.h

@@ -10,6 +10,7 @@
 #import "SDWebImageOperation.h"
 #import "SDWebImageDownloader.h"
 #import "SDImageCache.h"
+#import "SDWebImageTransformer.h"
 
 typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
     /**
@@ -183,6 +184,13 @@ SDWebImageManager *manager = [SDWebImageManager sharedManager];
 @property (strong, nonatomic, readonly, nullable) SDImageCache *imageCache;
 @property (strong, nonatomic, readonly, nullable) SDWebImageDownloader *imageDownloader;
 
+/**
+ The image transformer for manager. It's used for image transform after the image load finished and store the transformed image to cache, see `SDWebImageTransformer`.
+ Defaults to nil, which means no transform is applied.
+ @note This will affect all the load requests for this manager if you provide. However, you can pass `SDWebImageContextCustomTransformer` in context arg to explicitly use that transformer instead.
+ */
+@property (strong, nonatomic, nullable) id<SDWebImageTransformer> transformer;
+
 /**
  * The cache filter is a block used each time SDWebImageManager need to convert an URL into a cache key. This can
  * be used to remove dynamic part of an image URL.

SDWebImage/SDWebImageManager.m

@@ -8,8 +8,6 @@
 
 #import "SDWebImageManager.h"
 #import "NSImage+Additions.h"
-#import <objc/message.h>
-#import "SDWebImageTransformer.h"
 
 @interface SDWebImageCombinedOperation : NSObject <SDWebImageOperation>
 
@@ -156,6 +154,18 @@
     if (options & SDWebImageQueryDiskSync) cacheOptions |= SDImageCacheQueryDiskSync;
     if (options & SDWebImageTransformAnimatedImage) cacheOptions |= SDImageCacheTransformAnimatedImage;
     
+    // Image transformer
+    id<SDWebImageTransformer> transformer;
+    if ([context valueForKey:SDWebImageContextCustomTransformer]) {
+        transformer = [context valueForKey:SDWebImageContextCustomTransformer];
+    } else if (self.transformer) {
+        // Transformer from manager
+        transformer = self.transformer;
+        NSMutableDictionary<SDWebImageContextOption, id> *mutableContext = [NSMutableDictionary dictionaryWithDictionary:context];
+        [mutableContext setValue:transformer forKey:SDWebImageContextCustomTransformer];
+        context = [mutableContext copy];
+    }
+    
     __weak SDWebImageCombinedOperation *weakOperation = operation;
     operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key options:cacheOptions done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
         __strong __typeof(weakOperation) strongOperation = weakOperation;
@@ -240,9 +250,8 @@
 
                     if (options & SDWebImageRefreshCached && cachedImage && !downloadedImage) {
                         // Image refresh hit the NSURLCache cache, do not call the completion block
-                    } else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [context valueForKey:SDWebImageContextCustomTransformer]) {
+                    } else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && transformer) {
                         dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
-                            id<SDWebImageTransformer> transformer = [context valueForKey:SDWebImageContextCustomTransformer];
                             UIImage *transformedImage = [transformer transformedImageWithImage:downloadedImage forKey:key];
                             if (transformedImage && finished) {
                                 NSString *transformerKey = [transformer transformerKey];

SDWebImage/SDWebImageTransformer.h

@@ -11,8 +11,7 @@
 
 /**
  A transformer protocol to transform the image load from cache or from download.
- For cache, it store the transformed image to memory cache if the memory cache missed (Default).
- For download, it store the transformed image to disk cache and memory cache if the download finished (Default).
+ You can provide transformer to cache and manager (Through the `transformer` property or context option `SDWebImageContextCustomTransformer`).
  
  @note The transform process is called from a global queue in order to not to block the main queue.
  */
@@ -42,7 +41,7 @@
 // Separator for different transformerKey, for example, `image.png` |> flip(YES,NO) |> rotate(pi/4,YES) => 'image-SDWebImageFlippingTransformer(1,0)-SDWebImageRotationTransformer(0.78539816339,1).png'
 FOUNDATION_EXPORT NSString * _Nonnull const SDWebImageTransformerKeySeparator;
 
-// Pipeline transformer. Which you can bind multiple transformers together to let the image to be transformed one by one and generate the final image.
+// Pipeline transformer. Which you can bind multiple transformers together to let the image to be transformed one by one in order and generate the final image.
 @interface SDWebImagePipelineTransformer : NSObject <SDWebImageTransformer>
 
 @property (nonatomic, copy, readonly, nonnull) NSArray<id<SDWebImageTransformer>> *transformers;