Add one sync API for SDImageCache to directly get the image data from disk instead of image instance

SDWebImage/SDImageCache.h

@@ -178,6 +178,14 @@ typedef void(^SDWebImageCalculateSizeBlock)(NSUInteger fileCount, NSUInteger tot
  */
 - (BOOL)diskImageDataExistsWithKey:(nullable NSString *)key;
 
+/**
+ *  Query the image data for the given key synchronously.
+ *
+ *  @param key The unique key used to store the wanted image
+ *  @return The image data for the given key, or nil if not found.
+ */
+- (nullable NSData *)diskImageDataForKey:(nullable NSString *)key;
+
 /**
  * Operation that queries the cache asynchronously and call the completion when done.
  *
@@ -203,6 +211,7 @@ typedef void(^SDWebImageCalculateSizeBlock)(NSUInteger fileCount, NSUInteger tot
  * Query the memory cache synchronously.
  *
  * @param key The unique key used to store the image
+ * @return The image for the given key, or nil if not found.
  */
 - (nullable UIImage *)imageFromMemoryCacheForKey:(nullable NSString *)key;
 
@@ -210,6 +219,7 @@ typedef void(^SDWebImageCalculateSizeBlock)(NSUInteger fileCount, NSUInteger tot
  * Query the disk cache synchronously.
  *
  * @param key The unique key used to store the image
+ * @return The image for the given key, or nil if not found.
  */
 - (nullable UIImage *)imageFromDiskCacheForKey:(nullable NSString *)key;
 
@@ -217,6 +227,7 @@ typedef void(^SDWebImageCalculateSizeBlock)(NSUInteger fileCount, NSUInteger tot
  * Query the cache (memory and or disk) synchronously after checking the memory cache.
  *
  * @param key The unique key used to store the image
+ * @return The image for the given key, or nil if not found.
  */
 - (nullable UIImage *)imageFromCacheForKey:(nullable NSString *)key;
 

SDWebImage/SDImageCache.m

@@ -399,6 +399,18 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
     return exists;
 }
 
+- (nullable NSData *)diskImageDataForKey:(nullable NSString *)key {
+    if (!key) {
+        return nil;
+    }
+    __block NSData *imageData = nil;
+    dispatch_sync(self.ioQueue, ^{
+        imageData = [self diskImageDataBySearchingAllPathsForKey:key];
+    });
+    
+    return imageData;
+}
+
 - (nullable UIImage *)imageFromMemoryCacheForKey:(nullable NSString *)key {
     return [self.memCache objectForKey:key];
 }
@@ -459,7 +471,7 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
 }
 
 - (nullable UIImage *)diskImageForKey:(nullable NSString *)key {
-    NSData *data = [self diskImageDataBySearchingAllPathsForKey:key];
+    NSData *data = [self diskImageDataForKey:key];
     return [self diskImageForKey:key data:data];
 }