Add a convenience method to allow cancel on downloadToken

@@ -82,9 +82,16 @@ typedef SDHTTPHeadersDictionary * _Nullable (^SDWebImageDownloaderHeadersFilterB
 /**
  *  A token associated with each download. Can be used to cancel a download
  */
-@interface SDWebImageDownloadToken : NSObject
+@interface SDWebImageDownloadToken : NSObject <SDWebImageOperation>
 
+/**
+ The download's URL. This should be readonly and you should not modify
+ */
 @property (nonatomic, strong, nullable) NSURL *url;
+/**
+ The cancel token taken from `addHandlersForProgress:completed`. This should be readonly and you should not modify
+ @note use `-[SDWebImageDownloadToken cancel]` to cancel the token
+ */
 @property (nonatomic, strong, nullable) id downloadOperationCancelToken;
 
 @end

@@ -9,7 +9,23 @@
 #import "SDWebImageDownloader.h"
 #import "SDWebImageDownloaderOperation.h"
 
+@interface SDWebImageDownloadToken ()
+
+@property (nonatomic, weak, nullable) NSOperation<SDWebImageDownloaderOperationInterface> *downloadOperation;
+
+@end
+
 @implementation SDWebImageDownloadToken
+
+- (void)cancel {
+    if (self.downloadOperation) {
+        SDWebImageDownloadToken *cancelToken = self.downloadOperationCancelToken;
+        if (cancelToken) {
+            [self.downloadOperation cancel:cancelToken];
+        }
+    }
+}
+
 @end
 
 
@@ -258,6 +274,7 @@
         id downloadOperationCancelToken = [operation addHandlersForProgress:progressBlock completed:completedBlock];
 
         token = [SDWebImageDownloadToken new];
+        token.downloadOperation = operation;
         token.url = url;
         token.downloadOperationCancelToken = downloadOperationCancelToken;
     });

@@ -19,6 +19,7 @@ FOUNDATION_EXPORT NSString * _Nonnull const SDWebImageDownloadFinishNotification
 
 /**
  Describes a downloader operation. If one wants to use a custom downloader op, it needs to inherit from `NSOperation` and conform to this protocol
+ For the description about these methods, see `SDWebImageDownloaderOperation`
  */
 @protocol SDWebImageDownloaderOperationInterface<NSObject>
 
@@ -35,6 +36,8 @@ FOUNDATION_EXPORT NSString * _Nonnull const SDWebImageDownloadFinishNotification
 - (nullable NSURLCredential *)credential;
 - (void)setCredential:(nullable NSURLCredential *)value;
 
+- (BOOL)cancel:(nullable id)token;
+
 @end