Add a public API to allow user to invalidate URLSession used in SDWebImageDownloader to avoid memory leak on non-singleton instance

SDWebImage/SDWebImageDownloader.h

@@ -246,4 +246,11 @@ typedef SDHTTPHeadersDictionary * _Nullable (^SDWebImageDownloaderHeadersFilterB
  */
 - (void)createNewSessionWithConfiguration:(nonnull NSURLSessionConfiguration *)sessionConfiguration;
 
+/**
+ * Invalidates the managed session, optionally canceling pending operations.
+ * If you use custom downloader instead of shared downloader, you need call this method when you do not use it to avoid memory leak
+ * @param cancelPendingOperations Whether or not to cancel pending operations.
+ */
+- (void)invalidateSessionAndCancel:(BOOL)cancelPendingOperations;
+
 @end

SDWebImage/SDWebImageDownloader.m

@@ -107,6 +107,14 @@
                                             delegateQueue:nil];
 }
 
+- (void)invalidateSessionAndCancel:(BOOL)cancelPendingOperations {
+    if (cancelPendingOperations) {
+        [self.session invalidateAndCancel];
+    } else {
+        [self.session finishTasksAndInvalidate];
+    }
+}
+
 - (void)dealloc {
     [self.session invalidateAndCancel];
     self.session = nil;