Javascript 如何从调度队列循环返回，该循环连续4次向我发出错误警报？

我正在使用dispatch queue从服务器下载PDF文件，并在当前视图中加载为PDF预览，当有效的PDF url可用时，该预览工作正常

但当一个断开的URL链接来自服务器时，我调用了一个警报视图来向用户显示一条错误消息。 而此警报会弹出多次

有人能帮我从这个循环中返回吗

     - (void)viewPdf:(CDVInvokedUrlCommand*)command
    {
        if (command.arguments[0] == (id)[NSNull null])
        {
            [self.commandDelegate sendPluginResult:[CDVPluginResult resultWithStatus:CDVCommandStatus_ERROR] callbackId:command.callbackId];
            return;
        }

        NSString *requestIdStr;
        NSDictionary* options = [[NSDictionary alloc]init];

        if ([command.arguments count] > 0) {
            options = [command argumentAtIndex:0];
            requestIdStr = [options objectForKey:@"requestId"];
        }
        NSString *url = requestIdStr;

  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND,(unsigned long)NULL), ^(void)
               {
                   NSString *documentsDirectory = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) objectAtIndex:0];
                   NSString *file = [documentsDirectory stringByAppendingPathComponent:[NSString stringWithFormat:@"PDF.pdf"]];
                   NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
                   [request setURL:[NSURL URLWithString:url]];
                   [request setHTTPMethod:@"GET"];
                   [NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *error)
                    {
                        NSLog(@"Error... %@",error);//Console message shown below
                        NSLog(@"pdf downloaded, opening...");
                        NSData *pdf = data;
                        CGDataProviderRef provider = CGDataProviderCreateWithCFData((__bridge CFDataRef)pdf);
                        CGPDFDocumentRef document = CGPDFDocumentCreateWithProvider(provider);

                        if (document == nil) {
                            // error
                            NSLog(@"%@",@"error PDF broken link");


                         //***********  This alert Pops up multiple times ************//
                            UIAlertView * alert = [[UIAlertView alloc]initWithTitle:@"" message:@"Error while opening PDF" delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil, nil];
                            [alert show];
                           // return ;// I have used this "return " but no use
                           //Or am I calling this alertview view at wrong place
                          //*************************************** //

                        }
                        else
                        {
                            NSURL *localURL = [NSURL fileURLWithPath:file];
                            [pdf writeToFile:file options:NSDataWritingAtomic error:&error];
                            self.documentInteractionController = [UIDocumentInteractionController interactionControllerWithURL:localURL];
                            [self.documentInteractionController setDelegate:self];
                            [self.documentInteractionController presentPreviewAnimated:NO];
                         }
                         CGDataProviderRelease(provider);
                        CGPDFDocumentRelease(document);
                    }];
               });
 [self.commandDelegate sendPluginResult:[CDVPluginResult resultWithStatus:CDVCommandStatus_OK] callbackId:command.callbackId];
     }

下载无效url时的控制台消息：

错误。。。。。。Error Domain=NSURERRORDOMAIN Code=-1007“太多HTTP重定向”UserInfo=0x7aed5a80{NSERRORFAILINGURSTRINGKEY=，NSERRORFAILINGURKEY=，NSLocalizedDescription=太多HTTP重定向，NSUnderlyingError=0x7ae8ab80“太多HTTP重定向”} 找不到PDF标题：找不到“%PDF”

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感谢您的帮助！谢谢大家

尝试在主线程上显示警报并检查错误变量。