iPhone应用下载的NSURL连接未完成即终止

Question

我有这段代码可以从服务器上下载mp3。一切都是通过解析播客的表视图来设置的，mp3的链接是_entry.articleURL。几分钟后，iPhone终止了连接，我最终只下载了mp3的一小部分。你知道这是什么原因造成的吗？

-(void)didSelectRowAtIndexPath 
{     RSSEntry *entry = [_allEntries objectAtIndex:indexPath.row];

             self.nameit = entry.articleTitle;
             NSURL *url = [NSURL URLWithString:entry.articleUrl];    
             NSURLRequest *theRequest = [NSURLRequest requestWithURL:url cachePolicy:NSURLRequestReloadIgnoringLocalCacheData timeoutInterval:60];
             __block NSURLConnection *connection = [NSURLConnection connectionWithRequest:theRequest delegate:self];

             UIApplication *application = [UIApplication sharedApplication]; //Get the shared application instance

             __block UIBackgroundTaskIdentifier background_task; //Create a task object

             background_task = [application beginBackgroundTaskWithExpirationHandler: ^ {
                 // This code gets called when your app has been running in the background too long and the OS decides to kill it
                 // You might want to cancel your connection in this case, that way you won't receive delegate methods any longer.
                 [connection cancel];
                 [application endBackgroundTask: background_task]; //Tell the system that we are done with the tasks
                 background_task = UIBackgroundTaskInvalid; //Set the task to be invalid

                 //System will be shutting down the app at any point in time now
             }];

             self.backgroundTaskIdentifier = background_task;
             if (connection) {
                 receivedData = [[NSMutableData data] retain];
                 self.thetable = tableView;
                 self.thepath = indexPath;
             }
             else {
                 UIAlertView *cancelled = [[UIAlertView alloc] initWithTitle:@"Download Failed" message:@"Please check your network settings, and then retry the download." delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil];
                 [cancelled show];
                 [cancelled release];
             }
}

- (void) connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
[UIApplication sharedApplication].networkActivityIndicatorVisible = YES;
progress.hidden = NO;
downloadInProgress = YES;
RSSEntry *entry = [_allEntries objectAtIndex:thepath.row];

self.nameit = entry.articleTitle;
downloadlabel.text = [NSString stringWithFormat:@"%@", nameit];
[thebar addSubview:downloadlabel];
[receivedData setLength:0];
expectedBytes = [response expectedContentLength];
}

- (void) connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
[receivedData appendData:data];
float progressive = (float)[receivedData length] / (float)expectedBytes;
[progress setProgress:progressive];


}

- (void) connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
[UIApplication sharedApplication].networkActivityIndicatorVisible = NO;
[connection release];
UIAlertView *connectionfailed = [[UIAlertView alloc] initWithTitle:@"Download Failed" message:@"Please check your network settings, and then retry the download." delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil];
[connectionfailed show];
[connectionfailed release];
progress.hidden = YES;
downloadInProgress = NO;
[downloadlabel removeFromSuperview];
[thetable deselectRowAtIndexPath:thepath animated:YES]; 
[[UIApplication sharedApplication] endBackgroundTask:self.backgroundTaskIdentifier];
self.backgroundTaskIdentifier = UIBackgroundTaskInvalid;
[connection release];



}

- (NSCachedURLResponse *) connection:(NSURLConnection *)connection willCacheResponse:(NSCachedURLResponse *)cachedResponse {
return nil;
}

- (void) connectionDidFinishLoading:(NSURLConnection *)connection {
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);

NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *pdfPath = [documentsDirectory stringByAppendingPathComponent:[nameit stringByAppendingString:@".mp3"]];
NSLog(@"Succeeded! Received %d bytes of data",[receivedData length]);
[UIApplication sharedApplication].networkActivityIndicatorVisible = NO;
[receivedData writeToFile:pdfPath atomically:YES];
progress.hidden = YES;
downloadInProgress = NO;
[downloadlabel removeFromSuperview];

[thetable deselectRowAtIndexPath:thepath animated:YES]; 

[[UIApplication sharedApplication] endBackgroundTask:self.backgroundTaskIdentifier];
self.backgroundTaskIdentifier = UIBackgroundTaskInvalid;
}

问题是connectionDidFinishLoading似乎一直被调用，即使是不完整的。有什么想法吗？

Answer 1

我不确定你为什么要使用beginBackgroundTaskWithExpirationHandler来做这项工作--它的目的是为了一个完全不同的任务(在你的应用被移到后台之后做一些工作。您在前台使用它很可能是导致您的问题的原因。

我建议你去github等网站上寻找一个并发NSOperation示例项目，并用它们来做你的异步NSURLConnections。

此外，您还可以在回调中更新视图中的GUI元素。请记住，您必须在主线程上使用UIKit。如果您需要更新某些内容，只需使用一个块并将其分派到主队列进行更新。