ios - 如何使用异步请求下载文件？

Question

现在，当我的应用程序检测到服务器上的文件已更新时，它会下载文件，并且用户界面因下载时间而卡住。我的应用程序中有 ASIHTTPRequest 包装器，但我不知道如何将下载请求更改为异步。

我的代码:

- (void)downloadFileIfUpdated 
{
    NSString *urlString = @"http://www.mysite.com/data.plist";  
    NSLog(@"Downloading HTTP header from: %@", urlString);  
    NSURL *url = [NSURL URLWithString:urlString];  

    NSArray *documentPaths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
    NSString *cachedPath = [[documentPaths lastObject] stringByAppendingPathComponent:@"data.plist"];

    NSFileManager *fileManager = [NSFileManager defaultManager];  

    BOOL downloadFromServer = NO;  

    NSString *lastModifiedString = nil;
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
    [request setHTTPMethod:@"HEAD"];
    NSHTTPURLResponse *response;
    [NSURLConnection sendSynchronousRequest:request returningResponse:&response error: NULL];
    if ([response respondsToSelector:@selector(allHeaderFields)]) {
        lastModifiedString = [[response allHeaderFields] objectForKey:@"Last-Modified"];
    }

    NSDate *lastModifiedServer = nil;
    @try {
        NSDateFormatter *df = [[NSDateFormatter alloc] init];
        df.dateFormat = @"EEE',' dd MMM yyyy HH':'mm':'ss 'GMT'";
        df.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US"];
        df.timeZone = [NSTimeZone timeZoneWithAbbreviation:@"GMT"];
        lastModifiedServer = [df dateFromString:lastModifiedString];

    }
    @catch (NSException * e) {
    NSLog(@"Error parsing last modified date: %@ - %@", lastModifiedString, [e description]);
    }
  NSLog(@"lastModifiedServer: %@", lastModifiedServer);

    NSDate *lastModifiedLocal = nil;
    if ([fileManager fileExistsAtPath:cachedPath]) {
        NSError *error = nil;
        NSDictionary *fileAttributes = [fileManager attributesOfItemAtPath:cachedPath error:&error];
        if (error) {
            NSLog(@"Error reading file attributes for: %@ - %@", cachedPath, [error localizedDescription]);
        }
        lastModifiedLocal = [fileAttributes fileModificationDate];
         NSLog(@"lastModifiedLocal : %@", lastModifiedLocal);

        [activityIndicator stopAnimating];
    }

    // Download file from server if we don't have a local file
    if (!lastModifiedLocal) {
        downloadFromServer = YES;
    }
    // Download file from server if the server modified timestamp is later than the local modified timestamp
    if ([lastModifiedLocal laterDate:lastModifiedServer] == lastModifiedServer) {


        [activityIndicator startAnimating];


        downloadFromServer = YES;
    }

    if (downloadFromServer) {


        NSLog(@"Downloading new file from server");
        NSData *data = [NSData dataWithContentsOfURL:url];
        if (data) {
            // Save the data
            if ([data writeToFile:cachedPath atomically:YES]) {
                NSLog(@"Downloaded file saved to: %@", cachedPath);
            }

            // Set the file modification date to the timestamp from the server
            if (lastModifiedServer) {
                NSDictionary *fileAttributes = [NSDictionary dictionaryWithObject:lastModifiedServer forKey:NSFileModificationDate];
                NSError *error = nil;
                if ([fileManager setAttributes:fileAttributes ofItemAtPath:cachedPath error:&error]) {
                    NSLog(@"File modification date updated");
                    [NSThread detachNewThreadSelector:@selector(loadPList) toTarget:self withObject:nil];

                    [activityIndicator stopAnimating];



                }
                if (error) {
                NSLog(@"Error setting file attributes for: %@ - %@", cachedPath, [error localizedDescription]);
                }
            }
        }   


    }

}

Answer 1

 NSURL *url = [NSURL URLWithString:@"http://allseeing-i.com"];
 __block ASIHTTPRequest *request = [ASIHTTPRequest requestWithURL:url];
 [request setCompletionBlock:^{
    // Use when fetching text data
    NSString *responseString = [request responseString];

    // Use when fetching binary data
    NSData *responseData = [request responseData];
 }];
 [request setFailedBlock:^{
    NSError *error = [request error];
 }];
 [request startAsynchronous];

更多详情请查看http://allseeing-i.com/ASIHTTPRequest/How-to-use#using_blocks