NSURLConnection下载文件时，如何显示进度

这里以下载图片举例。
1，首先创建一个cconnection
downloadImage是一个线程函数，在子线程中下载图片。

//url 图片的url地址
- (void) downloadImage:(NSString*)url{ 
    self.uploadPool = [[NSAutoreleasePool alloc] init];
    self.characterBuffer = [NSMutableData data];
    done = NO;
    [[NSURLCache sharedURLCache] removeAllCachedResponses];

    NSMutableURLRequest *theRequest = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:url]];

    self.connection = [[NSURLConnection alloc] initWithRequest:theRequest delegate:self];
    [self performSelectorOnMainThread:@selector(httpConnectStart) withObject:nil waitUntilDone:NO];
    if (connection != nil) {
        do {
            [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
        } while (!done);
    }

    self.photo = [UIImage imageWithData:characterBuffer];

    [self performSelectorOnMainThread:@selector(fillPhoto) withObject:nil waitUntilDone:NO];

  // Release resources used only in this thread.
    self.connection = nil;
    [uploadPool release];
    self.uploadPool = nil;
}

2， 在NSURLConnection的delegate中接收数据
// Forward errors to the delegate.
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
    done = YES;
    [self performSelectorOnMainThread:@selector(httpConnectEnd) withObject:nil waitUntilDone:NO];
//NSLog(@"%@",[error localizedDescription]);
    [characterBuffer setLength:0];

}

// Called when a chunk of data has been downloaded.
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    // Process the downloaded chunk of data.

    //NSLog(@"%d", [data length]);

    received_ += [data length];
    [self performSelectorOnMainThread:@selector(updateProgress) withObject:nil waitUntilDone:NO];

    [characterBuffer appendData:data];

}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {

    //NSLog(@"%lld", received_);

    [self performSelectorOnMainThread:@selector(httpConnectEnd) withObject:nil waitUntilDone:NO];
    // Set the condition which ends the run loop.
    done = YES;
}
上面三个方法，第一个是连接出错的情况，第二个是接收数据，reveived_纪录一共接收了多少子节的数据。第三个是连接结束。
还有一个方法，我们可以从http返回的response中拿到一些连接的信息。这里我拿到了要下载的数据的大小，保存在"Content-Length"的字段中，详细的理论见HTTP协议。
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response{
    NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
    if(httpResponse && [httpResponse respondsToSelector:@selector(allHeaderFields)]){
        NSDictionary *httpResponseHeaderFields = [httpResponse allHeaderFields];

        total_ = [[httpResponseHeaderFields objectForKey:@"Content-Length"] longLongValue];

//NSLog(@"%lld", total_);
    }
}
有了要下载的数据的总大小，和已经下载的大小，进度自然就出来了。