使用NSConnection下载数据

1.创建NSConnection对象，设置委托对象

NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:[self urlString]]];
[NSURLConnection connectionWithRequest:request delegate:self];

2. NSURLConnection delegate委托方法
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response;
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error;
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data;
- (void)connectionDidFinishLoading:(NSURLConnection *)connection;

3. 实现委托方法
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
// store data
[self.receivedData setLength:0];            //通常在这里先清空接受数据的缓存
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
/* appends the new data to the received data */
[self.receivedData appendData:data];        //可能多次收到数据，把新的数据添加在现有数据最后
}

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
// 错误处理
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
// disconnect
[UIApplication sharedApplication].networkActivityIndicatorVisible = NO;
NSString *returnString = [[NSString alloc] initWithData:self.receivedData encoding:NSUTF8StringEncoding];
NSLog(returnString);
[self urlLoaded:[self urlString] data:self.receivedData];
firstTimeDownloaded = YES;
}