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];

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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)connection DidFinishLoading:(NSURLConnection *)connection;

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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];  //可多次收到数据，把新数据添加到现有数据最后面
}

- (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;
}