用urlconnection下载图片，接收的数据总大小为什么和一次接收到的数据的大小是一样的？

这样的话，做进度条的时候总是1.0 。
代码如下：

 -(void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response{
    [_downloadData setLength:0];
    _response = response;
}

-(void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data{
    [_downloadData appendData:data];
    if (_downloadProgress) {
        _totalBytesRead += (long long)data.length;
        _downloadProgress(data.length,_totalBytesRead,_response.expectedContentLength);
    }
}

输出如下：

 read:33841    totoalread:33841    totalexpected:33841
bytes read:33335    totoalread:67176    totalexpected:33335
bytes read:100142    totoalread:167318    totalexpected:100142

直接打印一下你获取的进度大小是多少，进度条是0-1.0之间。

[_downloadData setLength:0]; 你反复把它置0了啊