iOS AFNetworking downloadProgressBlock计算返回负数

我正在使用AFNetworking下载使用第三方类解析的数据.我曾经多次使用AFNetworking做类似的动作,但出于某种原因,当我调用downloadProgressBlock并进行计算以使用我的进度条时,数字会返回负数.见下文：

2013-09-22 16:04:06.036 Portland Police[2228:60b] Download = -31849.000000
2013-09-22 16:04:06.041 Portland Police[2228:60b] Download = -40537.000000
2013-09-22 16:04:06.042 Portland Police[2228:60b] Download = -44881.000000
2013-09-22 16:04:06.044 Portland Police[2228:60b] Download = -53569.000000
2013-09-22 16:04:06.046 Portland Police[2228:60b] Download = -62257.000000
2013-09-22 16:04:06.048 Portland Police[2228:60b] Download = -63705.000000
2013-09-22 16:04:06.085 Portland Police[2228:60b] Download = -70945.000000
2013-09-22 16:04:06.087 Portland Police[2228:60b] Download = -89769.000000
2013-09-22 16:04:06.089 Portland Police[2228:60b] Download = -94113.000000
2013-09-22 16:04:06.100 Portland Police[2228:60b] Download = -98457.000000
2013-09-22 16:04:06.104 Portland Police[2228:60b] Download = -102801.000000
2013-09-22 16:04:06.111 Portland Police[2228:60b] Download = 1.000000

以下是我的代码：

// Get the URL we are going to use to parse with
AFHTTPClient *httpClient = [[AFHTTPClient alloc] initWithBaseURL:[NSURL URLWithString:@"http://www.portlandonline.com/scripts/911incidents.cfm"]];
NSURLRequest *request = [httpClient requestWithMethod:@"GET" path:nil parameters:nil];
AFHTTPRequestOperation *operation = [httpClient HTTPRequestOperationWithRequest:request success:^(AFHTTPRequestOperation *operation, id responseObject) {

    NSLog(@"Response = %@", operation);

    // Empty our calls out
    [self.originalArray removeAllObjects];

    // Initiate our parser
    MWFeedParser *parser = [[MWFeedParser alloc] init];
    parser.delegate = self;
    [parser startParsingData:responseObject textEncodingName:[operation.response textEncodingName] withCompletionHandler:^(NSError *error, MWFeedParser *parser) {

        // If theres no error
        if (!error) {

            // Return the success block
            success(operation.request, operation.request.URL, self.calls);
        }
        else {

            // Return the failure block
            failure(operation.request, operation.request.URL, error);
        }
    }];

} failure:^(AFHTTPRequestOperation *operation, NSError *error) {

    NSLog(@"AFHTTPRequestOperation Failure: %@", error);

    // Return our failure block
    failure(operation.request, operation.request.URL, error);
}];
[operation setDownloadProgressBlock:^(NSUInteger bytesRead, long long totalBytesRead, long long totalBytesExpectedToRead) {

    NSLog(@"Download = %f", (float)totalBytesRead / totalBytesExpectedToRead);

}];
[operation start];

我怀疑你会发现问题是totalBytesExpectedToRead.具体来说,这来自NSURLResponse属性expectedContentLength,其中 as the docs say：

Return Value

The receiver’s expected content length, or NSURLResponseUnknownLength if the length can’t be determined.

Discussion

Some protocol implementations report the content length as part of the response, but not all protocols guarantee to deliver that amount of data. Clients should be prepared to deal with more or less data.

顺便说一下,这个常量NSURLResponseUnknownLength等于-1,这可以解释你的计算值.这只是系统通知您无法确定响应长度的方式.

因此,在最坏的情况下,您将获得-1作为预期的字节数.即使它不是-1,它也不完全可靠(特别是如果你编写了自己的服务器代码就会出现问题).您的应用必须正常处理向您的应用报告的所有expectedContentLength.大多数情况下,如果你得到一个非负值,你通常得到的值可以有效地用于“完成百分比”计算,但不要依赖它(例如,你可能会收到更多字节或者比expectedContentLength报告更少的字节).优雅地处理特殊的价值观.

例如,在设置我的进度条时,我做了类似的事情：

CGFloat progress;

if (expectedContentLength > 0 && progressContentLength <= expectedContentLength)
     progress = (CGFloat) progressContentLength / expectedContentLength;
else
     progress = (progressContentLength % 1000000l) / 1000000.0f;

这给了我一个进度条,如果我有一个很好的预期内容长度值,随着下载的进展缓慢进展到100％,但另外显示我进度条从下载的每1,000,000字节的0％进展到100％.后一种情况并不理想,但除非您的服务器报告准确的预期内容长度,否则您无法做很多事情.

或者,如果您从未获得可靠的expectedContentLength,则可以选择使用不确定的进度视图(例如UIActivityIndi​​catorView),这样可以完全避免此问题.

最重要的是,成功的进度计算取决于您的服务器是否为预期的字节提供合法值.如果你看看totalBytesExpectedToRead,我敢打赌你得到-1,结果看到你好奇的计算进度百分比值.