ios - 使用 NSURLSessionDownloadTask 只下载文件的一部分

Question

我正在尝试使用 NSURLSessionDownloadTask 下载一个大文件。但是，我只想下载该文件的前 10MB。我知道如何在下载 10MB 后取消操作，但如何强制调用 didFinishDownloadingToURL 以检索保存文件的位置？

-(void) downloadFileWithProgress
{
    NSURL * url = [NSURL URLWithString:@"https://s3.amazonaws.com/hayageek/downloads/SimpleBackgroundFetch.zip"];
    NSURLSessionConfiguration *defaultConfigObject = [NSURLSessionConfiguration defaultSessionConfiguration];
    NSURLSession *defaultSession = [NSURLSession sessionWithConfiguration: defaultConfigObject delegate:self delegateQueue: [NSOperationQueue mainQueue]];

    NSURLSessionDownloadTask * downloadTask =[ defaultSession downloadTaskWithURL:url];
    [downloadTask resume];

}
-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location
{
    NSLog(@"Temporary File :%@\n", location);
    NSError *err = nil;
    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSString *docsDir = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
 
    NSURL *docsDirURL = [NSURL fileURLWithPath:[docsDir stringByAppendingPathComponent:@"out1.zip"]];
    if ([fileManager moveItemAtURL:location
                             toURL:docsDirURL
                             error: &err])
    {
        NSLog(@"File is saved to =%@",docsDir);
    }
    else
    {
        NSLog(@"failed to move: %@",[err userInfo]);
    }
 
}
 
-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
    //You can get progress here
    NSLog(@"Received: %lld bytes (Downloaded: %lld bytes)  Expected: %lld bytes.\n",
          bytesWritten, totalBytesWritten, totalBytesExpectedToWrite);
}

Answer 1

如果服务器支持，最佳解决方案是使用 HTTP 范围请求而不是搞乱 NSURLSessionDownloadTask。

http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.35.2