NSURLSession: can't get resume data after calling cancelByProducingResumeData - ios

Question

I want to resume downloading after user cancels downloading or something wrong happens. But when I call cancelByProducingResumeData method, the resumeData is nil. So, I can't resume downloading. I'm sure the download link can be resumed, because our PC client can resume downloading of this link.
Here is my code. Here is the full project.
#import "ViewController.h"
#interface ViewController ()
{
NSURLSession *_session;
}
#property (weak, nonatomic) IBOutlet UIProgressView *progressView;
#property NSURLSessionDownloadTask *netTask;
#property NSData *resumeData;
#end
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
if (_session == nil) {
NSURLSessionConfiguration *confi = [NSURLSessionConfiguration defaultSessionConfiguration];
_session = [NSURLSession sessionWithConfiguration:confi delegate:self delegateQueue:[NSOperationQueue mainQueue]];
}
}
- (IBAction)startAction:(id)sender {
[self start];
}
- (IBAction)stopActon:(id)sender {
[self stop];
}
- (void)stop {
__weak typeof(self) vc = self;
[self.netTask cancelByProducingResumeData:^(NSData * _Nullable resumeData) {
vc.resumeData = resumeData;
vc.netTask = nil;
}];
}
- (void)start {
if (self.resumeData != nil) {
self.netTask = [_session downloadTaskWithResumeData:self.resumeData];
} else {
NSURL *downlaodURL = [NSURL URLWithString:#"http://sdl24.yunpan.cn/share.php?method=Share.download&cqid=37ef0df7c8155bacf55c237bd433ddd8&dt=24.02b6cbb4148de503fe35ddab08dac35b&e=1459317290&fhash=41181b28ff97806ef8469842b4a5eabc330a0c60&fname=feistudy%2B%2B%25E8%25AF%25AD%25E8%25A8%2580%25E5%25AD%25A6%25E4%25B9%25A0%25E6%2596%25B9%25E6%25B3%2595%25E8%25AE%25BA%25E8%25BF%25B0&fsize=87624815&nid=14471440239484082&st=e08142ab7c935cdd15ecc8851c82e819&xqid=22309244"];
self.netTask = [_session downloadTaskWithURL:downlaodURL];
}
[self.netTask resume];
}
- (NSString*)filePath
{
NSString *doc = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject];
NSString *path = [doc stringByAppendingPathComponent:#"p.rmvb"];
return path;
}
#pragma mark - NSURLSessionDownloadDelegate
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
didFinishDownloadingToURL:(NSURL *)location
{
dispatch_async(dispatch_get_main_queue(), ^{
NSFileManager *manager = [NSFileManager defaultManager];
[manager moveItemAtPath:location.path toPath:[self filePath] error:nil];
NSLog(#"locaton.path:%#", location.path);
NSLog(#"filePaht:%#",[self filePath]);
});
}
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
didWriteData:(int64_t)bytesWritten
totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
dispatch_async(dispatch_get_main_queue(), ^{
double progress = totalBytesWritten/(double)totalBytesExpectedToWrite;
NSLog(#"progress:%f",progress);
self.progressView.progress = progress;
});
}
#pragma mark - NSURLSessionTaskDelegate
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task
didCompleteWithError:(nullable NSError *)error
{
if (error) {
NSData *resumeData = error.userInfo[NSURLSessionDownloadTaskResumeData];
self.resumeData = resumeData;
}
}
#end

Answer 1

There's a long list of requirements for when resume data can be obtained. Among other things:
The response must contain an ETag or Last-Modified header.
I think the response must also contain an Accept-Ranges: bytes header
The temporary file must still exist (not low on disk space).
It must be an HTTP or HTTPS request.
The Last-Modified or ETag header must indicate that the file hasn't changed since it was last requested.
There might be other requirements that I'm forgetting, e.g. HTTP/1.1.