How can I check that an NSData blob is valid as resumeData for an NSURLSessionDownloadTask?

Question

I have an app that\'s using background downloads with the new NSURLSession APIs. When a download cancels or fails in such a way that NSURLSessionDownloadT

Answer 1

actually, the resume data is a plist file. it contains the follows key:

• NSURLSessionDownloadURL
• NSURLSessionResumeBytesReceived
• NSURLSessionResumeCurrentRequest
• NSURLSessionResumeEntityTag
• NSURLSessionResumeInfoTempFileName
• NSURLSessionResumeInfoVersion
• NSURLSessionResumeOriginalRequest

• NSURLSessionResumeServerDownloadDate so the steps u need to do are:

  1. check the data is a valid plist;
  2. check the plist have keys as above;
  3. check the temp file is exist;

Answer 2

This is the workaround suggested by Apple:

- (BOOL)__isValidResumeData:(NSData *)data{
    if (!data || [data length] < 1) return NO;

    NSError *error;
    NSDictionary *resumeDictionary = [NSPropertyListSerialization propertyListWithData:data options:NSPropertyListImmutable format:NULL error:&error];
    if (!resumeDictionary || error) return NO;

    NSString *localFilePath = [resumeDictionary objectForKey:@"NSURLSessionResumeInfoLocalPath"];
    if ([localFilePath length] < 1) return NO;

    return [[NSFileManager defaultManager] fileExistsAtPath:localFilePath];
}

Edit (iOS 7.1 is not NDA'd anymore): I got this from a Twitter exchange with an Apple engineer, he suggested what to do, and I wrote the above implementation

Answer 3

I have not found an answer to how to tell if the data is valid ahead of time.

However, I am presently working around the issue like so:

NSData *resumeData = ...;
NSURLRequest *originalURLRequest = ...;
NSURLSessionDownloadTask *downloadTask = nil;

@try {
    downloadTask = [session downloadTaskWithResumeData:resumeData];
}
@catch (NSException *exception) {
    if ([NSInvalidArgumentException isEqualToString:exception.name]) {
        downloadTask = [session downloadTaskWithRequest:originalURLRequest];
    } else {
        @throw exception; // only swallow NSInvalidArgumentException for resumeData
    }
}