checkacceptranges.mm

NSDictionary* headers = [(NSHTTPURLResponse *)response allHeaderFields];
if ([headers objectForKey:@"Accept-Ranges"]) {
    _allowResume = YES;
}
else
{
    _allowResume = NO;
}

resume.mm

-(void)resume
{
    //add range for resume but some of the server didn't support resume
    NSString *range = @"bytes=";
    range = [range stringByAppendingString:[[NSNumber numberWithInt:self.receiveBytes] stringValue]];
    range = [range stringByAppendingString:@"-"];
    NSURL* fileURL = self.request.URL.absoluteURL;
    
    self.resumeProgress = YES;//add trigger for except exceptedBytes
    
    
    self.request = [[NSMutableURLRequest alloc] initWithURL:fileURL cachePolicy:NSURLRequestReturnCacheDataElseLoad timeoutInterval:self.tmpTimeout];
    
    [self.request setValue:range forHTTPHeaderField:@"Range"];
    
    self.connection = [[NSURLConnection alloc] initWithRequest:self.request delegate:self startImmediately:YES];
}

sgdownloader.mm

SGdownloader *downloader = [[SGdownloader alloc] initWithURL:[NSURL URLWithString:@"http://myfile.com/file.jpg"] timeout:60];

[downloader startWithDownloading:(float progress,NSInteger percentage) {

    //progress for progress bar
    //percentage for download percentage

} onFinished:(NSData* fileData,NSString* fileName){

    //use NSData to write a file or image

}onFail (NSError* error){

    //on fail

}];