How can I use a progress bar in objective c mac

Question

How can I use a progress bar in objective c mac? I have a code that downloads a file and I want to be able to let the progress bar increase by 1. A bit further on the code (to download the file) it needs to increase again. And so on...
My code
-(IBAction) downloadButtonPressed:(id)sender;{
    //get the path to the image that we're going to download
    NSURL *url = [NSURL URLWithString:@"https://www.dropbox.com/s/l6o07m48npxknt4/Cluedo.zip?dl=1"];
    //get the path to documents folder where we're going to save our image
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *DocumentsDirectory = [paths objectAtIndex:0];
    NSString *filePath = [DocumentsDirectory stringByAppendingPathComponent:@"Cluedo.zip"];
    //Download the file to memory
    NSData *data = [NSData dataWithContentsOfURL:url];
    NSError *error = nil;
    //Save it on the Documents directory
    [data writeToFile:filePath options:NSDataWritingAtomic error:&error];
    NSFileManager* fm = [NSFileManager defaultManager];
    NSString* zipPath = filePath;
    NSString* targetFolder = @"/Applications"; //this it the parent folder
    //where your zip's content
    //goes to (must exist)
    //create a new empty folder (unzipping will fail if any
    //of the payload files already exist at the target location)
    [fm createDirectoryAtPath:targetFolder withIntermediateDirectories:NO
                   attributes:nil error:NULL];
    //now create a unzip-task
    NSArray *arguments = [NSArray arrayWithObject:zipPath];
    NSTask *unzipTask = [[NSTask alloc] init];
    [unzipTask setLaunchPath:@"/usr/bin/unzip"];
    [unzipTask setCurrentDirectoryPath:targetFolder];
    [unzipTask setArguments:arguments];
    [unzipTask launch];
    //[unzipTask waitUntilExit]; //remove this to start the task concurrently

Answer 1

Your code really isn't suitable for a progress bar, not a real one anyway. Your code will spend about 99.9% of its time in the method "dataWithContentsOfURL:url". That is not a method you want to use in the real world. It is only for demos or proof of concept exercises. All network operations should be asynchronous. That gives you the ability to gracefully recover when there is a failure and also to do fancy things like display a progress bar and tell the user when the download is complete.
Once you get the asynchronous download logic working, then you can worry about the progress bar. The first thing you will need is the size of the file. Without that, all you can ever do is an indeterminate progress bar or spinner. If you have the size of the file, then you can keep track of how much of the file you have downloaded and update your progress bar with the percentage complete. Make sure to throttle it to no more than incrementing by 1% at a time. Otherwise, a large file or a file transferred in many small chunks, would waste too much time updating the progress bar for no change.