Simple progress bar is not updating

Question

I am implementing a Cocoa Application which is just a simple progress bar that starts when I press a button.

The situation is: I can see Animation is Start and Stop when I press the button, but the progress bar will not update the value.

I had also tried the solution mentioned here but it doesn't work:
How do I update a progress bar in Cocoa during a long running loop?

Can someone help to see where is the problem in my source code?

Here is my source.

SimpleProgressBar.m

#import "SimpleProgressBar.h"

@implementation SimpleProgressBar
@synthesize progressBar;

int flag=0;

-(IBAction)startProgressBar:(id)sender{

    if(flag ==0){
        [self.progressBar startAnimation:sender];
        flag=1;
    }else{
        [self.progressBar stopAnimation:sender];
        flag=0;
    }
    [self.progressBar displayIfNeeded];
    [self.progressBar setDoubleValue:10.0];

    int i=0;

    for(i=0;i<100;i++){

        NSLog(@"progr: %f",(double)i);
        [self.progressBar setDoubleValue:(double)i];
        [self.progressBar setNeedsDisplay:YES];
    }


}

@end

SimpleProgressBar.h

#import < Foundation/Foundation.h >

@interface SimpleProgressBar : NSObject{

    __weak NSProgressIndicator *progressBar;

}

@property (weak) IBOutlet NSProgressIndicator *progressBar;

-(IBAction)startProgressBar:(id)sender;
@end

Thank you very much for any helpful answer.

Update:

Here is my porting from the solution and it doesn't work:

SimpleProgressBar.m

#import "SimpleProgressBar.h"

@implementation SimpleProgressBar
@synthesize progressBar;

int flag=0;

-(IBAction)startProgressBar:(id)sender{

    if(flag ==0){
        [self.progressBar startAnimation:sender];
        flag=1;
    }else{
        [self.progressBar stopAnimation:sender];
        flag=0;
    }
    [self.progressBar displayIfNeeded];
    [self.progressBar setDoubleValue:0.0];

    void(^progressBlock)(void);

    progressBlock = ^{

        [self.progressBar setDoubleValue:0.0];

        int i=0;

        for(i=0;i<100;i++){

            //double progr = (double) i / (double)100.0;
            double progr = (double) i;
            NSLog(@"progr: %f",progr);

             dispatch_async(dispatch_get_main_queue(),^{
             [self.progressBar setDoubleValue:progr];
             [self.progressBar setNeedsDisplay:YES];
             });

        }
    };

    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    dispatch_async(queue,progressBlock);

}

Answer 1

Update:

A couple of observations:

1. It strikes me that if you want to watch the NSProgressIndicator advance, you need to add a sleepForTimeInterval or else the for loop iterates so quickly that you won't see the progress indicator advance, but rather you'll just see it quickly end up in its final state. If you insert sleepForTimeInterval, you should see it progress:

   self.progressIndicator.minValue = 0.0;
   self.progressIndicator.maxValue = 5.0;
   [self.progressIndicator setIndeterminate:NO];
   
   self.progressIndicator.doubleValue = 0.001; // if you want to see it animate the first iteration, you need to start it at some small, non-zero value
   
   for (NSInteger i = 1; i <= self.progressIndicator.maxValue; i++)
   {
       [NSThread sleepForTimeInterval:1.0];
       [self.progressIndicator setDoubleValue:(double)i];
       [self.progressIndicator displayIfNeeded];
   }
   

   Or, if you wanted to do the for loop on a background thread, and dispatch the updates back to the main queue:

   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
       for (NSInteger i = 1; i <= self.progressIndicator.maxValue; i++)
       {
           [NSThread sleepForTimeInterval:1.0];
           dispatch_async(dispatch_get_main_queue(), ^{
               [self.progressIndicator setDoubleValue:(double)i];
               [self.progressIndicator displayIfNeeded];
           });
       }
   });
   

2. You are using startAnimation and stopAnimation, but according to the documentation each of these "does nothing for a determinate progress indicator," so these calls seem inappropriate for this situation.

---

My original answer, below, was predicated on the comment in the Threads and Your User Interface in the Threading Programming Guide, which says:

If your application has a graphical user interface, it is recommended that you receive user-related events and initiate interface updates from your application’s main thread. This approach helps avoid synchronization issues associated with handling user events and drawing window content. Some frameworks, such as Cocoa, generally require this behavior, but even for those that do not, keeping this behavior on the main thread has the advantage of simplifying the logic for managing your user interface.

But the answer below is (incorrectly) an iOS answer, so is not applicable.

Original answer:

Your for loop is running on the main thread, and thus UI updates won't appear until you yield back to the runloop. You're also going through that loop so quickly that even if you properly dispatched that to a background queue, you wouldn't experience the progress view changing as you iterate through your loop.

So, perform the loop on a secondary thread (e.g. via GCD or operation queue) and then dispatch UI updates back to the main thread, which is now free to do UI updates. So, using your theoretical example, you could do something like:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    for (int i = 0; i < 100; i++)
    {
        [NSThread sleepForTimeInterval:0.1];

        dispatch_async(dispatch_get_main_queue(), ^{
            [self.progressView setProgress: (CGFloat) (i + 1.0) / 100.0 animated:YES];
        });
    }
});

Note, having a loop that updates the progress view only makes sense if you're doing something slow enough for you to see the progress view change. In your original example, you're just looping from 0 to 99, updating the progress view. But that happens so quickly, that there's no point in a progress view in that case. That's why my above example not only employs a background queue for the loop, but also added a slight delay (via sleepForTimeInterval).

Let's consider a more realistic application of the progress view. For example, let's say I had an array, urls, of NSURL objects that represent items to be downloaded from the server. Then I might do something like:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    for (int i = 0; i < [urls count]; i++)
    {
        // perform synchronous network request (on main queue, you should only do asynchronous network requests, but on background queue, synchronous is fine, and in this case, needed)

        NSError *error = nil;
        NSURLResponse *response = nil;
        NSURLRequest *request = [NSURLRequest requestWithURL:urls[i]];
        NSData *data = [NSURLConnection sendSynchronousRequest:request returningResponse:response error:error];

        // got it; now update my model and UI on the basis of what I just downloaded

        dispatch_async(dispatch_get_main_queue(), ^{
            [self.progressView setProgress: (CGFloat) (i + 1.0) / [array count] animated:YES];
            // do additional UI/model updates here
        });
    }
});