Question

I am trying to solve a problem in Objective-C, but I don’t think the question is language specific.

I have to do some processing down in a model class that has no notion of UI. However, this processing takes some time and I want to let the user know the status via a progress bar.

My first attempt at this was defining a notion of a progress handler protocol/interface with some methods like

-startOperation;
-updateProgress:(double)currentValue ofMax:(double)maxValue
-endOperation;

This way my UI can implement that the the model need not know details about what goes on other than someone wants progress updates. Currently my UI unhides a progress bar, and updates it, then hides it when done. So far so good.

However, it turns out that sometimes this operation processing is very fast. Such that the UI updates result in a pretty disconcerting flicker as they execute. I don’t know if the operation will be fast or slow beforehand.

One idea I had was to force the operation to take at least a certain duration to avoid the UI changes being so jarring to the eye, but this seemed to put knowledge of the UI in the model class, which must be wrong.

This would seem to be a common issue with (hopefully) some known pattern.

How would you address this?

Answer 1

You can use NSTimer to delay the display of your progress bar until your operation had run for a given amount of time, say half a second:

-(void)startOperation {
    // Show the progress bar in 0.5 seconds
    if (!_timer) {
        _timer = [[NSTimer scheduledTimerWithTimeInterval:0.5 
                                                   target:self 
                                                 selector:@selector(showProgressBar:) 
                                                 userInfo:nil 
                                                  repeats:NO] retain];
    }
}

In -endOperation, you cancel the timer and hide progress bar:

-(void)endOperation {
    [_timer invalidate]; // cancel the timer
    [_timer release];
    _timer = nil;

    [self hideProgressBar];
}

If the operation completes in less than 0.5 seconds, the timer is canceled before the progress bar is displayed.