Determining Elapsed Time

I recently wrote a short block of code to display an activity indicator while waiting for an image to be written to the iPhone camera roll. The problem I ran into is that the time to write the image file varies, depending on the image size. In some cases the activity indicator would be visible for a few seconds, other times the activity indicator flashed on/off screen so quickly, if you blinked, you’d miss it.

I needed a way to display the activity indicator a consistent amount of time for each file save, regardless of how long it took to write the file. Here’s the workaround I came up with – I began by saving the start time and creating an activity indicator:

  // When did we start the image save ?
  // CFTimeInterval is a double (floating point number)
  CFTimeInterval startTime = CFAbsoluteTimeGetCurrent();    
  // Start the activity indicator
  UIActivityIndicatorView activityIndicator = [[UIActivityIndicatorView alloc]
                    100, 400, 30, 30];
  activityIndicator.activityIndicatorViewStyle = UIActivityIndicatorViewStyleGray;
  activityIndicator.autoresizingMask = (UIViewAutoresizingFlexibleLeftMargin |
                    UIViewAutoresizingFlexibleRightMargin |
                    UIViewAutoresizingFlexibleTopMargin |
  [activityIndicator sizeToFit];
  [self.view addSubview:activityIndicator];
  [activityIndicator startAnimating];

Next, I wrote the image to the camera roll and then checked for the amount of time elapsed:

  // Save image code here...
  // How much time has elapsed ?
  CFTimeInterval difference = CFAbsoluteTimeGetCurrent() - startTime;

The last check I needed was to see if the amount of time that has elapsed was greater or less than the minimum time I wanted to display the activity indicator. If less than the desired amount of time, sleep for the amount of time needed to hit the minimum display time.

  // Determine if some predetermined amount of time has passed.  
    [NSThread sleepForTimeInterval:SECONDS_TO_DISPLAY_SAVING_MSG - difference];
  // Remove the activity indicator and message
  [activityIndicator stopAnimating];  
  [activityIndicator removeFromSuperview];
  [activityIndicator release];

For my needs accuracy was not key. So even though CFTimeInterval is a double, I simply allow any rounding up or down to take place.