iPhone Development Blog

Creating objects can be very expensive in terms of performance. This is especially true for UIControls that are also added to a UIView. Instead create a number of objects once and reuse them when needed.

Here’s a code snippet from a book reader application that lays out each text paragraph as it’s own UILabel. Since each page has a varying number of paragraphs each with varying number of lines of text I initially created the necessary UILabels each time a page was rendered. Bad idea!

Now I have an array (often called an object pool) of UILabels that have been created and added to the page UIView. Initially the hidden property is set to YES for all these labels. When a page is rendered a UILabel is picked from the pool, initialized with the appropriate text, location and size. And the hidden property is set to NO to display the label.

// Add new text to existing labels
for (int i = 0; i < MAX_LABELS; i++) {
   UILabel *textLabel = [textLabels objectAtIndex:i];
   if (i < [currentPage.paragraphs count]) {
      NSString *paragraph = [currentPage.paragraphs objectAtIndex:i];
      CGSize textSize = [paragraph sizeWithFont:textFont
 						constrainedToSize:CGSizeMake(PARAGRAPH_WIDTH, 1000)
 						lineBreakMode:UILineBreakModeWordWrap];
      textLabel.font = textFont;
      textLabel.frame = CGRectMake(PARAGRAPH_LEFT_MARGIN, yOffset, PARAGRAPH_WIDTH, textSize.height);
      textLabel.hidden = NO;
      textLabel.text = paragraph;
      yOffset = yOffset + textSize.height + PARAGRAPH_SPACING;
   } else {
      textLabel.hidden = YES;
      textLabel.text = @"";
   }
}

 
In addition to speed performance improvements this solution also has the significant benefit of managing memory much better. 

written by Nick \\ tags: UILabel

The first rule of performance tuning is to measure your performance. If you don’t measure it, you don’t know if you’ve made any progress or if you’ve made it worse.

Here’s some very basic sample code you can use around the code you want to measure:

NSLog(@"Begin <your method name>");
NSDate *startTime = [NSDate date];

// Do something useful here

NSLog(@"End <your method name>");
NSTimeInterval elapsedTime = [startTime timeIntervalSinceNow];
NSLog([NSString stringWithFormat:@"Elapsed time: %f", -elapsedTime]);

Note that NSTimeInterval is specified in seconds and it has a sub-millisecond resolution. If your code segment executes in less than a millisecond then you should run your code in a loop and measure the elapsed time across 100 or 1,000 iterations to get meaningful measurements.

Important: Make sure that you do your performance testing on the actual device and not in the simulator. Some operations are 10x slower on a device than in the simulator, while others are actually faster on the device.

The NSLog statements will show up in the Xcode Console window if you launch the app on the device from Xcode. You can also view log statements as warnings in the Organizer window in the Console tab.

written by Nick \\ tags: NSDate, NSLog, NSTimeInterval