Why is allocating or initializing NSDateFormatter considered "expensive"?

Question

What do people mean when they say this is expensive? I create instances of a lot of transient objects just for intermediate storage (NSString and NSDate are typical ones). How do I know if my program use of NSDateFormatter is overdoing it?

Until now, I have tended to create what amounts to a singleton, but my preference would be to encapsulate it into some of the other objects it is associated with so I can use self references.

Short of running performance tests, I am looking for a better "rule-of-thumb" understanding of why I should or shouldn't do this.

Answer 1

When something like this is referred to as expensive, it doesn't necessarily mean that you should never do it, it just means avoid doing it in situations when you need to get out of a method as quickly as possible. For example, back when the iPhone 3G was the latest device, I was writing an application with a UITableView that formatted numbers for display in each cell (I might add, this was back when I was a beginner at iOS development). My first attempt was the following:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {

    NSString *reuseIdentifier = @"cell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:reuseIdentifier forIndexPath:indexPath];

    MyManagedObject *managedObject = [self.managedObjects objectAtIndex:indexPath.row];
    NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
    [numberFormatter setNumberStyle:NSNumberFormatterCurrencyStyle];

    [cell.textLabel setText:[managedObject title]];
    [cell.detailTextLabel setText:[numberFormatter stringFromNumber:[managedObject amount]]];

    return cell;
}

The scrolling performance of this code was terrible. The frame rate dropped to about 15 FPS because I was allocating a new NSNumberFormatter every time tableView:cellForRowAtIndexPath: was hit.

I fixed it by changing the code to this:

- (NSNumberFormatter *)numberFormatter {

    if (_numberFormatter != nil) {
        return _numberFormatter;
    }

    _numberFormatter = [[NSNumberFormatter alloc] init];
    [_numberFormatter setNumberStyle:NSNumberFormatterCurrencyStyle];

    return _numberFormatter;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {

    NSString *reuseIdentifier = @"cell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:reuseIdentifier forIndexPath:indexPath];

    MyManagedObject *managedObject = [self.managedObjects objectAtIndex:indexPath.row];
    NSNumberFormatter *numberFormatter = [self numberFormatter];

    [cell.textLabel setText:[managedObject title]];
    [cell.detailTextLabel setText:[numberFormatter stringFromNumber:[managedObject amount]]];

    return cell;
}

The difference here is that I've lazily loaded the NSNumberFormatter into an ivar, so that each run of tableView:cellForRowAtIndexPath: no longer allocates a new instance. This simple change pushed the scrolling performance back up to about 60 FPS.

This specific example isn't quite as relevant anymore, as the newer chips are capable of handling the allocation without affecting scrolling performance, but it's always better to be as efficient as possible.