Compare method

Either you implement a compare-method for your object:

- (NSComparisonResult)compare:(Person *)otherObject {
    return [self.birthDate compare:otherObject.birthDate];
}

NSArray *sortedArray = [drinkDetails sortedArrayUsingSelector:@selector(compare:)];

NSSortDescriptor (better)

or usually even better:

NSSortDescriptor *sortDescriptor;
sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"birthDate"
                                           ascending:YES];
NSArray *sortedArray = [drinkDetails sortedArrayUsingDescriptors:@[sortDescriptor]];

You can easily sort by multiple keys by adding more than one to the array. Using custom comparator-methods is possible as well. Have a look at the documentation.

Blocks (shiny!)

There's also the possibility of sorting with a block since Mac OS X 10.6 and iOS 4:

NSArray *sortedArray;
sortedArray = [drinkDetails sortedArrayUsingComparator:^NSComparisonResult(Person *a, Person *b) {
    return [a.birthDate compare:b.birthDate];
}];

Performance

The -compare: and block-based methods will be quite a bit faster, in general, than using NSSortDescriptor as the latter relies on KVC. The primary advantage of the NSSortDescriptor method is that it provides a way to define your sort order using data, rather than code, which makes it easy to e.g. set things up so users can sort an NSTableView by clicking on the header row.

As mentioned in another answer, for some finite amount of time, as your app is being launched, Default.png is displayed and you have no control over it. However, if in your AppDelegate, you create a new view that displays the same Default.png, you can create a seamless transition from the original Default.png to a view that you can add a progress bar to.

Now, presumably, you have created a view or similar and you are updating a progress bar every so often in order to give the user some feedback. The challenge here is that your view is only drawn when it gets called to do a drawRect. If, however, you go from AppDelegate to some initialization code to a viewcontroller's viewDidLoad, without the run loop getting a chance to figure out which views need to have drawRect called on, then your view will never display its status bar.

Therefore in order to accomplish what you want, you have to either make sure that drawRect gets called, such as by pushing off a lot of your initialization code into other threads or timer tasks, or you can force the drawing by calling drawRect yourself, after setting up contexts and such.

If you go with the background tasks, then make sure your initialization code is thread-safe.