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Active state, normally this won’t matter, but in case it’s purged and must be relaunched, your users will appreciate starting off in the same place.

Background Inactive

Use applicationWillEnterForeground:/UIApplicationWillEnterForeground to undo anything you did when switching from Inactive to Background. For example, here you can reestablish persistent network connections.

Inactive Active

Use applicationDidBecomeActive:/UIApplicationDidBecomeActive to undo anything you did when switching from Active to Inactive. Note that if your app is a game, this probably does not mean dropping out of pause straight to the game; you should let your users do that on their own. Also keep in mind that this method and notification are used when an app is freshly launched, so anything you do here must work in that context as well.

There is one special consideration for the Inactive Background transition. Not only does it have the longest description in the previous list, but it’s also probably the most codeand time-intensive transition in applications because of the amount of bookkeeping you may want your app to do. When this transition is underway, the system won’t give you the benefit of an unlimited amount of time to save your changes here. It gives you about five seconds. If your app takes longer than that to return from the delegate method (and handle any notifications you’ve registered for), then your app will be summarily purged from memory and pushed into the Not Running state! If this seems unfair, don’t worry, because there is a reprieve available. While handling that delegate method or notification, you can ask the system to perform some additional work for you in a background queue, which buys you some extra time. We’ll demonstrate that technique in the next section.

Handling the Inactive State

The simplest state change your app is likely to encounter is from Active to Inactive and then back to Active. You may recall that this is what happens if your iPhone receives an SMS message while your app is running and displays it for the user. In this section, we’re going to make State Lab do something visually interesting so that you can see what happens if you ignore that state change, and then we’ll show you how to fix it.

We’ll add a UILabel to our display and make it move using Core Animation, which is a really nice way of animating objects in iOS.

Start by adding a UILabel as an instance variable and property in BIDViewController.h:

@interface BIDViewController : UIViewController

@property (strong, nonatomic) UILabel *label;

@end

Then do the usual memory-management work for this property in BIDViewController.m:

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548CHAPTER 15: Grand Central Dispatch, Background Processing, and You

@implementation BIDViewController

@synthesize label;

.

.

.

- (void)viewDidUnload { [super viewDidUnload];

//Release any retained subviews of the main view.

//e.g. self.myOutlet = nil;

self.label = nil;

}

Now, let’s set up the label when the view loads. Add the bold lines shown here to the viewDidLoad method:

- (void)viewDidLoad { [super viewDidLoad];

// Do any additional setup after loading the view, typically from a nib.

CGRect bounds = self.view.bounds;

CGRect labelFrame = CGRectMake(bounds.origin.x, CGRectGetMidY(bounds) - 50, bounds.size.width, 100);

self.label = [[UILabel alloc] initWithFrame:labelFrame]; label.font = [UIFont fontWithName:@"Helvetica" size:70]; label.text = @"Bazinga!";

label.textAlignment = UITextAlignmentCenter; label.backgroundColor = [UIColor clearColor]; [self.view addSubview:label];

}

It’s time to set up some animation. We’ll define two methods: one to rotate the label to an upside-down position and one to rotate it back to normal. Let’s declare these methods in an class extension at the top of the file, just before the class’s

@implementation begins:

@interface BIDViewController ()

-(void)rotateLabelUp;

-(void)rotateLabelDown;

@end

The method definitions themselves can then be inserted anywhere within the

@implementation block:

- (void)rotateLabelDown {

[UIView animateWithDuration:0.5 animations:^{

label.transform = CGAffineTransformMakeRotation(M_PI);

}

completion:^(BOOL finished){ [self rotateLabelUp];

}];

}

- (void)rotateLabelUp {

[UIView animateWithDuration:0.5 animations:^{

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label.transform = CGAffineTransformMakeRotation(0);

}

completion:^(BOOL finished){ [self rotateLabelDown];

}];

}

This deserves a bit of explanation. UIView defines a class method called animateWithDuration:animations:completion:, which sets up an animation. Any animatable attributes that we set within the animations block don’t have an immediate effect on the receiver. Instead, Core Animation will smoothly transition that attribute from its current value to the new value we specify. This is what’s called an implicit animation and is one of the main features of Core Animation. The final completion block lets us specify what will happen after the animation is complete.

So, each of these methods sets the label’s transform property to a particular rotation, specified in radians. Each also sets up a completion block to just call the other method, so the text will continue to animate back and forth forever.

Finally, we need to set up a way to kick-start the animation. For now, we’ll do this by adding this line at the end of viewDidLoad (but we’ll change this later, for reasons we’ll describe at that time):

[self rotateLabelDown];

Now, build and run the app. You should see the Bazinga! label rotate back and forth (see Figure 15–4).

To test the Active Inactive transition, you really need to once again run this on an actual iPhone and send an SMS message to it from elsewhere. Unfortunately, there’s no way to simulate this behavior in any version of the iOS simulator that Apple has released so far. If you don’t yet have the ability to build and install on a device or don’t have an iPhone, you won’t be able to try this for yourself, but please follow along as best you can!

Build and run the app on an iPhone, and see that the animation is running along. Now, send an SMS message to the device. When the system alert comes up to show the message, you’ll see that the animation keeps on running! That may be slightly comical, but it’s probably irritating for a user. We will use transition notifications to stop our animation when this occurs.

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550	CHAPTER 15: Grand Central Dispatch, Background Processing, and You

Figure 15–4. The State Lab application doing its label rotating magic

Our controller class will need to have some internal state to keep track of whether it should be animating at any given time. For this purpose, let’s add an ivar to BIDViewController.m. Because this simple BOOL doesn't need to be accessed by any outside classes, we skip the header and add it to the class extension we created earlier.

@interface BIDViewController ()

@property (assign, nonatomic) BOOL animate;

-(void)rotateLabelUp;

-(void)rotateLabelDown;

@end

@implementation BIDViewController @synthesize label;

@synthesize animate;

Since our class isn’t the application delegate, we can’t just implement the delegate methods and expect them to work. Instead, we sign up to receive notifications from the application when the execution state changes. Do this by adding a few lines at the top of the viewDidLoad method in BIDViewController.m:

- (void)viewDidLoad { [super viewDidLoad];

// Do any additional setup after loading the view, typically from a nib.

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[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationWillResignActive)

name:UIApplicationWillResignActiveNotification object:[UIApplication sharedApplication]];

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationDidBecomeActive)

name:UIApplicationDidBecomeActiveNotification object:[UIApplication sharedApplication]];

CGRect bounds = self.view.bounds;

.

.

.

This sets up these two notifications to each call a method in our class at the appropriate time. Define these methods anywhere you like inside the @implementation block:

- (void)applicationWillResignActive {

NSLog(@"VC: %@", NSStringFromSelector(_cmd)); animate = NO;

}

- (void)applicationDidBecomeActive {

NSLog(@"VC: %@", NSStringFromSelector(_cmd)); animate = YES;

[self rotateLabelDown];

}

Here, we’ve included the same method logging as before, just so you can see where the methods occur in the Xcode console. We added the preface "VC: " to distinguish this call from the NSLog() calls in the delegate (VC is for view controller). The first of these methods just turns off the animate flag. The second turns the flag back on, and then actually starts up the animations again. For that first method to have any effect, we need to add some code to check the animate flag and keep on animating only if it’s enabled.

- (void)rotateLabelUp {

[UIView animateWithDuration:0.5 animations:^{

label.transform = CGAffineTransformMakeRotation(0);

}

completion:^(BOOL finished){ if (animate) {

[self rotateLabelDown];

}

}];

}

We added this to the completion block of rotateLabelUp, and only there, so that our animation will stop only when the text is right-side up.

Now, build and run the app again, and see what happens. Chances are, you’ll see some flickery madness, with the label rapidly flipping up and down, not even rotating! The reason for this is simple but perhaps not obvious (though we did hint at it earlier).

Remember that we started up the animations at the end of viewDidLoad by calling rotateLabelDown? Well, we’re now calling rotateLabelDown in

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