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phone is being held upright in portrait orientation. A positive value for y indicates some force is being exerted in the opposite direction, which could mean the phone is being held upside down or that the phone is being moved in a downward direction.

Keeping the diagram in Figure 19–1 in mind, let’s look at some accelerometer results (see Figure 19–3). Note that, in real life, you will almost never get values this precise, as the accelerometer is sensitive enough to sense even tiny amounts of motion, and you will usually pick up at least some tiny amount of force on all three axes. This is realworld physics, not high-school physics.

Figure 19–3. Idealized acceleration values for different device orientations

Probably the most common usage of the accelerometer in third-party applications is as a controller for games. We’ll create a program that uses the accelerometer for input a little later in the chapter, but first, we’ll look at another common accelerometer use: detecting shakes.

Detecting Shakes

Like a gesture, a shake can be used as a form of input to your application. For example, the drawing program GLPaint, which is one of the iOS sample code projects, lets users erase drawings by shaking their iOS device, sort of like an Etch A Sketch.

Detecting shakes is relatively trivial. All it requires is checking for an absolute value on one of the axes that is greater than a set threshold. During normal usage, it’s not uncommon for one of the three axes to register values up to around 1.3 g, but getting values much higher than that generally requires intentional force. The accelerometer seems to be unable to register values higher than around 2.3 g (at least in our experience), so you don’t want to set your threshold any higher than that.

To detect a shake, you could check for an absolute value greater than 1.5 for a slight shake and 2.0 for a strong shake, like this:

- (void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration {

if (fabsf(acceleration.x) > 2.0

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||fabsf(acceleration.y) > 2.0

||fabsf(acceleration.z) > 2.0) { // Do something here...

}

}

This method would detect any movement on any axis that exceeded two g-forces.

You could implement more sophisticated shake detection by requiring the user to shake back and forth a certain number of times to register as a shake, like so:

- (void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration {

static NSInteger shakeCount = 0; static NSDate *shakeStart;

NSDate *now = [[NSDate alloc] init];

NSDate *checkDate = [[NSDate alloc] initWithTimeInterval:1.5f sinceDate:shakeStart];

if ([now compare:checkDate] == NSOrderedDescending || shakeStart == nil) {

shakeCount = 0;

shakeStart = [[NSDate alloc] init];

}

if (fabsf(acceleration.x) > 2.0

||fabsf(acceleration.y) > 2.0

||fabsf(acceleration.z) > 2.0) { shakeCount++;

if (shakeCount > 4) {

// Do something shakeCount = 0;

shakeStart = [[NSDate alloc] init];

}

}

}

This method keeps track of the number of times the accelerometer reports a value above 2.0, and if it happens four times within a 1.5-second span of time, it registers as a shake.

Baked-In Shaking

There’s actually another way to check for shakes—one that’s baked right into the responder chain. Remember back in Chapter 17 how we implemented methods like touchesBegan:withEvent: to detect touches? Well, iOS also provides three similar responder methods for detecting motion:

When motion begins, the motionBegan:withEvent: method is sent to the first responder and then on through the responder chain, as discussed in Chapter 17.

When the motion ends, the motionEnded:withEvent: method is sent to the first responder.

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If the phone rings, or some other interrupting action happens during the shake, the motionCancelled:withEvent: message is sent to the first responder.

This means that you can actually detect a shake without using CMMotionManager directly. All you need to do is override the appropriate motion-sensing methods in your view or view controller, and they will be called automatically when the user shakes the phone. Unless you specifically need more control over the shake gesture, you should use the baked-in motion detection rather than the manual method described previously, but we thought we would show you the manual method in case you ever do need more control.

Now that you have the basic idea of how to detect shakes, we’re going to break your phone.

Shake and Break

OK, we’re not really going to break your phone, but we’ll write an application that detects shakes, and then makes your phone look and sound like it broke as a result of the shake.

When you launch the application, the program will display a picture that looks like the iPhone home screen (see Figure 19–4). Shake the phone hard enough, though, and your poor phone will make a sound that you never want to hear coming out of a consumer electronics device. What’s more, your screen will look like the one shown in Figure 19–5. Why do we do these evil things? Not to worry. You can reset the iPhone to its previously pristine state by touching the screen.

Figure 19–4. The ShakeAndBreak application looks innocuous enough…

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Figure 19–5. . . . but handle it too roughly and—oh no!

NOTE: Just for completeness, we’ve included a modified version of ShakeAndBreak in the project archives based on the built-in shake-detection method. You’ll find it in the project archive in a folder named 19 - ShakeAndBreak - Motion Methods. The magic is in the

BIDViewController‘s motionEnded:withEvent: method.

Create a new project in Xcode using the Single View Application template. Call the new project ShakeAndBreak. In the 19 - ShakeAndBreak folder of the project archive, we’ve provided the two images and the sound file you need for this application. Drag home.png, homebroken.png, and glass.wav to your project. There’s also an icon.png file in that folder. Add that to the project as well.

Next, expand the Supporting Files folder, and select ShakeAndBreak-Info.plist to bring up the property list editor. We need to add an entry to the property list to tell our application not to use a status bar. Start off by right-clicking (or control-clicking) anywhere in the property list editor, and selecting the Show Raw Keys/Values option from the context menu so you can see the real names of the configurations we’re setting. Single-click any row in the property list, and press enter to add a new row. Change the new row’s Key to UIStatusBarHidden. The Value of this row will default to NO. Change it to YES. Finally,

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expand the array entry named CFBundleIconFiles, and press enter to add a new String item. Type icon.png in the Value column (see Figure 19–6).

Figure 19–6. Changing the value for CFBundleIconFiles (shown highlighted) and for UIStatusBarHidden (last line in the property list)

Now, let’s start creating our view controller. We’re going to need to create an outlet to point to an image view so that we can change the displayed image. We’ll also need a couple of UIImage instances to hold the two pictures, a sound ID to refer to the sound, and a Boolean to keep track of whether the screen needs to be reset. Single-click BIDViewController.h, and add the following code:

#import <UIKit/UIKit.h>

#import <CoreMotion/CoreMotion.h> #import <AudioToolbox/AudioToolbox.h>

#define	kAccelerationThreshold	1.7
#define	kUpdateInterval	(1.0f / 10.0f)

@interface BIDViewController : UIViewController

<UIAccelerometerDelegate>

@property (weak, nonatomic) IBOutlet UIImageView *imageView; @property (strong, nonatomic) CMMotionManager *motionManager; @property (assign, nonatomic) BOOL brokenScreenShowing; @property (assign, nonatomic) SystemSoundID soundID; @property (strong, nonatomic) UIImage *fixed;

@property (strong, nonatomic) UIImage *broken;

@end

Save the header file. Now, select BIDViewController.xib to edit the file in Interface Builder. Click the View icon to select the view, and then bring up the attributes inspector and change the Status Bar popup under Simulated Metrics from Gray to None. Next, drag an Image View over from the library to the view in the layout area. The image view should automatically resize to take up the full window, so just place it so that it sits perfectly within the window.

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Control-drag from the File’s Owner icon to the image view, and select the imageView outlet. Then save the nib file.

Next, select the BIDViewController.m file, and add the following code near the top of the file:

#import "BIDViewController.h"

@implementation BIDViewController

@synthesize imageView; @synthesize motionManager; @synthesize brokenScreenShowing; @synthesize soundID;

@synthesize fixed; @synthesize broken;

.

.

.

Give the viewDidLoad method the following implementation:

- (void) viewDidLoad { [super viewDidLoad];

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

NSString *path = [[NSBundle mainBundle] pathForResource:@"glass" ofType:@"wav"];

NSURL *url = [NSURL fileURLWithPath:path]; AudioServicesCreateSystemSoundID((__bridge CFURLRef)url,

&soundID);

self.fixed = [UIImage imageNamed:@"home.png"]; self.broken = [UIImage imageNamed:@"homebroken.png"];

imageView.image = fixed;

self.motionManager = [[CMMotionManager alloc] init]; motionManager.accelerometerUpdateInterval = kUpdateInterval; NSOperationQueue *queue = [[NSOperationQueue alloc] init]; [motionManager startAccelerometerUpdatesToQueue:queue

withHandler: ^(CMAccelerometerData *accelerometerData, NSError *error){

if (error) {

[motionManager stopAccelerometerUpdates]; } else {

if (!brokenScreenShowing) {

CMAcceleration acceleration = accelerometerData.acceleration; if (acceleration.x > kAccelerationThreshold

||acceleration.y > kAccelerationThreshold

||acceleration.z > kAccelerationThreshold) {

[imageView performSelectorOnMainThread:@selector(setImage:) withObject:broken

waitUntilDone:NO];

AudioServicesPlaySystemSound(soundID); brokenScreenShowing = YES;

}

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}

}

}];

}

Insert the following lines of code into the existing viewDidUnload method:

- (void)viewDidUnload

{

[super viewDidUnload];

//Release any retained subviews of the main view.

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

self.imageView = nil; self.motionManager = nil; self.fixed = nil; self.broken = nil;

}

And finally, add the following new method at the bottom of the file:

.

.

.

#pragma mark -

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event { imageView.image = fixed;

brokenScreenShowing = NO;

}

@end

The first method we implement, and the one where most of the interesting things happen, is viewDidLoad. First, we create an NSURL object pointing to our sound file, load it into memory, and save the assigned identifier in the soundID instance variable. In order to satisfy the requirements for ARC, we need to tell the compiler how to manage the memory for the NSURL object before passing it off to

AudioServicesCreateSystemSoundID(), which we do by casting it using the __bridge qualifier.

NSString *path = [[NSBundle mainBundle] pathForResource:@"glass" ofType:@"wav"];

NSURL *url = [NSURL fileURLWithPath:path]; AudioServicesCreateSystemSoundID((__bridge CFURLRef)url,

&soundID);

NOTE: New to the __bridge qualifier? It’s discussed in Chapter 7. In a nutshell, it is used to

safely bridge to ARC.

We then load the two images into memory.

self.fixed = [UIImage imageNamed:@"home.png"]; self.broken = [UIImage imageNamed:@"homebroken.png"];

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Finally, we set imageView to show the unbroken screenshot and set brokenScreenShowing to NO to indicate that the screen does not currently need to be reset.

imageView.image = fixed; brokenScreenShowing = NO;

Then we create a CMMotionManager and an NSOperationQueue (just as we’ve done before), and start up the accelerometer, sending it a block to be run each time the accelerometer value changes.

self.motionManager = [[CMMotionManager alloc] init]; motionManager.accelerometerUpdateInterval = kUpdateInterval; NSOperationQueue *queue = [[NSOperationQueue alloc] init]; [motionManager startAccelerometerUpdatesToQueue:queue

withHandler:

If the block finds accelerometer values high enough to trigger the break, it makes imageView switch over to the broken image and starts playing the breaking sound. Note that imageView is a member of the UIImageView class, which, like most parts of UIKit, is meant to run only in the main thread. Since the block may be run in another thread, we force the imageView update to happen on the main thread.

^(CMAccelerometerData *accelerometerData, NSError *error){ if (error) {

[motionManager stopAccelerometerUpdates]; } else {

if (!brokenScreenShowing) {

CMAcceleration acceleration = accelerometerData.acceleration; if (acceleration.x > kAccelerationThreshold

||acceleration.y > kAccelerationThreshold

||acceleration.z > kAccelerationThreshold) {

[imageView performSelectorOnMainThread:@selector(setImage:) withObject:broken

waitUntilDone:NO];

AudioServicesPlaySystemSound(soundID); brokenScreenShowing = YES;

}

}

}

}];

The last method is one you should be quite familiar with by now. It’s called when the screen is touched. All we do in that method is to set the image back to the unbroken screen and set brokenScreenShowing back to NO.

imageView.image = fixed; brokenScreenShowing = NO;

Finally, add the CoreMotion.framework as well as the AudioToolbox.framework so that we can play the sound file. You can link the frameworks into your application by following the instructions from earlier in the chapter.

Compile and run the application, and take it for a test drive. For those of you who don’t have the ability to run this application on your iOS device, you might want to give the shake-event-based version a try. The simulator does not simulate the accelerometer

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