The Accelerometer

CSE 391 Fall 2011Tony Scarlatos

The Three Axes on the iPhone

Example Values: Portrait

Y = - 1.0 (gravity)

Z = 0.0

X = 0.0

Example Values: Landscape Right

Y = 0.0 Z = 0.0

X = 1.0 (negative if landscape left)

Example Values: Tilt 45 degrees X & Y

Y = - 0.5

Z = 0.0

X = 0.5

Example Values: Tilt 45 degrees Y & Z

Y = - 0.5

Z = - 0.5

X = 0.0

Uses for accelerometer values

• Detect orientation change– Ex. Portrait to Landscape

• Detect a “Shake”– Often used to clear fields or undo an action

• Use the Accelerometer as a control– Commonly used in games

Detecting and Managing Orientation

• If you want your interface to adapt to different orientations, such as rotating from portrait to landscape, you’ll have to edit the shouldAutorotateToInterfaceOrientation method in your view controller to be specific about which orientations you want to support.

• In the code that follows on the next slide, the supported orientations are Portrait, LandscapeLeft and LandscapeRight.

• In the simulator you can change orientations from the Hardware drop down menu.

The shouldAutorotateToInterfaceOrientation method

// Override to allow orientations other than the default portrait orientation.- (BOOL)shouldAutorotateToInterfaceOrientation:

(UIInterfaceOrientation)interfaceOrientation {

// Return YES for supported orientations

if (interfaceOrientation == UIInterfaceOrientationPortrait || //two vertical lines denote an OR statementinterfaceOrientation == UIInterfaceOrientationLandscapeLeft ||interfaceOrientation == UIInterfaceOrientationLandscapeRight)return YES;

else {return NO;}

}

Locking out an orientationIt seems like this is a good place to remind you of the code we used to force the app into landscape mode in an earlier demo:

-(BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation {

if ((interfaceOrientation==UIInterfaceOrientationPortrait)||(interfaceOrientation==UIInterfaceOrientationPortraitUpsideDown))

// NO locks out the portrait mode of the iPhone. The initial landscape orientation is also set in the summary panel for the app target.{return NO;}

if ((interfaceOrientation==UIInterfaceOrientationLandscapeLeft)||(interfaceOrientation==UIInterfaceOrientationLandscapeRight)){return YES;}

}

Interface Builder Autosizing Panel

If you want to compensate for the changes to your interface because of supporting multiple orientations, you can select those UI elements in IB, and in the Autosizing tab of the Inspector panel you can adjust their properties. You can choose which sides of the frame you want anchor the element to (struts), or if you want the element to stretch to fit the new orientation (springs).

Interface Builder Autosizing Panel

SPRINGS stretchThe UI element tothe sides of the view

The preview windowshows the effects of yourchanges to the UI element

Origin establishesthe location of the

element in the view

Fields for entering discretevalues for size and positionof the UI element

Detecting a Motion Event (Shake)

• The first thing to do to detect a motion event is to declare that the view controller can be the first responder.

• The next thing to do is activate that status using the viewDidAppear method.

• Then you can respond to either the motionBegan or motionEnded methods (or both).

• You can test the event in the simulator in the Hardware drop down menu which simulates a shake event.

• The code is written in the implementation file of the view controller, and it simply changes the background color of the view when the phone is shaken. It follows on the next slide.

The Shake Event- (BOOL)canBecomeFirstResponder {

return YES;}

- (void)viewDidAppear:(BOOL)animated { [super viewDidAppear:animated];[self becomeFirstResponder];

}

- (void)motionBegan:(UIEventSubtype)motion withEvent:(UIEvent *)event {if (event.subtype == UIEventSubtypeMotionShake) {

self.view.backgroundColor = [UIColor redColor];}

}

Accessing Accelerometer data• The UIAccelerometer object contains the combined data from the 3

sensors (axes) on the iPhone.• It is a “singleton” object – it always exists. You don’t have to create or

release it, you just need to request it.• The method used to access the data is sharedAccelerometer. Typically

you set the delegate of the object to your view controller (self).• The other thing that you will need to set is the sampling rate using the

updateInterval method. A typical sampling rate is 30 times per second.• You can then use the data in your app by employing the didAccelerate

method.• In the code that follows on the next 3 slides we set up a View-based

Application that displays accelerometer data in three labels and three progress bars.

The mainViewController.h file@interface MainViewController : UIViewController <UIAccelerometerDelegate> { IBOutlet UILabel *labelX; IBOutlet UILabel *labelY; IBOutlet UILabel *labelZ;

IBOutlet UIProgressView *progressX; IBOutlet UIProgressView *progressY; IBOutlet UIProgressView *progressZ;

UIAccelerometer *accelerometer;

}

@property (nonatomic, retain) IBOutlet UILabel *labelX;@property (nonatomic, retain) IBOutlet UILabel *labelY;@property (nonatomic, retain) IBOutlet UILabel *labelZ;

@property (nonatomic, retain) IBOutlet UIProgressView *progressX;@property (nonatomic, retain) IBOutlet UIProgressView *progressY;@property (nonatomic, retain) IBOutlet UIProgressView *progressZ;

@property (nonatomic, retain) UIAccelerometer *accelerometer;

The mainViewController.m file (1)

@synthesize labelX;@synthesize labelY;@synthesize labelZ;

@synthesize progressX;@synthesize progressY;@synthesize progressZ;

@synthesize accelerometer;

The mainViewController.m file (2)- (void)viewDidLoad { [super viewDidLoad]; self.accelerometer = [UIAccelerometer sharedAccelerometer];self.accelerometer.updateInterval = .1;self.accelerometer.delegate = self;}

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

labelX.text = [NSString stringWithFormat:@"%@%f", @"X: ", acceleration.x]; labelY.text = [NSString stringWithFormat:@"%@%f", @"Y: ", acceleration.y]; labelZ.text = [NSString stringWithFormat:@"%@%f", @"Z: ", acceleration.z];

self.progressX.progress = ABS(acceleration.x);self.progressY.progress = ABS(acceleration.y);self.progressZ.progress = ABS(acceleration.z);}

Testing the accelerometer app• The iPhone simulator does not natively support data from the

accelerometer, but there are a couple of plug-ins for Xcode that allow your device to send that information to the simulator wirelessly.

• The free app, AccSim, is available on the App Store. The files it depends upon (which have to be added to your Xcode project) are available at: http://www.brianhpratt.net/cms/index.php?page=accsim

• A more robust app that also shares location and touch event data with the simluator is iSimulate. It’s available on the App Store for $15.99. It has a library which has to be embedded in your Xcode project that is available here: http://www.vimov.com/isimulate/sdk/

• The advantage of both of these apps is that you don’t have to install your project on your device to be able to test your code. They also provide a convenient way to make video trailers of your apps that use accelerometer input, like games.