cs193p – Lecture #13 Application Lifecycle and Core Motion

Please note, this blog entry is from a previous course. You might want to check out the current one.

NSNotification

Notifications provide a way to react to asynchronous events. e.g.:

[swift]
NSNotificationCenter.defaultCenter()

func addObserverForName(String, // name of the "radio station"
object: AnyObject?, // broadcaster (or nil for "anyone")
queue: NSOperationQueue?) // queue to execute the closure on
{ (notification: NSNotification) -> Void in
let info: [NSObject:AnyObject]? = notification.userInfo // notification-specific information
}
[/swift]

Application Lifecycle

An app has four different states:

When in foreground, the app can either be inactive (running, but no UI events) or active (running, receiving and processing UI events).

In background code can run, but only for a limited time and without UI events.

When suspended, no code is running, and the app can be killed at any time.

When launching, the app goes from not running to inactive – the application delegate is called with didFinishLaunchingWithOptions, UIApplicationDidFinishLaunchingNotification is sent to any observer – and then to active – the application delegate is called with applicationDidBecomeActive, UIApplicationDidBecomeActiveNotification is sent to any observer.

When switching to another app, the app goes from active to inactive – the application delegate is called with applicationWillResignActive,UIApplicationWillResignActiveNotification is sent to any observer – to background – the application delegate is called with applicationDidEnterBackground, UIApplicationDidEnterBackgroundNotification is sent to any observer – and then to suspended.

When an app switches from background to foreground, the application delegete is called with applicationWillEnterForeground, UIApplicationWillEnterForegroundNotification is sent to any observer.

When an app gets killed it goes from suspended to not running – without any notification.

Other methods of the delegation methods are

• local notifications – timers set to go off at certain times to wake the app if needed
  [swift]
  func (un)registerForRemoteNotifications()
  func scheduleLocalNotification(UILocalNotification)
  func registerUserNotificationSettings(UIUserNotificationSettings)
  [/swift]
• state restoration – saving the state of the UI to be able to restore it even if the app was killed
• data protection – protecting files when the device’s screen is locked
• open URL – opening registered for URLs
  [swift]
  func openURL(NSURL)
  func canOpenURL(NSURL) -> Bool
  [/swift]
• background fetching – fetching and receiving results in the background
  [swift]
  func setMinimumBackgroundFetchInterval(NSTimeInterval) // UIApplicationBackgroundFetchIntervalMinimum
  func backgroundTaskWithExpirationHandler(handler: () -> Void) -> UIBackgroundTaskIdentifier
  endBackgroundTask(UIBackgroundTaskIdentifier)
  [/swift]
• …
  [swift]
  var networkActivityIndicatorVisible: Bool
  var backgroundTimeRemaining: NSTimeInterval { get }
  preferredContentSizeCategory: String { get }
  var applicationState: UIApplicationState { get }
  [/swift]

The just learned – including manipulating Info.plist – is shown in the Airdrop demo Trax.

Core Motion

1. Check which hardware is available.
   [swift]
   var {accelerometer,gyro,magnetometer,deviceMotion}Available: Bool
   [/swift]
2. Start sampling ans poll the motion manager for the latest result.
   [swift]
   func start{Accelerometer,Gyro,Magnetometer,DeviceMotion}Updates()
   var {accelerometer,gyro,magnetometer,deviceMotion}Active: Bool
   var accelerometerData: CMAccelerometerData
   var acceleration: CMAcceleration
   var gravity: CMAcceleration
   var userAcceleration: CMAcceleration
   struct CMAcceleration {
   var x: Double // in g (9.8m/s/s)
   var y: Double // in g
   var z: Double // in g
   }

   var gyroData: CMGyroData
   var rotationRate: CMRotationRate
   struct CMRotationRate {
   var x: Double // in radians/s
   var y: Double // in radians/s
   var z: Double // in radians/s
   }
   var attitude: CMAttitude
   class CMAttitude: NSObject {
   var roll: Double
   var pitch: Double
   var yaw: Double
   }
   var magneticField: CMCalibratedMagneticField
   struct CMCalibratedMagneticField {
   var field: CMMagneticField
   var accuracy: CMMagneticFieldCalibrationAccuracy
   }

   var magnetometerData: CMMagnetometerData
   var magneticField: CMMagneticField
   struct CMMagneticField {
   var x: Double // in microteslas
   var y: Double // in microteslas
   var z: Double // in microteslas
   }
   [/swift]

3. Stop sampling.
   [swift]
   func stop{Accelerometer,Gyro,Magnetometer,DeviceMotion}Updates()
   [/swift]

or:

2. Set the rate for receiving new values
   [swift]
   var accelerometerUpdateInterval: NSTimeInterval
   var gyroUpdateInterval: NSTimeInterval
   var magnetometerUpdateInterval: NSTimeInterval
   var deviceMotionUpdateInterval: NSTimeInterval
   [/swift]
3. Register a closure to use those values
   [swift]
   func startAccelerometerUpdatesToQueue(queue: NSOperationQueue!,
   withHandler: CMAccelerometerHandler)
   typealias CMAccelerationHandler = (CMAccelerometerData!, NSError!) -> Void

   func startDeviceMotionUpdatesToQueue(queue: NSOperationQueue!,
   withHandler: CMDeviceMotionHandler)
   typealias CMDeviceMotionHandler = (CMDeviceMotion!, NSError!) -> Void
   [/swift]

Core Motion is demonstrated in the Bouncer demo.

The lecture and its slides are available via iTunes named “13. Application Lifecycle and Core Motion”. The code for Trax is available on GitHub and at Stanford. The code for Bouncer is available on GitHub and at Stanford