You are previewing iOS 6 Programming Cookbook.

iOS 6 Programming Cookbook

Cover of iOS 6 Programming Cookbook by Vandad Nahavandipoor Published by O'Reilly Media, Inc.
  1. iOS 6 Programming Cookbook
  2. Preface
    1. Audience
    2. Organization of This Book
    3. Additional Resources
    4. Conventions Used in This Book
    5. Using Code Examples
    6. We’d Like to Hear from You
    7. Safari® Books Online
    8. Acknowledgments
  3. 1. The Basics
    1. 1.0. Introduction
    2. 1.1. Creating a Simple iOS App in Xcode
      1. Problem
      2. Solution
      3. Discussion
    3. 1.2. Understanding Interface Builder
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    4. 1.3. Compiling iOS Apps
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    5. 1.4. Running iOS Apps on the Simulator
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    6. 1.5. Running iOS Apps on iOS Devices
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    7. 1.6. Packaging iOS Apps for Distribution
      1. Problem
      2. Solution
      3. Discussion
    8. 1.7. Declaring Variables in Objective-C
      1. Problem
      2. Solution
      3. Discussion
    9. 1.8. Allocating and Making Use of Strings
      1. Problem
      2. Solution
      3. Discussion
    10. 1.9. Comparing Values in Objective-C with an if Statement
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    11. 1.10. Implementing Loops with for Statements
      1. Problem
      2. Solution
      3. Discussion
    12. 1.11. Implementing while Loops
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    13. 1.12. Creating Custom Classes
      1. Problem
      2. Solution
      3. Discussion
    14. 1.13. Defining Functionality for Classes
      1. Problem
      2. Solution
      3. Discussion
    15. 1.14. Defining Two or More Methods with the Same Name
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    16. 1.15. Allocating and Initializing Objects
      1. Problem
      2. Solution
      3. Discussion
    17. 1.16. Adding Properties to Classes
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    18. 1.17. Moving from Manual Reference Counting to Automatic Reference Counting
      1. Problem
      2. Solution
      3. Discussion
    19. 1.18. Typecasting with Automatic Reference Counting
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    20. 1.19. Delegating Tasks with Protocols
      1. Problem
      2. Solution
      3. Discussion
    21. 1.20. Determining Whether Instance or Class Methods Are Available
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    22. 1.21. Determining Whether a Class Is Available at Runtime
      1. Problem
      2. Solution
      3. Discussion
    23. 1.22. Allocating and Making Use of Numbers
      1. Problem
      2. Solution
      3. Discussion
    24. 1.23. Allocating and Making Use of Arrays
      1. Problem
      2. Solution
      3. Discussion
    25. 1.24. Allocating and Making Use of Dictionaries
      1. Problem
      2. Solution
      3. Discussion
    26. 1.25. Allocating and Making Use of Sets
      1. Problem
      2. Solution
      3. Discussion
    27. 1.26. Creating Bundles
      1. Problem
      2. Solution
      3. Discussion
    28. 1.27. Loading Data from the Main Bundle
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    29. 1.28. Loading Data from Other Bundles
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    30. 1.29. Sending Notifications with NSNotificationCenter
      1. Problem
      2. Solution
      3. Discussion
    31. 1.30. Listening for Notifications Sent from NSNotificationCenter
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
  4. 2. Implementing Controllers and Views
    1. 2.0. Introduction
    2. 2.1. Displaying Alerts with UIAlertView
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    3. 2.2. Creating and Using Switches with UISwitch
      1. Problem
      2. Solution
      3. Discussion
    4. 2.3. Customizing the UISwitch
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    5. 2.4. Picking Values with UIPickerView
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    6. 2.5. Picking the Date and Time with UIDatePicker
      1. Problem
      2. Solution
      3. Discussion
    7. 2.6. Implementing Range Pickers with UISlider
      1. Problem
      2. Solution
      3. Discussion
    8. 2.7. Customizing the UISlider
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    9. 2.8. Grouping Compact Options with UISegmentedControl
      1. Problem
      2. Solution
      3. Discussion
    10. 2.9. Customizing the UISegmentedControl
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    11. 2.10. Presenting and Managing Views with UIViewController
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    12. 2.11. Presenting Sharing Options with UIActivityViewController
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    13. 2.12. Implementing Navigation with UINavigationController
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    14. 2.13. Manipulating a Navigation Controller’s Array of View Controllers
      1. Problem
      2. Solution
      3. Discussion
    15. 2.14. Displaying an Image on a Navigation Bar
      1. Problem
      2. Solution
      3. Discussion
    16. 2.15. Adding Buttons to Navigation Bars Using UIBarButtonItem
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    17. 2.16. Presenting Multiple View Controllers with UITabBarController
      1. Problem
      2. Solution
      3. Discussion
    18. 2.17. Displaying Static Text with UILabel
      1. Problem
      2. Solution
      3. Discussion
    19. 2.18. Customizing the UILabel
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    20. 2.19. Accepting User Text Input with UITextField
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    21. 2.20. Displaying Long Lines of Text with UITextView
      1. Problem
      2. Solution
      3. Discussion
    22. 2.21. Adding Buttons to the User Interface with UIButton
      1. Problem
      2. Solution
      3. Discussion
    23. 2.22. Displaying Images with UIImageView
      1. Problem
      2. Solution
      3. Discussion
    24. 2.23. Creating Scrollable Content with UIScrollView
      1. Problem
      2. Solution
      3. Discussion
    25. 2.24. Loading Web Pages with UIWebView
      1. Problem
      2. Solution
      3. Discussion
    26. 2.25. Presenting Master-Detail Views with UISplitViewController
      1. Problem
      2. Solution
      3. Discussion
    27. 2.26. Enabling Paging with UIPageViewController
      1. Problem
      2. Solution
      3. Discussion
    28. 2.27. Displaying Popovers with UIPopoverController
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    29. 2.28. Displaying Progress with UIProgressView
      1. Problem
      2. Solution
      3. Discussion
    30. 2.29. Listening and Reacting to Keyboard Notifications
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    31. 2.30. Constructing and Displaying Styled Texts
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
  5. 3. Auto Layout and the Visual Format Language
    1. 3.0. Introduction
    2. 3.1. Placing UI Components in the Center of the Screen
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    3. 3.2. Defining Horizontal and Vertical Constraints with the Visual Format Language
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    4. 3.3. Utilizing Cross View Constraints
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    5. 3.4. Configuring Auto Layout Constraints in Interface Builder
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
  6. 4. Constructing and Using Table Views
    1. 4.0. Introduction
    2. 4.1. Instantiating a Table View
      1. Problem
      2. Solution
      3. Discussion
    3. 4.2. Assigning a Delegate to a Table View
      1. Problem
      2. Solution
      3. Discussion
    4. 4.3. Populating a Table View with Data
      1. Problem
      2. Solution
      3. Discussion
    5. 4.4. Receiving and Handling Table View Events
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    6. 4.5. Using Different Types of Accessories in a Table View Cell
      1. Problem
      2. Solution
      3. Discussion
    7. 4.6. Creating Custom Table View Cell Accessories
      1. Problem
      2. Solution
      3. Discussion
    8. 4.7. Displaying Hierarchical Data in Table Views
      1. Problem
      2. Solution
      3. Discussion
    9. 4.8. Enabling Swipe Deletion of Table View Cells
      1. Problem
      2. Solution
      3. Discussion
    10. 4.9. Constructing Headers and Footers in Table Views
      1. Problem
      2. Solution
      3. Discussion
    11. 4.10. Displaying Context Menus on Table View Cells
      1. Problem
      2. Solution
      3. Discussion
    12. 4.11. Moving Cells and Sections in Table Views
      1. Problem
      2. Solution
      3. Discussion
    13. 4.12. Deleting Cells and Sections from Table Views
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    14. 4.13. Utilizing the UITableViewController for Easy Creation of Table Views
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    15. 4.14. Displaying a Refresh Control for Table Views
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
  7. 5. Storyboards
    1. 5.0. Introduction
    2. 5.1. Creating a Project with Storyboards
      1. Problem
      2. Solution
      3. Discussion
    3. 5.2. Adding a Navigation Controller to a Storyboard
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    4. 5.3. Passing Data From One Screen to Another
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    5. 5.4. Adding a Storyboard to an Existing Project
      1. Problem
      2. Solution
      3. Discussion
  8. 6. Concurrency
    1. 6.0. Introduction
    2. 6.1. Constructing Block Objects
      1. Problem
      2. Solution
      3. Discussion
    3. 6.2. Accessing Variables in Block Objects
      1. Problem
      2. Solution
      3. Discussion
    4. 6.3. Invoking Block Objects
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    5. 6.4. Dispatching Tasks to Grand Central Dispatch
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    6. 6.5. Performing UI-Related Tasks with GCD
      1. Problem
      2. Solution
      3. Discussion
    7. 6.6. Executing Non-UI Related Tasks Synchronously with GCD
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    8. 6.7. Executing Non-UI Related Tasks Asynchronously with GCD
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    9. 6.8. Performing Tasks After a Delay with GCD
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    10. 6.9. Performing a Task Only Once with GCD
      1. Problem
      2. Solution
      3. Discussion
    11. 6.10. Grouping Tasks Together with GCD
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    12. 6.11. Constructing Your Own Dispatch Queues with GCD
      1. Problem
      2. Solution
      3. Discussion
    13. 6.12. Running Tasks Synchronously with Operations
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    14. 6.13. Running Tasks Asynchronously with Operations
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    15. 6.14. Creating Dependency Between Operations
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    16. 6.15. Creating Timers
      1. Problem
      2. Solution
      3. Discussion
    17. 6.16. Creating Concurrency with Threads
      1. Problem
      2. Solution
      3. Discussion
    18. 6.17. Invoking Background Methods
      1. Problem
      2. Solution
      3. Discussion
    19. 6.18. Exiting Threads and Timers
      1. Problem
      2. Solution
      3. Discussion
  9. 7. Core Location and Maps
    1. 7.0. Introduction
    2. 7.1. Creating a Map View
      1. Problem
      2. Solution
      3. Discussion
    3. 7.2. Handling the Events of a Map View
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    4. 7.3. Pinpointing the Location of a Device
      1. Problem
      2. Solution
      3. Discussion
    5. 7.4. Displaying Pins on a Map View
      1. Problem
      2. Solution
      3. Discussion
    6. 7.5. Displaying Pins with Different Colors on a Map View
      1. Problem
      2. Solution
      3. Discussion
    7. 7.6. Displaying Custom Pins on a Map View
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    8. 7.7. Converting Meaningful Addresses to Longitude and Latitude
      1. Problem
      2. Solution
      3. Discussion
    9. 7.8. Converting Longitude and Latitude to a Meaningful Address
      1. Problem
      2. Solution
      3. Discussion
  10. 8. Implementing Gesture Recognizers
    1. 8.0. Introduction
    2. 8.1. Detecting Swipe Gestures
      1. Problem
      2. Solution
      3. Discussion
    3. 8.2. Detecting Rotation Gestures
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    4. 8.3. Detecting Panning and Dragging Gestures
      1. Problem
      2. Solution
      3. Discussion
    5. 8.4. Detecting Long Press Gestures
      1. Problem
      2. Solution
      3. Discussion
    6. 8.5. Detecting Tap Gestures
      1. Problem
      2. Solution
      3. Discussion
    7. 8.6. Detecting Pinch Gestures
      1. Problem
      2. Solution
      3. Discussion
  11. 9. Networking, JSON, XML, and Twitter
    1. 9.0. Introduction
    2. 9.1. Downloading Asynchronously with NSURLConnection
      1. Problem
      2. Solution
      3. Discussion
    3. 9.2. Handling Timeouts in Asynchronous Connections
      1. Problem
      2. Solution
      3. Discussion
    4. 9.3. Downloading Synchronously with NSURLConnection
      1. Problem
      2. Solution
      3. Discussion
    5. 9.4. Modifying a URL Request with NSMutableURLRequest
      1. Problem
      2. Solution
      3. Discussion
    6. 9.5. Sending HTTP GET Requests with NSURLConnection
      1. Problem
      2. Solution
      3. Discussion
    7. 9.6. Sending HTTP POST Requests with NSURLConnection
      1. Problem
      2. Solution
      3. Discussion
    8. 9.7. Sending HTTP DELETE Requests with NSURLConnection
      1. Problem
      2. Solution
      3. Discussion
    9. 9.8. Sending HTTP PUT Requests with NSURLConnection
      1. Problem
      2. Solution
      3. Discussion
    10. 9.9. Serializing Arrays and Dictionaries into JSON
      1. Problem
      2. Solution
      3. Discussion
    11. 9.10. Deserializing JSON into Arrays and Dictionaries
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    12. 9.11. Integrating Twitter Functionality into Your Apps
      1. Problem
      2. Solution
      3. Discussion
    13. 9.12. Parsing XML with NSXMLParser
      1. Problem
      2. Solution
      3. Discussion
  12. 10. Audio and Video
    1. 10.0. Introduction
    2. 10.1. Playing Audio Files
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    3. 10.2. Handling Interruptions while Playing Audio
      1. Problem
      2. Solution
      3. Discussion
    4. 10.3. Recording Audio
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    5. 10.4. Handling Interruptions while Recording Audio
      1. Problem
      2. Solution
      3. Discussion
    6. 10.5. Playing Audio over Other Active Sounds
      1. Problem
      2. Solution
      3. Discussion
    7. 10.6. Playing Video Files
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    8. 10.7. Capturing Thumbnails from a Video File
      1. Problem
      2. Solution
      3. Discussion
    9. 10.8. Accessing the Music Library
      1. Problem
      2. Solution
      3. Discussion
  13. 11. Address Book
    1. 11.0. Introduction
    2. 11.1. Requesting Access to the Address Book
      1. Problem
      2. Solution
      3. Discussion
    3. 11.2. Retrieving a Reference to an Address Book
      1. Problem
      2. Solution
      3. Discussion
    4. 11.3. Retrieving All the People in the Address Book
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    5. 11.4. Retrieving Properties of Address Book Entries
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    6. 11.5. Inserting a Person Entry into the Address Book
      1. Problem
      2. Solution
      3. Discussion
    7. 11.6. Inserting a Group Entry into the Address Book
      1. Problem
      2. Solution
      3. Discussion
    8. 11.7. Adding Persons to Groups
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    9. 11.8. Searching the Address Book
      1. Problem
      2. Solution
      3. Discussion
    10. 11.9. Retrieving and Setting a Person’s Address Book Image
      1. Problem
      2. Solution
      3. Discussion
  14. 12. Files and Folder Management
    1. 12.0. Introduction
    2. 12.1. Finding the Paths of the Most Useful Folders on Disk
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    3. 12.2. Writing to and Reading from Files
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    4. 12.3. Creating Folders on Disk
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    5. 12.4. Enumerating Files and Folders
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    6. 12.5. Deleting Files and Folders
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    7. 12.6. Securing Files on Disk
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    8. 12.7. Saving Objects to Files
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
  15. 13. Camera and the Photo Library
    1. 13.0. Introduction
    2. 13.1. Detecting and Probing the Camera
      1. Problem
      2. Solution
      3. Discussion
    3. 13.2. Taking Photos with the Camera
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    4. 13.3. Taking Videos with the Camera
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    5. 13.4. Storing Photos in the Photo Library
      1. Problem
      2. Solution
      3. Discussion
    6. 13.5. Storing Videos in the Photo Library
      1. Problem
      2. Solution
      3. Discussion
    7. 13.6. Retrieving Photos and Videos from the Photo Library
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    8. 13.7. Retrieving Assets from the Assets Library
      1. Problem
      2. Solution
      3. Discussion
    9. 13.8. Editing Videos on an iOS Device
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
  16. 14. Multitasking
    1. 14.0. Introduction
    2. 14.1. Detecting the Availability of Multitasking
      1. Problem
      2. Solution
      3. Discussion
    3. 14.2. Completing a Long-Running Task in the Background
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    4. 14.3. Receiving Local Notifications in the Background
      1. Problem
      2. Solution
      3. Discussion
    5. 14.4. Playing Audio in the Background
      1. Problem
      2. Solution
      3. Discussion
    6. 14.5. Handling Location Changes in the Background
      1. Problem
      2. Solution
      3. Discussion
    7. 14.6. Saving and Loading the State of Multitasking iOS Apps
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    8. 14.7. Handling Network Connections in the Background
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    9. 14.8. Handling Notifications Delivered to a Waking App
      1. Problem
      2. Solution
      3. Discussion
    10. 14.9. Responding to Changes in App Settings
      1. Problem
      2. Solution
      3. Discussion
    11. 14.10. Opting Out of Multitasking
      1. Problem
      2. Solution
      3. Discussion
  17. 15. Core Data
    1. 15.0. Introduction
    2. 15.1. Creating a Core Data Model with Xcode
      1. Problem
      2. Solution
      3. Discussion
    3. 15.2. Generating Class Files for Core Data Entities
      1. Problem
      2. Solution
      3. Discussion
    4. 15.3. Creating and Saving Data Using Core Data
      1. Problem
      2. Solution
      3. Discussion
    5. 15.4. Reading Data from Core Data
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    6. 15.5. Deleting Data from Core Data
      1. Problem
      2. Solution
      3. Discussion
    7. 15.6. Sorting Data in Core Data
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    8. 15.7. Boosting Data Access in Table Views
      1. Problem
      2. Solution
      3. Discussion
    9. 15.8. Implementing Relationships in Core Data
      1. Problem
      2. Solution
      3. Discussion
  18. 16. Dates, Calendars, and Events
    1. 16.0. Introduction
    2. 16.1. Retrieving the List of Calendars
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    3. 16.2. Adding Events to Calendars
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    4. 16.3. Accessing the Contents of Calendars
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    5. 16.4. Removing Events from Calendars
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    6. 16.5. Adding Recurring Events to Calendars
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    7. 16.6. Retrieving the Attendees of an Event
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    8. 16.7. Adding Alarms to Calendars
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    9. 16.8. Handling Event Changed Notifications
      1. Problem
      2. Solution
      3. Discussion
    10. 16.9. Presenting Event View Controllers
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    11. 16.10. Presenting Event Edit View Controllers
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
  19. 17. Graphics and Animations
    1. 17.0. Introduction
    2. 17.1. Enumerating and Loading Fonts
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    3. 17.2. Drawing Text
      1. Problem
      2. Solution
      3. Discussion
    4. 17.3. Constructing, Setting, and Using Colors
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    5. 17.4. Drawing Images
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    6. 17.5. Constructing Resizable Images
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    7. 17.6. Drawing Lines
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    8. 17.7. Constructing Paths
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    9. 17.8. Drawing Rectangles
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    10. 17.9. Adding Shadows to Shapes
      1. Problem
      2. Solution
      3. Discussion
    11. 17.10. Drawing Gradients
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    12. 17.11. Displacing Shapes Drawn on Graphic Contexts
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    13. 17.12. Scaling Shapes Drawn on Graphic Contexts
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    14. 17.13. Rotating Shapes Drawn on Graphic Contexts
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    15. 17.14. Animating and Moving Views
      1. Problem
      2. Solution
      3. Discussion
    16. 17.15. Animating and Scaling Views
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    17. 17.16. Animating and Rotating Views
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
  20. 18. Core Motion
    1. 18.0. Introduction
    2. 18.1. Detecting the Availability of an Accelerometer
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    3. 18.2. Detecting the Availability of a Gyroscope
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    4. 18.3. Retrieving Accelerometer Data
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    5. 18.4. Detecting Shakes on an iOS Device
      1. Problem
      2. Solution
      3. Discussion
    6. 18.5. Retrieving Gyroscope Data
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
  21. 19. iCloud
    1. 19.0. Introduction
    2. 19.1. Setting Up Your App for iCloud
      1. Problem
      2. Solution
      3. Discussion
    3. 19.2. Storing and Synchronizing Dictionaries in iCloud
      1. Problem
      2. Solution
      3. Discussion
    4. 19.3. Creating and Managing Folders for Apps in iCloud
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    5. 19.4. Searching for Files and Folders in iCloud
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    6. 19.5. Storing User Documents in iCloud
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    7. 19.6. Managing the State of Documents in iCloud
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    8. 19.7. Handling Conflicts in iCloud Documents
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
  22. 20. Pass Kit
    1. 20.0. Introduction
    2. 20.1. Creating Pass Kit Certificates
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    3. 20.2. Creating Pass Files
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    4. 20.3. Providing Icons and Images for Passes
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    5. 20.4. Preparing Your Passes for Digital Signature
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    6. 20.5. Signing Passes Digitally
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    7. 20.6. Distributing Passes Using Email
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    8. 20.7. Distributing Passes Using Web Services
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    9. 20.8. Enabling Your iOS Apps to Access Passes on iOS Devices
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
    10. 20.9. Interacting with Passbook Programmatically
      1. Problem
      2. Solution
      3. Discussion
      4. See Also
  23. Index
  24. About the Author
  25. Colophon
  26. Copyright
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1.17. Moving from Manual Reference Counting to Automatic Reference Counting

Problem

You want to learn about Automatic Reference Counting, Apple’s new Compiler solution to solving the headache that programmers had to deal with when working with objects and memory management in Objective-C.

Note

Automatic Reference Counting eliminates many of the manual reference counting issues that ultimately resulted in iOS apps that would crash here and there and would be very unstable when deployed on user devices. ARC removes this headache by leaving most of the memory management complexity to the compiler.

Solution

Study the new storage attributes introduced with the latest LLVM compiler: strong, weak, and unsafe_unretained.

Discussion

To use Automatic Reference Counting (ARC) in the latest LLVM compiler, we need to deal with storage that is strong, weak, or unsafe and unretained. Any object under ARC is managed with one of these storage attributes. Here is a short explanation for each one:

strong

An object of this type is automatically retained at runtime and will be valid until the end of its scope, where it will automatically be released. For those familiar with Objective-C’s traditional way of memory management, this keyword is similar to the retain keyword.

weak

This is zeroing weak referencing. If a variable is defined with this keyword, when the object to which this variable points gets deallocated, this value will get set to nil. For instance, if you have a strong string property and a weak string property and set the weak property’s value to the strong property’s value, when the strong property gets deallocated, the weak property’s value will get set to nil.

unsafe_unretained

This is simply pointing one variable to another. This will not retain the object into the new variable, it will simply assign the object to the variable.

By default, all local variables are strong variables. In contrast, properties must explicitly specify their storage attribute. In other words, the compiler won’t assume that all properties without a storage attribute are by default strong properties. So do make sure that you specify the storage attributes for your properties. Let’s have a look at an example of the strong storage attribute. Let’s assume we have two properties called string1 and string2:

#import <UIKit/UIKit.h>

@interface Moving_from_Manual_Reference_Counting_to_ARCAppDelegate
           : UIResponder <UIApplicationDelegate>

@property (strong, nonatomic) UIWindow *window;

@property (nonatomic, strong) NSString *string1;
@property (nonatomic, strong) NSString *string2;

@end

Now if we initialize the string1 property with the string value of String 1 and assign this property’s value to the string2 property, we will see that with the strong storage attribute, the string2 property will keep its value even after string1 is deallocated:

#import "Moving_from_Manual_Reference_Counting_to_ARCAppDelegate.h"

@implementation Moving_from_Manual_Reference_Counting_to_ARCAppDelegate

- (BOOL)            application:(UIApplication *)application
  didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{  
  self.string1 = @"String 1";
  self.string2 = self.string1;
  self.string1 = nil;

  NSLog(@"String 2 = %@", self.string2);

  self.window = [[UIWindow alloc] initWithFrame:
                 [[UIScreen mainScreen] bounds]];

  self.window.backgroundColor = [UIColor whiteColor];
  [self.window makeKeyAndVisible];
  return YES;
}

Note

Memory allocated for an object is disposed of when all strong variables pointing to that memory are deallocated.

The output of this program is this:

String 2 = String 1

the strong, weak, and unsafe_unretained are most frequently used when declaring properties. You can take advantage of these storage specifiers even when declaring local variables, but you need to change the specifiers a bit. The strong specifier’s inline equivalent is __strong, weak specifier’s inline equivalent is __weak, and unsafe_unretained specifier’s inline equivalent is __unsafe_unretained. (Note that each of those keywords begins with two underline characters.) Here is an example:

- (BOOL)            application:(UIApplication *)application
  didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{

  /* All local variables are by default strong, so just emphasize that. We
   really don't have to mention __strong for the first variable but
   to make it clear, we will set it. No harm in doing so. */
  __strong NSString *yourString = @"Your String";
  __weak   NSString *myString = yourString;
  yourString = nil;
  __unsafe_unretained NSString *theirString = myString;

  /* All pointers will be nil at this time */
  self.window = [[UIWindow alloc] initWithFrame:
                 [[UIScreen mainScreen] bounds]];

  self.window.backgroundColor = [UIColor whiteColor];
  [self.window makeKeyAndVisible];
  return YES;
}

the unsafe_unretained storage specifier is truly unsafe, as its name implies. The reason for it being unsafe is if the object to which an unsafe_unretained variable points gets deallocated, this variable will not get set to nil and will point to a dangling location in the memory. Accessing this location might cause your application to crash. To avoid this, you should be using the zeroing weak referencing storage specifier, weak or its inline equivalent __weak.

Let’s see an example for zeroing weak referencing. Let’s change our string2 property’s storage specifier to weak instead of strong:

#import <UIKit/UIKit.h>

@interface Moving_from_Manual_Reference_Counting_to_ARCAppDelegate
           : UIResponder <UIApplicationDelegate>

@property (strong, nonatomic) UIWindow *window;

@property (nonatomic, strong) NSString *string1;
@property (nonatomic, weak) NSString *string2;

@end

When our app starts for the first time, we will initialize the strong string1 property and will assign string1 to string2. We will then set the value of the string1 property to nil. Then we will wait. This is absolutely crucial. If immediately after setting the value of string1 to nil, you print out the value of string2, chances are that you will get incorrect results instead of nil. So you need to make sure that your app’s run loop has gotten rid of all invalidated objects. In order to achieve this, we will print the value of strong2 when our app gets sent to the background. (This is caused by the user pressing the home button on their iOS device.) Once we’re running in the background, we know that the run loop has already gotten rid of invalidated objects in the memory and the results that we will get will be accurate:

- (BOOL)            application:(UIApplication *)application
  didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{

  self.string1 = [[NSString alloc] initWithUTF8String:"String 1"];
  self.string2 = self.string1;
  self.string1 = nil;

  /* All pointers will be nil at this time */

  self.window = [[UIWindow alloc] initWithFrame:
                 [[UIScreen mainScreen] bounds]];

  self.window.backgroundColor = [UIColor whiteColor];
  [self.window makeKeyAndVisible];
  return YES;
}

- (void)applicationDidEnterBackground:(UIApplication *)application{
  NSLog(@"String 2 = %@", self.string2);
}

Now run this app, wait a second or two, and press the Home button on the device/simulator. You will notice that the following results will get printed to the console window:

String 2 = (null)

This easily proved that the zeroing weak references work perfectly under ARC. Now to check how dangerous the unsafe_unretained storage specifier is, let’s go ahead and change the string2 property’s storage specifier to unsafe_unretained and repeat the exact same practice as we did for the weak property:

#import <UIKit/UIKit.h>

@interface Moving_from_Manual_Reference_Counting_to_ARCAppDelegate
           : UIResponder <UIApplicationDelegate>

@property (strong, nonatomic) UIWindow *window;

@property (nonatomic, strong) NSString *string1;
@property (nonatomic, unsafe_unretained) NSString *string2;

@end

Now if you leave the implementation of your app delegate as we had implemented it in the previous example (printing the value of string2 property when our app gets sent to the background), and you repeat the same procedure and open your app and send it to the background, you will crash! This means that when our app was sent to the background, we tried to print out the contents of an invalidated memory location that the string2 property was pointing to. Since the string2 property was unsafe and unretained, it didn’t know that the object that it was pointing to (in string1) was already deallocated when string1 was set to nil.

In addition to the aforementioned three storage specifiers, we can also use the __autoreleasing specifier. This storage specifier is most handy when we want to pass an object by reference to a method. This is required for when you want to call a method and leave that method responsible for allocating, initializing, and returning an instance of a class. The caller method will then have no responsibility at all to release the returned instance and will leave it to the runtime to decide when it is best to release the allocated instance of that class (handled by autorelease pools). For instance, if you have a method that needs to pass an error of type NSError to the caller method, the caller method will pass an uninitialized and unallocated instance of NSError to this method. This means the caller didn’t allocate memory for this error variable, so our method should do so. To do this, you must specify that this error parameter needs to be automatically released by the runtime when the right time comes:

- (void) generateErrorInVariable:(__autoreleasing NSError **)paramError{

  NSArray *objects = [[NSArray alloc] initWithObjects:@"A simple error", nil];

  NSArray *keys =
  [[NSArray alloc] initWithObjects:NSLocalizedDescriptionKey, nil];

  NSDictionary *errorDictionary = [[NSDictionary alloc] initWithObjects:objects
                                                                forKeys:keys];

  *paramError = [[NSError alloc] initWithDomain:@"MyApp"
                                           code:1
                                       userInfo:errorDictionary];
}

- (BOOL)            application:(UIApplication *)application
  didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{

  NSError *error = nil;
  [self generateErrorInVariable:&error];

  NSLog(@"Error = %@", error);

  self.window = [[UIWindow alloc] initWithFrame:
                 [[UIScreen mainScreen] bounds]];

  self.window.backgroundColor = [UIColor whiteColor];
  [self.window makeKeyAndVisible];
  return YES;
}

In this example, the application:didFinishLaunchingWithOptions: method didn’t allocate the instance of NSError; the generateErrorInVariable method did. But for the compiler to understand the scope of the error object, the generateErrorInVariable method mentioned to the compiler that the object which will be created into its error parameter needs to be automatically released if it is no longer needed.

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