Data persistence in iOS Development

Ways to persist data
*NSUserDefaults -- save user data in property list. Non-sensitive info, e.g. user settings
*SQLite 3 -- embedded data in iOS
*Core Data -- object oriented for SQLite3
*Keychain access -- for sensitive information

----------------------------
NSUserDefaults

@implementation ViewController

- (void)viewDidLoad

{

    [super viewDidLoad];

    // Read from NSUserDefaults object

    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];

    

    _myTextField.text = [defaults objectForKey:@"TextFieldText"];

    

    NSNumber *switchNumValue = [defaults objectForKey:@"SwitchValue"];

    _mySwitch.on = [switchNumValue boolValue];

    

}

- (void)didReceiveMemoryWarning

{

    [super didReceiveMemoryWarning];

    // Dispose of any resources that can be recreated.

}

- (IBAction)saveButtonPressed:(id)sender {

    

    // Get a reference to NSUserDefaults object

    // alloc init doesn't work because NSUserDefaults is a singleton

    // every single app only has 1 instance of NSUserDefaults

    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];

    

    [defaults setObject:_myTextField.text forKey:@"TextFieldText"];

    [defaults setObject:[NSNumber numberWithBool:_mySwitch.on] forKey:@"SwitchValue"];

    // need to cast the switch value to number because NSUserDefaults only accept string or int

    

    [defaults synchronize]; // this is to write changes to disk

}

@end

---------------------------------
Keychain access

References:
http://software-security.sans.org/blog/2011/01/05/using-keychain-to-store-passwords-ios-iphone-ipad/
http://developer.apple.com/library/ios/#documentation/Security/Conceptual/keychainServConcepts/02concepts/concepts.html%23//apple_ref/doc/uid/TP30000897-CH204-TP9 

"KeyChainItemWrapper" package downloadable from Apple
http://developer.apple.com/library/ios/#samplecode/GenericKeychain/Listings/Classes_KeychainItemWrapper_h.html

After the login button is pressed

KeychainItemWrapper *keychainItem = [AppDelegate keychainItem];

[keychainItem setObject:username forKey:(__bridge id)(kSecAttrAccount)];

[keychainItem setObject:password forKey:(__bridge id)(kSecValueData)];

AppDelegate.h

// declare static method for KeyChainItemWrapper class

+ (KeychainItemWrapper *)keychainItem;

AppDelegate.m

// Instantiate KeyChainItemWrapper class as a singleton through AppDelegate

static KeychainItemWrapper *_keychainItem;

@implementation AppDelegate

+ (KeychainItemWrapper *)keychainItem{

    return _keychainItem;

}