help with singleton's

I am trying to create a singleton User class in my app, here's the code:

#import "User.h"
#import "Login.h"
#import "SFHFKeychainUtils.h"

// Constants
static NSString* const kDBUserCurrentUserIDDefaultsKey = @"kDBUserCurrentUserIDDefaultsKey";

// Current User singleton
static User* currentUser = nil;

@implementation User
@synthesize username = _username;
@synthesize password = _password;
@synthesize delegate = _delegate;


- (id)init
{
    self = [super init];
    if (self) {
        // Initialization code here.
    }
    return self;
}

+ (NSString*)primaryKeyProperty {
    return @"username";
}

+ (User*)currentUser {
    if (nil == currentUser) {

        id username = [[NSUserDefaults standardUserDefaults] objectForKey:@"kApplicationUserNameKey"];
        if (!username) {
            currentUser = [self new];
        } else{
             NSLog(@"CURRENT USER");
            return self;
        }

        [currentUser retain];
    }

    return currentUser;
}

+ (void)setCurrentUser:(User*)user {
    [user retain];
    [currentUser release];
    currentUser = user;
}

/**
 * Implementation of a RESTful login pattern. We construct an object loader addressed to
 * the /login resource path and POST the credentials. The target of the object loader is
 * set so that the login response gets mapped back into this object, populating the
 * properties according to the mappings declared in elementToPropertyMappings.
 */
- (void)loginWithUsername:(NSString*)username andPassword:(NSString*)password delegate:(NSObject<DBUserAuthenticationDelegate>*)delegate {
    _delegate = delegate;

    //[RKObjectManager sharedManager].client.username = username;
    //[RKObjectManager sharedManager].client.password = password;

    self.username = username;
    self.password = password;

    RKObjectMapping * userMapping = [[RKObjectManager sharedManager].mappingProvider objectMappingForKeyPath:@"LoginViewController"];
    [[RKObjectManager sharedManager] loadObjectsAtResourcePath:@"/account/verify.json" objectMapping:userMapping delegate:self];
}

/**
* Implementation of a RESTful logout pattern. We POST an object loader to
* the /logout resource path. This destroys the remote session
*/
- (void)logout/*:(NSObject<DBUserAuthenticationDelegate>*)delegate */{  
    NSError * error = nil;
    [[NSUserDefaults standardUserDefaults] setValue:nil forKey:@"kApplicationUserNameKey"];
    [[NSUserDefaults standardUserDefaults] synchronize];
    [SFHFKeychainUtils deleteItemForUsername:self.username andServiceName:@"convore" error:&error];

    NSLog(@"LOGGING OUT");
    if ([self.delegate respondsToSelector:@selector(userDidLogout:)]) {
        [self.delegate userDidLogout:self];
    }

    [[NSNotificationCenter defaultCenter] postNotificationName:@"DBUserDidLogoutNotification" object:nil];
}

- (void)loginWasSuccessful {
    // Upon login, we become the current user
    [User setCurrentUser:self];
    NSError * error = nil;

    // Persist the username for recovery later
    [[NSUserDefaults standardUserDefaults] setValue:self.username forKey:@"kApplicationUserNameKey"];
    [[NSUserDefaults standardUserDefaults] synchronize];
    [SFHFKeychainUtils storeUsername:self.username andPassword:self.password forServiceName:@"convore" updateExisting:TRUE error:&error];

    // Inform the delegate
    if ([self.delegate respondsToSelector:@selector(userDidLogin:)]) {
        [self.delegate userDidLogin:self];
    }

    [[NSNotificationCenter defaultCenter] po开发者_运维技巧stNotificationName:@"DBUserDidLoginNotification" object:self];
}

- (void)request:(RKRequest*)request didLoadResponse:(RKResponse*)response 
{
    NSLog(@"Loaded payload: %@", [response bodyAsString]);
}

- (void)objectLoader:(RKObjectLoader*)objectLoader didLoadObject:(id)object
{
    if ([objectLoader wasSentToResourcePath:@"/account/verify.json"]) {
        Login * login = (Login *) object;
        if ([login.username length] > 0)
            [self loginWasSuccessful];
    }
}


- (void)objectLoader:(RKObjectLoader *)objectLoader didFailWithError:(NSError*)error {  
    if ([objectLoader wasSentToResourcePath:@"/account/verify.json"]) {
         NSLog(@"Encountered an error: %@", error); 
        // Login failed
        if ([self.delegate respondsToSelector:@selector(user:didFailLoginWithError:)]) {
            [self.delegate user:self didFailLoginWithError:error];
        }
    } 
}

- (BOOL)isLoggedIn {
    return self.username != nil;
    //return self.singleAccessToken != nil;
}

- (void)dealloc {
    _delegate = nil;
    [_password release];
    [_passwordConfirmation release];
    [super dealloc];
}

@end

The issue is that whenever I tried to access currentUser it always breaks down. I first called the loginWithUsernameandPassword and then tried calling the currentUser, but when I call the currentUser on logout, it gives me an error:

calling this:

if ([[User currentUser] isLoggedIn])

gives me:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+[User isLoggedIn]: unrecognized selector sent to class 0x1a671c'

seems that currentUser is nil, why is this?

Quick Singleton 101 (I wish I had this when I started, lol. Everyone just pointed me to the docs which didn't help much). The name of the singleton is going to be "Singleton"

//Singleton.h
#import <Foundation/Foundation.h>

@interface SingletonManager : NSObject
{
    NSDictionary* randomDictionary; //just using a dictionary for demonstrative purposes. You can make this a string or whatever you want.
}

+ (Singleton*)sharedSingleton;

@property (nonatomic, retain) NSDictionary *randomDictionary;

@end

And now the .m

//Singleton.m
#import "Singleton.h"

static Singleton *sharedSingleton = nil;

@implementation Singleton

@synthesize randomDictionary;

#pragma mark Singleton Method
+ (Singleton*)sharedSingleton
{
    @synchronized(self)
    {
        if(sharedSingleton == nil)
        {
            sharedSingleton = [[super allocWithZone:NULL] init];
        }
    }
    return sharedSingleton;
}
@end

And to set/get, first import the singleton in whatever class you need: #import "Singleton.h", then grab the singleton with Singleton *singletonManager = [Singleton sharedSingleton]; and then you can do whatever you need to as necessary. i.e. to get the description of the NSDictionary you would call [[singletonManager randomDictionary] description];

Now this is using ARC, so if you are not you'd just have to make sure you manage your memory correctly. Enjoy.

You need to get the singleton object before you can call a method on it.

if ( [[User currentUser] isLoggedIn] ) {
  // Magic happens here
}

You aren't coding your singleton properly.

+ (User *) currentUser {
    @synchronized (self) {

       if (currentUser == nil) {
            currentUser = [[self alloc] init];
       }
       return currentUser;
   }
}

The answer is really a combo of the two answers from XCodeDev and Matthieu Cormier. You need to "protect" your init the way the code sample says so new versions of the object are not created. Otherwise, its not a real singleton. More info on Singleton pattern.

Also, just because its a singleton doesn't mean you can access it with just class methods after you initialize it. You still need to get the instance you initialized, otherwise you cannot do operations that require certain values only in the instance.