Singleton dictionary is empty, and I don't see why

Question

I have an iOS app that matches incoming text fields to standard fields used to import records. My problem is that a NSMutableDictionary that uses those fields is empty! Here is the code that saves the mapping:

-(void)mapUserFields: (id) sender  {   //  move contents of each textField when user has finished entering it

    SingletonDictionary *sd = [SingletonDictionary sharedDictionary];

    UITextField *tf = (UITextField *)sender;  //  textfield contains the pointer to user's half of the equation
    int tagValue = (int)tf.tag;  //  get the tag value

    [sd.dictionaryOfUserIndexes setObject:tf.text forKey:[NSString stringWithFormat:@"%d", tagValue]];  //  value found in textField id'd by tag

    NSLog(@"\nfield.text: %@  tagValue: %d  nsd.count: %d\n",tf.text, tagValue, sd.dictionaryOfUserIndexes.count);

}

This is the result of the NSLog:

field.text: 1 tagValue: 38 nsd.count: 0

This is the definition of the singleton in the .h file:

@property (nonatomic, retain) NSMutableDictionary *dictionaryOfUserIndexes;

This is the code to initialize the singleton in the .m file:

//--  SingletonDictionaryOfUserIDs  --
+ (id) sharedDictionary  {

    static dispatch_once_t dispatchOncePredicate = 0;
    __strong static id _sharedObject = nil;
    dispatch_once(&dispatchOncePredicate, ^{
        _sharedObject = [[self alloc] init];
    });

    return _sharedObject;
}

-(id) init {
    self = [super init];
    if (self) {
        dictionaryOfUserIndexes = [NSMutableDictionary new];
    }
    return self;
}

@end

I believe my problem is because the sd.dictionaryOfUserIndexes is not initialized, but I am not sure if that's true, and if so, how to initialize it (I tried several different variants, all of which created build errors). I looked on SO and Google, but found nothing that addresses this particular issue. Help would be greatly appreciated!

Answer 1

There are a few things we could improve in this code, but the only thing wrong with it is the reference to dictionaryOfUserIndexes in the init method. The code as posted wouldn't compile, unless: (a) you have a line like:

@synthesize dictionaryOfUserIndexes = dictionaryOfUserIndexes;

so that the backing variable is named without the default _ prefix, or (b) you refer to the ivar with the default prefix, as in:

_dictionaryOfUserIndexes = [NSMutableDictionary new];

The other way -- preferable in most every context except within an init method -- is to use the synthesized setter, like:

self.dictionaryOfUserIndexes = [NSMutableDictionary new];

But with that change alone (so it will compile) your code runs fine, adds a value to the dictionary and logs an incremented count.