Singleton - Reading from a Data Store - objective-c

Question

I'm trying to do the right thing by not using global variables in my Xcode project. I've successfully created a singleton. I have a simple data store which I named "myContactsStore" that contains an array, with each object in the array having only 3 instance variables (name, phoneNum, and eMail). I have no problem creating, modifying, saving etc. the data in the array when I'm executing the view controller that created the array.
My problem is trying to access the data store from another view controller. I'm halfway there, as proven by my ability to print the contents of the entire test array from another view controller by using the following code in a for loop:
NSLog(#"%#", myContactsStore.description);
Here's the output:
"Mary, 0938420839, PaulDoe#Mac.com",
"John, 9932097372, PaulDoe#Mac.com",
"Mary, 0726756893, RedCat#iwon.com",
"Mary, 8556327199, xxxbct#mac.com",
"John, 0640848317, xxxbct#mac.com"
How do I access just one instance variables? For example, I want to create a read-only array in another view controller that contains just the email addresses of every contact in the "myContactsStore" array. I've tried several things, but I'm new at this and I must be missing something very basic.
Thanks for you help and any code example you might have the time to include.

Answer 1

While Singletons are an easy way to share data across classes they cause cohesion problems in your overall design where your classes start to "import the world". The issue is coming up with exactly the right dependency for a given class and this is very easily missed when you try to design from classes instead of designing from use cases. You want to ask yourself, "what data does this class use?" You then create a protocol (abstraction) that gives the class the view of the data it wants. In your case one class writes to the data store. It doesn't need to create the data store nor does either class need to know that the data is maintained in an array. Follow these steps exactly in order and follow directly or you'll miss my point. Try a protocol in a separate .h file like the following:
#protocol MyDataStore <NSObject> {
}
Import this header in your 1st class that creates the contacts and declare a property of the protocol's type.
#import "MyDataStore.h"
#interface MyContactCreator : UIViewController
#property (nonatomic, retain) id<MyDataStore> dataStore;
-(id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil dataStore:(id<MyDataStore>)aDataStore;
#end
I threw in a custom init method in case you are currently instantiating your view controllers programmatically instead of via InterfaceBuilder. In your implementation you would do something like this:
#implementation MyContactCreator
//other methods...
-(id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil dataStore:(id<MyDataStore>)aDataStore
{
self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
if (self) {
self.dataStore = aDataStore;
}
return self;
}
//other methods...
#end
If you are using Interface Builder to create your view controller you can drag a custom object into play and call it something like "MyDataStoreImpl". The idea here is that you are giving the datastore to the view controller instead of it creating it directly and knowing about it. Also you want to defer worrying about how the data store works until you really need to. Later in your view controller where you create the contacts you would use the data store to create them. Assuming the info comes from standard screen elements you would write code like this:
-(void) addContactTapped:(id)sender
{
[self.datastore createContactWithName:txtNameField.text phoneNumber:txtPhoneField.text email:txtEmailField.text];
}
Your editor would scream at you (with little red marks like what your 2nd grade teacher would use on your spelling homework) because the datastore doesn't respond to the message you are sending. You go back and add that method to the protocol:
#protocol MyDataStore <NSObject> {
-(void) createContactWithName:(NSString*)aName phoneNumber:(NSString*)aPhoneNumber email:(NSString*)anEmail;
}
In your other view controller class that wants the list of email addresses you would import the same data store protocol. You would also declare a datasource property identical as what we did above using a complimentary custom init method or Interface Builder to pass the datasource in. This view controller (assuming it's a table view controller) would probably have some methods like:
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
return 1;
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
return [self.datasource numberOfContacts];
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
NSString *email = [self.datasource emailForContactNumber:indexPath.row];
UITableViewCell *cell = //create tableview cell with the email string
return cell;
}
Your editor will start screaming with the little red lines and all. This is where you go and add more methods to the protocol.
#protocol MyDataStore <NSObject> {
-(void) createContactWithName:(NSString*)aName phoneNumber:(NSString*)aPhoneNumber email:(NSString*)anEmail;
-(NSInteger) numberOfContacts;
-(NSString*) emailForContactNumber:(NSInteger)index;
}
The little red lines go away and finally you can begin thinking about how the contacts are stored and retrieved. Create a separate class called MyDataStoreImpl which extends NSObject and imports and follows the "MyDataStore" protocol. Fill out implementations of all of the methods and you should be up and running. It could be as simple as storing NSDisctionary objects containing the contact info in an internal NSMutableArray property.
- (id)init {
self = [super init];
if (self) {
self.allContacts = [NSMutableArray array];
}
return self;
}
-(void) createContactWithName:(NSString*)aName phoneNumber:(NSString*)aPhoneNumber email:(NSString*)anEmail;
{
NSDictionary *newContact = [NSDictionary dictionaryWithObjectsAndKeys:
aName, #"name", aPhoneNumber, #"phone", anEmail, #"email",
nil];
[self.allConstacts addObject:newContact];
}
-(NSInteger) numberOfContacts;
{
return [self.allContacts count];
}
-(NSString*) emailForContactNumber:(NSInteger)index;
{
[[self.allContacts objectAtIndex:index] valueForKey:#"email"];
}
The advantages here are many. You can later re-implement the datasource to read/write from a plist file, network server, or database without touching any of your controllers. Also, your app will be easier to optimize for performance because you can design the read write methods to pull directly from a source instead of naively copying data from one array to another as you would if you were worrying about how it is managed too early. All of the above thrown together without testing and likely has errors but given to illustrate a point of how to properly share data between controllers without Singletons while maintaining a testable and easily maintainable codebase.

Answer 2

You can stick this to any class to make it a singleton:
+(MySingleton *)singleton {
static dispatch_once_t pred;
static MySingleton *shared = nil;
dispatch_once(&pred, ^{
shared = [[MySingleton alloc] init];
shared.someVar = someValue;
});
return shared;
}
-(void) dealloc {
abort();
[someVar release];
[super dealloc];
}
Better yet, you can add the class as an ivar of the application delegate, which is a singleton you already have. You can get a reference to it like this:
// AppDelegate or whatever name
AppDelegate *delegate = (AppDelegate*)[[UIApplication sharedApplication] delegate];
Then you access the datastore as an ivar on that delegate, and optionally implement one of the several persistence technologies available on iOS, basically plist files through direct file access or NSCoding or even NSUserDefaults (which you shouldn't but it's handy for small tasks), Core Data, or SQLite.
If you are using this data store class only to pass data between controllers, you can do so directly instead. Example:
CitySelectionVC *citySel = [[CitySelectionVC alloc] initWithCities:self.cities];
[self.navigationController pushViewController:citySel animated:TRUE];
[citySel release];

Answer 3

I always use SynthesizeSingleton.h by Mike Gallagher. He has a really informative article relating to this topic here. You should really check it out. It makes the creation of Singleton classes really easy.

Answer 4

Assuming your myContactsStore object has a method contactsArray which returns the internal NSArray object, you can do this:
NSArray *emails = [myContactsStore.contentsArray valueForKey:#"email"];
NSLog(#"output: %#", emails);
Which should output:
output: (
"PaulDoe#Mac.com",
"PaulDoe#Mac.com",
"RedCat#iwon.com",
"xxxbct#mac.com",
"xxxbct#mac.com"
)

Answer 5

You simply have to enumerate through the array and retrieve the data you want. Like so:
NSMutableArray *emails = [[NSMutableArray alloc] initWithCapacity:myContactsStore.count];
for (ContactClassName *contact in myContactsStore) {
NSString *email = contact.eMail; // I'm assuming your ivar eMail is also a property
if (email) [emails addObject:email];
}
You now have an NSMutableArray containing just the list of emails. If you want to make this list immutable simply do:
NSArray *emailList = [emails copy];