Question

I read enough information about singleton and delegation. So, I think I understand what is singleton. About delegation I still confuse. I understand conception of delegation, but I need to create my protocol for understanding delegation.

Ok, I create singleton for work with my entities from CoreData. Maybe I wrong and it is not singleton, tell me please about it. My singleton is FetchData.

Fetchdata.h

#import <Foundation/Foundation.h>

@interface FetchData : NSObject <UIApplicationDelegate>

+(FetchData*) fetchData;

-(NSArray*)fetchLogin:(NSString*)name;
-(BOOL)newGroup:(NSString*)group forLogin:(NSString*)login;
-(NSMutableArray*)contactsForGroup:(NSString*)group;
-(BOOL)newContact:(NSString*)name surname:(NSString*)surname withDatas:(NSArray*)array;
//other methods 

@end

Fetchdata.m

#import "FetchData.h"
#import "Contact.h"
#import "Login.h"
#import "Group.h"
#import "AppDelegate.h"

@interface FetchData ()
@property (nonatomic, strong) NSEntityDescription *loginEntity;
@property (nonatomic, strong) NSEntityDescription* groupEntity;
@property (nonatomic, strong) NSManagedObjectContext* context;
@property (nonatomic, strong) NSEntityDescription* contactEntity;
@property (nonatomic, strong) AppDelegate* appDelegate;
//other properties
@end

@implementation FetchData
@synthesize //my properties

+(FetchData*) fetchData
{
 static  FetchData* fetchData = nil;
 if (!fetchData) 
    fetchData = [[super allocWithZone:nil]init];
 return fetchData;
}

+(id)allocWithZone:(NSZone *)zone
{
 return [self fetchData];
}

//implementation my methods
@end

So, it is very easy to work with CoreData now for me. I need only import FetchData and simply use methods for create/delete/change/add/sort…

SomeClass.m

#import "FetchData.h"
#define fetching [FetchData fetchData]

But I think that I can use for my aim delegation. Or maybe it is the best decesion as compared with singleton. So I want to remake singleton for delegation. And I need help with this question. What I must do?

If I understand correctly I need create protocol with all my methods from FetchData.h, FetchData.m I can leave without changes. And in SomeClass I need import FetchData and add my protocol. Like:

#import <Foundation/Foundation.h>

@protocol FetchingDelegate

//all methods from FetchData.h

@end

@interface FetchData : NSObject
@property (nonatomic, strong) id <FetchingDelegate> delegate;
@end

FetchData.m

@interface FetchData()
//all properties without changing
@end

@implementation FetchData
@synthesize //all properties and delegate

//implementation of methods
@end

SomeClass

#import "FetchData.h"

@interface SomeClass : NSObject <FetchingDelegate>
@end

@implementation SomeClass

-(void)viewDidLoad
{
  FetchData* fetching = [FetchData new]
  fetching.delegate = self
}
//and now I can use any methods from protocol like [fetching anyMethod]
//such I used with singleton

Answer 1

The idea of a singleton is that your entire app can access this one class. Multiple view controllers may need data coming from your database. In your case, I would change your fetchData method (and maybe change its name as it doesn’t really follow convention now):

+(FetchData*) fetchData
{
    static FetchData *fetchData;
    dispatch_once_t token;
    dispatch_once(&token, ^{
        if (!fetchData) 
            fetchData = [super init];
    }
 return fetchData;
}

Delegates are meant for one-on-one communication, meaning that one object has a delegate and sends any messages to that one particular delegate.

That means that a singleton and delegation don’t go well together. The singleton is made to send messages to multiple receivers, while the delegation pattern is meant for one-on-one communication. So you have two options: you could either not use a singleton and use the delegation pattern, or you could use a singleton, and use NSNotificationCenter to notify observers of changes.