Objective-C Singletons – an alternative pattern

Singletons are supposed to be bad practice in any programming language. One of the arguments against their use is that they are difficult to re-use, they hide dependencies, they’re hard to sub-class, and loads of other reasons that I can’t remember since I fell sleep reading them…

Bullshit. There are clear cases where a Singleton makes sense, for instance when you are only ever going to use one instance of a class – for example an application’s data model. The key is to make your singleton re-usable and interchangeable and to provide a façade to the implementation by means of an interface – “write to the interface”. This ensures that the class is not really a Singleton, but that you are accessing a single default instance of the class. This is a subtle difference.

Here’s the canonical way of declaring a Singleton in Objective-C:

@interface MySingleton : NSObject

+ (MySingleton *)sharedInstance;

@end

@implementation MySingleton

+ (MySingleton *)sharedInstance {
    static dispatch_once_t once;
    static MySingleton *instance;
    dispatch_once(&once, ^{
        instance = [[MySingleton alloc] init];
    });
    return instance;
}

@end

The dispatch_once call ensures that the instantiation is thread-safe. That’s it. It’s limited but it works. However, this pattern is the one that the critics shoot down. Here’s a better pattern…

We’re going to create a re-usable Data Model Manager class (DataModelManager) where we can access just one instance (the Singleton), via a Factory class, of an implementation of an Objective-C Protocol and still have the freedom to swap out the implementation if needed (and sub-class, too).

Here’s the Protocol for the Data Model Manager and the Interface that will serve as the factory class:

// The Protocol declaring the instance methods that we need for our DataModelManager class
@protocol DataModelManager

-(void)setup;
// Other method declarations...

@end

// Factory class that returns a shared DataModelManager type class
@interface DataModelManagerFactory : NSObject

+ (id)sharedInstance;

@end

Any required instance methods for the Data Model Manager should be declared in the Protocol. This is equivalent to a Java Interface. We are declaring those methods that any implementation of the DataModelManager protocol has to implement (required). The DataModelManagerFactory interface returns the (Singleton) shared instance of DataModelManager. The implementation is opaque to the user, and is in fact interchangeable. What we have declared so far is the public API. The consumer doesn’t need to know any more than that.

Here’s the implementation:

@implementation DataModelManagerFactory

// Factory class method
+ (id)sharedInstance {
    // Retrieve class name from a config file or inject it...
    static NSString *className = @"TheDataModelManager";
    static dispatch_once_t once;
    static id instance;
    dispatch_once(&once, ^{
        instance = [[NSClassFromString(className) alloc] init];
    });
    return instance;
}

@end

We’re using the same traditional Singleton pattern as before, except that we’re declaring the class name of the actual implementation of the DataModelManager protocol in an NSString. This could be declared and retrieved from an external configuration file, or passed as a parameter to the DataModelManagerFactory when initialising. Or hard-coded if you ever need to refactor and provide a different implementation of DataModelManager.

Here’s the interface declaration and implementation of TheDataModelManager:

// Implementation of DataModelManager Protocol
@interface TheDataModelManager : NSObject
@end

@implementation TheDataModelManager

- (void)setup {
    NSLog(@"Setting up TheDataModelManager");
}

@end

If we want to swap out a different implementation class, all we need to do is change the name of the class string in the Factory and provide a matching implementation class of the same name. So if we changed it to this:

    static NSString *className = @"AnotherDataModelManager";

We would implement it differently:

// Another implementation of DataModelManager Protocol
@interface AnotherDataModelManager : NSObject
@end

@implementation AnotherDataModelManager

- (void)setup {
    NSLog(@"Setting up AnotherDataModelManager");
}

@end

This pattern ensures that you’re not locked in to a single, static implementation of your Singleton. As the consumer, you only care about the methods that the Singleton declares in its Protocol. The actual implementation can be changed both at compile time and dynamically at run-time if the name of the class is provided to the Factory class. And, as a bonus, it’s easy to substitute a mock class for unit testing.

Let’s modify the Factory class so we can change the class name:

@interface DataModelManagerFactory : NSObject

+ (void)setClassName: (NSString *)className;
+ (id)sharedInstance;

@end

@implementation DataModelManagerFactory

NSString *_className = @"DefaultDataModelManager";

+ (void)setClassName: (NSString *)className {
    _className = className;
}

+ (id)sharedInstance {
    static dispatch_once_t once;
    static id instance;
    dispatch_once(&once, ^{
        instance = [[NSClassFromString(_className) alloc] init];
    });
    return instance;
}

@end

Now we can initialise the DataModelManagerFactory with the class name of our mock DataModelManager class, “MockDataModelManager”:

    [DataModelManagerFactory setClassName:@"MockDataModelManager"];
    id dataModelManager = [DataModelManagerFactory sharedInstance];
    [dataModelManager setup];

All we need do now is implement the MockDataModelManager class.

There’s refinements that could be made to this pattern, and improvements. For example, you could pass a Class type instead of a Class string. It’s the basis for a useful pattern.

You could argue that it’s not really a Singleton pattern at all, but that you are declaring a single access point (the sharedInstance factory class method) to an instance.

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