Creating a singleton for my Core Data default stack in Swift

Question

I am trying to use a singleton for Core Data. Previously, I've been successfully able to do it by creating a class CoreDataStack.h/.m, calling the default stack method below, and its respective managed object context, in Objective-C, and works very well:

//RETURNS CoreDataStack
+ (instancetype)defaultStack {
    static CoreDataStack *defaultStack;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        defaultStack = [[self alloc]init];
    });
    
    return defaultStack;
}

However, I am using a Swift project, and I've been struggling to convert this into the latest Swift syntax. How would I go about creating this? This is my attempt so far:

class func defaultStack() -> Self {
    var defaultStack: CoreDataStack
    var onceToken: dispatch_once_t = 0
    dispatch_once(&onceToken) {
       defaultStack = self.init()
    }
    return defaultStack
}

and my Xcode generated error:

Answer 1

To create a singleton, use Krakendev's single-line singleton code:

class CoreDataStack {

    // Here you declare all your properties

    static let sharedInstance = User()

    private init() {
        // If you have something to do at the initialization stage
        // you can add it here. It will only be called once. Guaranteed.
    }

    // Add the rest of your methods here

}

You will call your methods and properties as CoreDataStack.sharedInstance().property and CoreDataStack.sharedInstance().method(). I recommend using something shorter instead of sharedInstance, like service.

This solution applies in general, not only in your Core Data case.