Singleton Pattern in Swift

A singleton object ensures that no matter how many times you create or retrieve an instance of a class, you always get the same object. Whether dealing with audio effects or network utilities, singletons give your app a unified way to access shared resources or services.

Singleton in Objective-C

In OC, singletons are implemented by calling the initialization method inside a dispatch_once block.

+ (instancetype)sharedInstance {
    static id _sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _sharedInstance = [[self alloc] init];
    });

    return _sharedInstance;
}

Singleton in Swift

In Swift, singletons are implemented using a lazily initialized static class property (which is guaranteed to be initialized only once). This is thread-safe even under concurrent access.

class Singleton {
    static let sharedInstance = Singleton()
}

If you need additional setup during initialization, you can use a closure:

class Singleton {
    static let sharedInstance: Singleton = {
        let instance = Singleton()
        // setup code
        return instance
    }()
}

This section is largely translated from Apple's official documentation:

Singleton