Additionally, singletons can be unit tested by using dependency injection. #SWIFT SHARE SINGLETON THROUGHOUT PROJECT CODE#A global variable in your code can’t be easily subclassed, so that’s why you use a singleton class. Use a singleton when your code requires no more than one instance of a class (i.e., the CEO in the company)Īnd when it must be accessible from anywhere in your code (i.e., the file system)Īnother use case is subclassing. That’s complex, but what it boils down to is: When the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code There must be exactly one instance of a class, and it must be accessible to clients from a well-known access point #SWIFT SHARE SINGLETON THROUGHOUT PROJECT SOFTWARE#When do you use singletons? The book Design Patterns: Elements Of Reusable Object-Oriented Software by the Gang of Four has the following to say. Managing state poorly is the single biggest reason for singleton misuse. The API class can switch between two states:Ī state in which isRequestPending is falseĪ state in which isRequestPending is trueĪs you’ll learn in the next section, state and singletons can wreak all sorts of havoc on your code. You can switch from one state to the other. You can see “state” as a feeling: you’re either happy or you’re sad, or you’re angry, and so on. The API class now manages something called state. There’s something else we need to discuss. The isRequestPending boolean is set to false again, and the request data is processed.Īnd here’s how we can use the API singleton anywhere in our code: online data has been downloaded into the app. This function is invoked when the API request returns, i.e. The API class also has an onReturnAPIRequest() function. In the function, you can see that a request can only be made when no other request is currently pending. Imagine that we can use this function to get some data back from a webservice API, like Twitter’s. The API class also has a makeAPIRequest() function. This is where the danger begins… See how the isRequestPending boolean ensures that only one API request can be done at a time? (Note that isRequestPending is an instance property.) The API class now has an isRequestPending property. It’s still a singleton, and it still uses those static let shared = API() and private init() bits of code. After all, you can’t create more of it.Īnd now we’ve ensured the API class conforms to the two attributes of a singleton: class API In other words, you cannot create an instance of API outside of the API class! This ensures that the API object we’ve created is the only instance in our code. This private keyword ensures that the API class can only be initialized within the API class. The class initializer init() is marked with private. We’re sort of creating an API object that can be accessed through the API class. What’s interesting is that the shared property initializes an instance of API within the API class. Compare this to a normal instance property, which can only be accessed via an instance of a class. That means we can access the shared property via the class API.
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