Chapter 6: Singleton Pattern

大綱

• When should you use it?

• Playground example

• What should you be careful about?

• Tutorial project

• Key points

When should you use it?

• The singleton pattern restricts a class to only one instance.

  • This pattern is extremely common in iOS app development, as Apple makes extensive use of it.

• When should you use it?

  • Use the singleton pattern when having more than one instance of a class would cause problems, or when it just wouldn’t be logical.

  • Use the singleton plus pattern if a shared instance is useful most of the time, but you also want to allow custom instances to be created.

    • An example of this is FileManager, which handles everything to do with filesystem access. There is a “default” instance which is a singleton, or you can create your own. You would usually create your own if you’re using it on a background thread.”

Playground example

• Singleton pattern

let app = UIApplication.shared

public class MySingleton {
  static let shared = MySingleton()
  private init() { }
}

let mySingleton = MySingleton.shared

• Singleton Plus pattern

// MARK: - Singleton Plus
let defaultFileManager = FileManager.default
let customFileManager = FileManager()

public class MySingletonPlus {
  static let shared = MySingletonPlus()
  public init() { }
}

let singletonPlus = MySingletonPlus.shared
let singletonPlus2 = MySingletonPlus()

What should you be careful about?

• The singleton pattern is very easy to overuse.

• If you encounter a situation where you’re tempted to use a singleton, first consider other ways to accomplish your task.

  • For example, singletons are not appropriate if you’re simply trying to pass information from one view controller to another. Instead, consider passing models via an initializer or property.

• If you determine you actually do need a singleton, consider whether a singleton plus makes more sense.

  • Will having more than one instance cause problems?

  • Will it ever be useful to have custom instances?

  • Your answers will determine whether its better for you to use a true singleton or singleton plus.

Tutorial project

Key points

• The singleton pattern restricts a class to only one instance.

• The singleton plus pattern provides a “default” shared instance but also allows other instances to be created too.

• Be careful about overusing this pattern! Before you create a singleton, consider other ways to solve the problem without it. If a singleton really is best, prefer to use a singleton plus over a singleton.