Lazy property initialization in Swift

Question

How would you implement the following pattern in Swift?

The Container class is initialized with a JSON array that contains dictionaries. These dictionar

Answer 1

You can use Lazy Stored Properties in Swift to implement the Lazy Instantiation pattern. This is done by adding the @lazy attribute before the declaration of the stored property.

There are two things to keep in mind:

• Lazy properties must be initialized when declared
• Lazy properties can only be used on members of a struct or a class (hence why we need to use a DataManager)

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Here's some code you can throw into a Playground to see how the @lazy attribute works

// initialize your lazily instantiated data
func initLazyData() -> String[] {
    return ["lazy data"]
}

// a class to manage the lazy data (along with any other data you want)
class DataManager {
    @lazy var lazyData = initLazyData()

    var otherData = "Other data"
}

// when we create this object, the "lazy data" array is not initialized
let manager = DataManager()

// even if we access another property, the "lazy data" array stays nil
manager.otherData += ", more data"
manager

// as soon as we access the "lazy data" array, it gets created
manager.lazyData
manager

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For more information, you can check out the Lazy Stored Properties section on the Properties page of the Swift Programming Language Guide. Note that that link is to pre-release documentation.