When to use lazy properties

Carson, great question!

I'm sure you read the Apple documentation, and noticed that it doesn't say anything about when not to use it.

"Lazy properties are useful when the initial value for a property is dependent on outside factors whose values are not known until after an instance’s initialization is complete. Lazy properties are also useful when the initial value for a property requires complex or computationally expensive setup that should not be performed unless or until it is needed."

https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Properties.html

I suppose we could conclude that we shouldn't use them except in cases like the ones mentioned.

I also found one commentator on lazy properties noting a possible problem: "Accessing the lazy property is a mutating operation because the property’s initial value is set on the first access. When a struct (which is a value type) contains a lazy property, any owner of the struct that accesses the lazy property must therefore declare the struct as a variable, too, because accessing the property means potentially mutating its container. "

His example is:

struct Image {
    lazy var metadata: [String:AnyObject] = {
        // Load image file and parse metadata, expensive
        // ...
        return ...
    }()
}

And he points out that this requires any user of the struct to create images as vars, not constants:

let image = Image()  //error

http://oleb.net/blog/2015/12/lazy-properties-in-structs-swift/

I couldn't find much else in the way of any warnings.

Another commentator seems in favor!

"As you can see, using lazy loading is easier than ever in Swift. Use it whenever possible, since it’ll save memory and make your code more efficient."

https://www.natashatherobot.com/swift-lazy/

Looks like "best practice" is still evolving! Your question may help that process.