What are lazy variables?

Swift version: 5.6

Paul Hudson @twostraws October 7th 2021

It's very common in iOS to want to create complex objects only when you need them, largely because with limited computing power at your disposal you need to avoid doing expensive work unless it's really needed.

Swift has a mechanism built right into the language that enables just-in-time calculation of expensive work, and it is called a lazy variable. These variables are created using a function you specify only when that variable is first requested. If it's never requested, the function is never run, so it does help save processing time.

I don't want to produce a complicated example because that would rather defy the point, so instead I've built a simple (if silly!) one: imagine you want to calculate a person's age using the Fibonacci sequence. This sequence goes 0, 1, 1, 2, 3, 5, 8, 13, 21, and so on – each number is calculated by adding the previous two numbers in the sequence. So if someone was aged 8, their Fibonacci sequence age would be 21, because that's at position 8 in the sequence.

I chose this because the most common pedagogical way to teach the Fibonacci sequence is using a function like this one:

func fibonacci(of num: Int) -> Int {
    if num < 2 {
        return num
    } else {
        return fibonacci(of: num - 1) + fibonacci(of: num - 2)
    }
}

That function calls itself, which makes it a recursive function, and actually it's quite slow. If you try to calculate the Fibonacci value of something over, say, 21, expect it to be slow in a playground!

Anyway, we want to create a Person struct that has an age property and a fibonacciAge property, but we don't want that second one to be evaluated unless it's actually used. So, create this struct now:

struct Person {
    var age = 16

    lazy var fibonacciOfAge: Int = {
        fibonacci(of: self.age)
    }()

    func fibonacci(of num: Int) -> Int {
        if num < 2 {
            return num
        } else {
            return fibonacci(of: num - 1) + fibonacci(of: num - 2)
        }
    }
}

There are five important things to note in that code:

The lazy property is marked as lazy var. You can't make it lazy let because lazy properties must always be variables.
Because the actual value is created by evaluation, you need to declare its data type up front. In the case of the code above, that means declaring the property as Int.
Once you've set your data type, you need to use an open brace ("{") to start your block of code, then "}" to finish.
You need to use self inside the function. You don’t need to use [weak self] or similar, because the closure is immediately applied and therefore won’t cause a retain cycle.
You need to end your lazy property with (), because what you're actually doing is making a call to the function you just created.

Once that code is written, you can use it like this:

var singer = Person()
print(singer.fibonacciOfAge)

Remember, the point of lazy properties is that they are computed only when they are first needed, after which their value is saved. This means if you create 1000 singers and never touch their fibonacciOfAge property, your code will be lightning fast because that lazy work is never done.