In Swift, the lazy keyword changes a property so that it is only calculated when it's accessed. This not only makes it useful for performance reasons, it's also incredibly useful for initializing your class when a non-optional property depends on another property for initialization.

Let's look at an example:

class C {
    var x: UIView
    var y: UIView = {
        var v = UIView()
        x.addSubview(v)
        return v
    }()

    init() {
        x = UIView()
    }
}

This class definition compiles with an error because the value of x is used when computing the value of y:

error: instance member 'x' cannot be used on type 'C'

This wouldn't work even if you change the definition of x to:

var x = UIView()

There's an easy fix — just change y to be lazy:

class C {
    var x: UIView
    lazy var y: UIView = {
        var v = UIView()
        x.addSubview(v)
        return v
    }()

    init() {
        x = UIView()
    }
}

Because lazy properties are only calculated when they are first used — and you don't use them in your initializer before x is initialized, this is a very simple fix.

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