What is the advantage of closure stored property Initialisation?

Question

What is the difference and the advantages/disadvantages of this code when initialising a property of a class as:

1.

let menuBar:MenuBar = {
        let mb = MenuBar()
        return mb
    }()

and:

2.

let menuBar = MenuBar()

Answer 1

Both of the code snippets declare and initialize stored properties, but in the first one it is initialized by closure. The reason why you should set a stored property with a closure is: there is a requirement(s) to do customization (calling a method for instance); Adapted from The Swift Programming Language (Swift 4.1) - Initialization: Setting a Default Property Value with a Closure or Function:

If a stored property’s default value requires some customization or setup, you can use a closure or global function to provide a customized default value for that property. Whenever a new instance of the type that the property belongs to is initialized, the closure or function is called, and its return value is assigned as the property’s default value.

Which means that you would be able to do:

let menuBar:MenuBar = {
    let mb = MenuBar()
    // for example, you'd need to call "doSomething" method
    // before returning the instance:
    menuBar.doSomething()
    return mb
}()

Note that in the body of the stored property closure, you would be not able to able to use the other properties in your class/struct since they considered as not being initialized yet. Example:

struct MyType {
    let myString = "My String!"
    let myInt: Int = {
        let anInt = 101

        // this won't work
        print(myString)

        return anInt
    }()
}

The result of the above code snippet is getting a compile-time error:

error: instance member 'myString' cannot be used on type 'MyType' print(myString)

---

Furthermore at some point, it would be recommended to declare your property as lazy:

lazy var menuBar:MenuBar = {
     let mb = MenuBar()
     // for example, you'd need to call "doSomething" method
     // before returning the instance:
     menuBar.doSomething()
     return mb
 }()

Means that:

A lazy stored property is a property whose initial value is not calculated until the first time it is used. You indicate a lazy stored property by writing the lazy modifier before its declaration.