Lazy stored property in Swift

Question

I am having some confusion in lazy stored property. I read many tutorials and found this, but I could not understand this in real scenario. Can anyone please clear out few things,

A lazy stored property is a property whose initial value is not calculated until the first time it is used...
You must always declare a lazy property as a variable (with the var keyword), because its initial value may not be retrieved until after instance initialization completes...
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.
Where should we use lazy stored property

Thanks

What is the issue you are facing with lazy stored properties? These statements are very clear in their meaning. — PGDev
– PGDev, Commented May 29, 2017 at 10:29

michip96 · Accepted Answer · 2017-05-29 10:55:00Z

4

Mike Buss wrote an article about lazy initialization in Swift http://mikebuss.com/2014/06/22/lazy-initialization-swift/

One example of when to use lazy initialization is when the initial value for a property is not known until after the object is initialized or when the computation of the property is computationally expensive.

Here are two examples for both cases from the post:

In the first example we don't know which value personalized greeting should have. We need to wait until the object is initialized to know the correct greeting for this person.

class Person {

var name: String

lazy var personalizedGreeting: String = {
    [unowned self] in
    return "Hello, \(self.name)!"
    }()

init(name: String) {
        self.name = name
    }
}

The second example covers the case of an expensive computation. Imagine a MathHelper class which should give you values for pi and other important constants. You don't need to calculate all constants if you only use a subset of them.

class MathHelper {

lazy var pi: Double = {
    // Calculate pi to a crazy number of digits
    return resultOfCalculation
    }()

}

answered May 29, 2017 at 10:55

michip96

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2 Comments

Kautsya Kanu Over a year ago

How to change personalizedGreeting If I change name?? In same code..

michip96 Over a year ago

You may want to take a look at this question. You have multiple ways to trigger a reinitialization of your lazy attributes.

Shubham Singh · Accepted Answer · 2021-06-03 21:34:26Z

4

Explained well!! I want to add a simple example to understand it in a better way.

Lazy property is a stored property whose memory will be allocated only when the variable is actually used.

class Car{
    lazy var brand: String = "BMW" // Memory not allocated
}

let aCar = Car()
print(aCar.brand) // Memory allocated as it is getting used here

answered Jun 3, 2021 at 21:34

Shubham Singh

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