What is the use of “static” keyword if “let” keywo

Question

I'm little bit confused about using static keyword in swift. As we know swift introduces let keyword to declare immutable objects. Like declaring the id of a table view cell which most likely won't change during its lifetime. Now what is the use of static keyword in some declaration of struct like:

struct classConstants
{
    static let test = "test"
    static var totalCount = 0
}

whereas let keyword do the same.In Objective C we used static to declare some constant like

static NSString *cellIdentifier=@"cellId";

Besides which makes me more curious is the use of static keyword along with let and also var keyword. Can anybody explain me where to use this static keyword? More importantly do we really need static in swift?

Answer 1

to see the difference between type properties and / or methods and class properties and / or methods, please look at this self explanatory example from apple docs

class SomeClass {
    static var storedTypeProperty = "Some value."
    static var computedTypeProperty: Int {
        return 27
    }
    class var overrideableComputedTypeProperty: Int {
        return 107
    }
}

Static properties may only be declared on type, not globally. In other words static property === type property in Swift. To declare type property you have to use static keyword.

Answer 2

I will break them down for you:

• var : used to create a variable
• let : used to create a constant
• static : used to create type properties with either let or var. These are shared between all objects of a class.

Now you can combine to get the desired out come:

• static let key = "API_KEY" : type property that is constant
• static var cnt = 0 : type property that is a variable
• let id = 0 : constant (can be assigned only once, but can be assigned at run time)
• var price = 0 : variable

So to sum everything up var and let define mutability while static and lack of define scope. You might use static var to keep track of how many instances you have created, while you might want to use just varfor a price that is different from object to object. Hope this clears things up a bit.

Example Code:

class MyClass{
    static let typeProperty = "API_KEY"
    static var instancesOfMyClass = 0
    var price = 9.99
    let id = 5

}

let obj = MyClass()
obj.price // 9.99
obj.id // 5

MyClass.typeProperty // "API_KEY"
MyClass.instancesOfMyClass // 0

Answer 3

"The let keyword defines a constant" is confusing for beginners who are coming from C# background (like me). In C# terms, you can think of "let" as "readonly" variable.

(answer to How exactly does the “let” keyword work in Swift?)

Use both static and let to define constant

public static let pi = 3.1416            // swift

public const double pi = 3.1416;         // C#
public static final double pi = 3.1416   // Java     

Whenever I use let to define constant, it feels like I am using readonly of C#. So, I use both static and let to define constant in swift.

Answer 4

A static variable is shared through all instances of a class. Throw this example in playground:

class Vehicle {
    var car = "Lexus"
    static var suv = "Jeep"
}

// changing nonstatic variable
Vehicle().car // Lexus
Vehicle().car = "Mercedes"
Vehicle().car // Lexus

// changing static variable
Vehicle.suv // Jeep
Vehicle.suv = "Hummer"
Vehicle.suv // Hummer

When you change the variable for the static property, that property is now changed in all future instances.

Answer 5

Static Variables are belong to a type rather than to instance of class. You can access the static variable by using the full name of the type.

Code:

class IOS {

  var iosStoredTypeProperty = "iOS Developer"

  static var swiftStoredTypeProperty = "Swift Developer"

 }

 //Access the iosStoredTypeProperty by way of creating instance of IOS Class

let iOSObj = IOS()

print(iOSObj.iosStoredTypeProperty)  // iOS Developer


 //print(iOSObj.swiftStoredTypeProperty) 
 //Xcode shows the error 
 //"static member 'swiftStoredTypeProperty' cannot be used on instance of type IOS”


 //You can access the static property by using full name of the type
print(IOS.swiftStoredTypeProperty)  // Swift Developer

Hope this helps you..