Using Self in Swift

S## The Swift self keyword

Programmers familiar with other object-oriented programming languages may be in the habit of prefixing references to properties and methods with self to indicate that the method or property belongs to the current class instance. The Swift programming language also provides the self property type for this purpose and it is, therefore, perfectly valid to write code that reads as follows:

class MyClass {
    var myNumber = 1
 
    func addTen() {
        self.myNumber += 10
    }
}

In this context, the self prefix indicates to the compiler that the code is referring to a property named myNumber which belongs to the MyClass class instance. When programming in Swift, however, it is no longer necessary to use self in most situations since this is now assumed to be the default for references to properties and methods. “The Swift Programming Language” guide published by Apple says, “In practice you don’t need to write self in your code very often.” The function from the above example, therefore, can also be written as follows with the self reference omitted:

func addTen() {
    myNumber += 10
}

Using self in closure expressions

In most cases, the use of self is optional in Swift. With that being said, one situation where it is still necessary to use self is when referencing a property or method from within a closure expression. The use of self, for example, is mandatory in the following closure expression:

document?.openWithCompletionHandler({(success: Bool) -> Void in
    if success {
        self.ubiquityURL = resultURL
    }
})

It is also necessary to use self to resolve ambiguity, such as when a function parameter has the same name as a class property. In the following code, for example, the first print statement will output the value passed through to the function via the myNumber parameter, while the second print statement outputs the number assigned to the myNumber class property (in this case 10):