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How to use local variable observers

Swift version: 5.1

Paul Hudson    @twostraws   

You should already be familiar with the concept of property observers in Swift – those willSet and didSet blocks you can attach to property on classes and structs. Well, those same blocks can be attached to local and global variables as well, allowing you to respond to changes easily.

The syntax is identical: create your variable, give it an initial value, then provide willSet and/or didSet closures inside braces, like this:

var name = "Taylor Swift" {
    didSet {
        print("Name changed to \(name)!")
    }
}

name = "Justin Bieber"

That will print “Name changed to Justin Bieber!” when run.

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This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.

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