NEW: Start my new Ultimate Portfolio App course with a free Hacking with Swift+ trial! >>

Property observers

Property observers let you run code before or after any property changes. To demonstrate this, we’ll write a Progress struct that tracks a task and a completion percentage:

struct Progress {
    var task: String
    var amount: Int
}

We can now create an instance of that struct and adjust its progress over time:

var progress = Progress(task: "Loading data", amount: 0)
progress.amount = 30
progress.amount = 80
progress.amount = 100

What we want to happen is for Swift to print a message every time amount changes, and we can use a didSet property observer for that. This will run some code every time amount changes:

struct Progress {
    var task: String
    var amount: Int {
        didSet {
            print("\(task) is now \(amount)% complete")
        }
    }
}

You can also use willSet to take action before a property changes, but that is rarely used.

Subscribe to Hacking with Swift+

Sponsor Hacking with Swift and reach the world's largest Swift community!

BUY OUR BOOKS
Buy Pro Swift Buy Swift Design Patterns Buy Testing Swift Buy Hacking with iOS Buy Swift Coding Challenges Buy Swift on Sundays Volume One Buy Server-Side Swift (Vapor Edition) Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Advanced iOS Volume Three Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with macOS Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Server-Side Swift (Kitura Edition) Buy Beyond Code

Was this page useful? Let us know!

Average rating: 4.7/5

Link copied to your pasteboard.