NEW: Nominations are now open for the 2019 Swift Community Awards! >>

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.

SAVE 20% ON iOS CONF SG The largest iOS conference in Southeast Asia is back in Singapore for the 5th time in January 2020, now with two days of workshops plus two days of talks on SwiftUI, Combine, GraphQL, and more! Save a massive 20% on your tickets by clicking on this link.

MASTER SWIFT NOW
Buy Testing Swift Buy Practical iOS 12 Buy Pro Swift Buy Swift Design Patterns Buy Swift Coding Challenges Buy Server-Side Swift (Vapor Edition) Buy Server-Side Swift (Kitura Edition) Buy Hacking with macOS Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with Swift Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let us know!

Average rating: 4.3/5