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

How to send state updates manually using objectWillChange

Paul Hudson    @twostraws   

Updated for Xcode 12.5

Although using @Published is the easiest way to control state updates, you can also do it by hand if you need something specific. For example, you might want the view to refresh only if you’re happy with the values you’ve been given.

All observable objects automatically get access to an objectWillChange property, which itself has a send() method we can call whenever we want observing views to refresh.

For example:

// Create an observable object class that announces 
// changes to its only property
class UserAuthentication: ObservableObject {
    var username = "Taylor" {
        willSet {
            objectWillChange.send()
        }
    }
}

struct ContentView: View {
    // Create an instance of our object
    @StateObject var user = UserAuthentication()

    var body: some View {
        VStack(alignment: .leading) {
            TextField("Enter your name", text: $user.username)
            Text("Your username is: \(user.username)")
        }
    }
}

Notice how we have a willSet property observer attached to the username property of UserAuthentication, allowing us to can run code whenever that value changes. In our example code, we call objectWillChange.send() whenever username changes, which is what tells the objectWillChange publisher to put out the news that our data has changed so that any subscribed views can refresh.

Tip: This example is no different from using @Published on the property, but now that we have a custom call to objectWillChange.send() we can add extra functionality – we could save the value to disk, for example.

Hacking with Swift is sponsored by ViRE

SPONSORED ViRE offers discoverable way of working with regex. It provides really readable regex experience, code complete & cheat sheet, unit tests, powerful replace system, step-by-step search & replace, regex visual scheme, regex history & playground. ViRE is available on Mac & iPad.

Download on the App Store

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

Similar solutions…

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

 
Unknown user

You are not logged in

Log in or create account
 

Link copied to your pasteboard.