How can I run an action when a state changes?

enum SectionType: String, CaseIterable {
    case top = "Top"
    case best = "Best"
}

struct ContentView : View {
    @State private var selection: Int = 0

    var body: some View {
        SegmentedControl(selection: $selection) {
            ForEach(SectionType.allCases.identified(by: \.self)) { type in
                Text(type.rawValue).tag(type)
            }
        }
    }
}

How do I run code (e.g print("Selection changed to \(selection)") when the $selection state changes? I looked through the docs and I couldn't find anything.

iOS 14.0+

You can use the onChange(of:perform:) modifier, like so:

struct ContentView: View {
    
    @State private var isLightOn = false

    var body: some View {
        Toggle("Light", isOn: $isLightOn)
            .onChange(of: isLightOn) { value in
                if value {
                    print("Light is now on!")
                } else {
                    print("Light is now off.")
                }
            }
    }
}

iOS 13.0+

The following as an extension of Binding, so you can execute a closure whenever the value changes.

extension Binding {
    
    /// When the `Binding`'s `wrappedValue` changes, the given closure is executed.
    /// - Parameter closure: Chunk of code to execute whenever the value changes.
    /// - Returns: New `Binding`.
    func onUpdate(_ closure: @escaping () -> Void) -> Binding<Value> {
        Binding(get: {
            wrappedValue
        }, set: { newValue in
            wrappedValue = newValue
            closure()
        })
    }
}

Used like so for example:

struct ContentView: View {
    
    @State private var isLightOn = false
    
    var body: some View {
        Toggle("Light", isOn: $isLightOn.onUpdate(printInfo))
    }
    
    private func printInfo() {
        if isLightOn {
            print("Light is now on!")
        } else {
            print("Light is now off.")
        }
    }
}

^{This example doesn't need to use a separate function. You only need a closure.}

You can't use didSet observer on @State but you can on an ObservableObject property.

import SwiftUI
import Combine

final class SelectionStore: ObservableObject {
    var selection: SectionType = .top {
        didSet {
            print("Selection changed to \(selection)")
        }
    }

    // @Published var items = ["Jane Doe", "John Doe", "Bob"]
}

Then use it like this:

import SwiftUI

enum SectionType: String, CaseIterable {
    case top = "Top"
    case best = "Best"
}

struct ContentView : View {
    @ObservedObject var store = SelectionStore()

    var body: some View {
        List {
            Picker("Selection", selection: $store.selection) {
                ForEach(FeedType.allCases, id: \.self) { type in
                    Text(type.rawValue).tag(type)
                }
            }.pickerStyle(SegmentedPickerStyle())

            // ForEach(store.items) { item in
            //     Text(item)
            // }
        }
    }
}

Here is another option if you have a component that updates a @Binding. Rather than doing this:

Component(selectedValue: self.$item, ...)

you can do this and have a little greater control:

Component(selectedValue: Binding(
    get: { self.item },
    set: { (newValue) in
              self.item = newValue
              // now do whatever you need to do once this has changed
    }), ... )

This way you get the benefits of the binding along with the detection of when the Component has changed the value.

SwiftUI 1 & 2 (iOS 13 & 14)

You can use onReceive:

import Combine
import SwiftUI

struct ContentView: View {
    @State private var selection = false

    var body: some View {
        Toggle("Selection", isOn: $selection)
            .onReceive(Just(selection)) { selection in
                // print(selection)
            }
    }
}

In iOS 14 there is now a onChange modifier you can use like so:

SegmentedControl(selection: $selection) {
    ForEach(SectionType.allCases.identified(by: \.self)) { type in
        Text(type.rawValue).tag(type)
    }
}
.onChange(of: selection) { value in
    print("Selection changed to \(selection)")
}