iOS13_onChange.swift

import SwiftUI
import Combine

public struct ChangeObserver<V: Equatable>: ViewModifier {
    public init(newValue: V, action: @escaping (V) -> Void) {
        self.newValue = newValue
        self.newAction = action
    }

    private typealias Action = (V) -> Void

    private let newValue: V
    private let newAction: Action

    @State private var state: (V, Action)?

    public func body(content: Content) -> some View {
        if #available(iOS 14, *) {
            assertionFailure("Please don't use this ViewModifer directly and use the `onChange(of:perform:)` modifier instead.")
        }
        return content
            .onAppear()
            .onReceive(Just(newValue)) { newValue in
                if let (currentValue, action) = state, newValue != currentValue {
                    action(newValue)
                }
                state = (newValue, newAction)
            }
    }
}

extension View {
    @_disfavoredOverload
    @ViewBuilder public func onChange<V>(of value: V, perform action: @escaping (V) -> Void) -> some View where V: Equatable {
        if #available(iOS 14, *) {
            onChange(of: value, perform: action)
        } else {
            modifier(ChangeObserver(newValue: value, action: action))
        }
    }
}

Ok I can see my issue. I thought it was a shadowed property but actually that wasn't it.

Essentially I've also ported the .task(id:) modifier on top of this. However it was there that the task id change was triggering correctly, however the value appeared to always be the previous value.

I've now identified its because in my TaskModifier implementation, I had passed my id in as a @State property which was a mistake. Removing that property wrapper/annotation now fixed my issue.

So the onChange implementation (both yours and mine) were not at all at fault. My mistake 👍

What is the purpose of the onAppear() on line 22?

Also i'm getting [SwiftUI] Modifying state during view update, this will cause undefined behavior. on line 27, which is fixable by adding .receive(on: DispatchQueue.main) operator to the Just on line 13

-> .onReceive(Just(newValue).receive(on: DispatchQueue.main)) { newValue in

+1 To what is the purpose of onAppear

@KoCMoHaBTa @kuanfajardo I don't remember well but it was probably a workaround to some bug.

This is such a beautiful solution...... wow......

Great solution! Saved me. Just one question: What's the idea of putting action into state? Seems, the action never changes within the struct lifecycle

@subtranix my backports library actually has some improvements and is far more battle tested against a lot more projects id suggest checking that out.

https://github.com/shaps80/SwiftUIBackports/blob/main/Sources/SwiftUIBackports/Shared/OnChange/OnChange.swift