In iOS 17, Apple deprecated the onChange(of:perform) view modifier replacing it with two new variations.

Pre-iOS 17 onChange

Before iOS 17, you use the onChange(of:perform:) view modifier to run an action when a property of a view changes. For example, here’s a list view that scrolls to the last item each time the count of items in the list changes:

struct ListView: View {
  @EnvironmentObject private var store: ItemStore

  var count: Int {
    store.itemCount
  }

  var body: some View {
    ScrollViewReader { proxy in
      List {
        ForEach(store.items) { item in
          NavigationLink(value: item) { ItemRow(item: item) }
        }
      }
      .onChange(of: count) { newCount in
        if let lastID = store.lastItemID() {
          withAnimation {
            proxy.scrollTo(lastID)
          }
        }
      }
    }
  }
}

The closure has a single argument for the new value of the count. If I only want to scroll when the count increases I need to capture the value before the state changes:

.onChange(of: count) { [count] newCount in
  if newCount > count,
   let lastID = store.lastItemID {
    withAnimation {
      proxy.scrollTo(lastID)
    }
  }
}

Note: The closure captures the value before the change.

onChange in iOS 17

Once you update the minimum deployment target of your app to iOS 17 Xcode shows a deprecation warning for any use of the old onChange handler:

You have two choices depending on whether you want the value that’s changed or not. For situations where you don’t care about the value use the closure with zero arguments:

.onChange(of: count) {
  if let lastID = store.lastItemID {
    withAnimation {
      proxy.scrollTo(lastID)
    }
  }
}

The alternate version gives you both the old and new values of the tracked value:

.onChange(of: count, initial: true) { oldCount, newCount in
  if newCount > oldCount,
   let lastID = store.lastItemID {
    withAnimation {
      proxy.scrollTo(lastID)
    }
  }
}

Initial Action

Both these new methods have a boolean initial property that defaults to false. When true the action runs when the view initially appears. For example, if I want to scroll to the end of the list when the view appears:

.onChange(of: count, initial: true) { oldCount, newCount in
  if newCount >= oldCount,
   let lastID = store.lastItemID {
    withAnimation {
      proxy.scrollTo(lastID)
    }
  }
}

Note that on the initial run the old and new values are the same.