Scroll SwiftUI List to new selection

Question

If you have a SwiftUI List with that allows single selection, you can change the selection by clicking the list (presumably this makes it the key responder) and then using the arrow keys. If that selection reaches the end of the visible area, it will scroll the whole list to keep the selection visible.

However, if the selection object is updated in some other way (e.g. using a button), the list will not be scrolled.

Is there any way to force the list to scroll to the new selection when set programmatically?

Example app:

import SwiftUI

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

    func changeSelection(_ by: Int) {
        switch self.selection {
        case .none:
            self.selection = 0
        case .some(let sel):
            self.selection = max(min(sel + by, 20), 0)
        }
    }

    var body: some View {
        HStack {
            List((0...20), selection: $selection) {
                Text(String($0))
            }
            VStack {
                Button(action: { self.changeSelection(-1) }) {
                    Text("Move Up")
                }
                Button(action: { self.changeSelection(1) }) {
                    Text("Move Down")
                }
            }
        }
    }
}

Answer 1

I tried several solutions, one of them I'm using in my project (I need horizontal paging for 3 lists). And here are my observations:

1. I didn't find any methods to scroll List in SwiftUI, there is no mention about it in documentation yet;
2. You may try ScrollView (my variant below, here is other solution), but these things might look monstroid;
3. Maybe the best way is to use UITableView: tutorial from Apple and try scrollToRowAtIndexPath method (like in this answer).

As I wrote, here is my example, which, of course, requires refinement. First of all ScrollView needs to be inside GeometryReader and you can understand the real size of content. The second thing is that you need to control your gestures, which might be difficult. And the last one: you need to calculate current offset of ScrollViews's content and it could be other than in my code (remember, I tried to give you example):

struct ScrollListView: View {

    @State var selection: Int?
    @State private var offset: CGFloat = 0
    @State private var isGestureActive: Bool = false

    func changeSelection(_ by: Int) {
        switch self.selection {
        case .none:
            self.selection = 0
        case .some(let sel):
            self.selection = max(min(sel + by, 30), 0)
        }
    }

    var body: some View {

        HStack {

            GeometryReader { geometry in

                VStack {
                    ScrollView(.vertical, showsIndicators: false) {

                        ForEach(0...29, id: \.self) { line in
                            ListRow(line: line, selection: self.$selection)
                                .frame(height: 20)
                        }

                        }
                    .content.offset(y: self.isGestureActive ? self.offset : geometry.size.height / 4 - CGFloat((self.selection ?? 0) * 20))
                    .gesture(DragGesture()
                        .onChanged({ value in
                            self.isGestureActive = true
                            self.offset = value.translation.width + -geometry.size.width * CGFloat(self.selection ?? 1)

                        })
                    .onEnded({ value in
                        DispatchQueue.main.async { self.isGestureActive = false }
                    }))

                }

            }


            VStack {
                Button(action: { self.changeSelection(-1) }) {
                    Text("Move Up")
                }
                Spacer()
                Button(action: { self.changeSelection(1) }) {
                    Text("Move Down")
                }
            }
        }
    }
}

of course you need to create your own "list row":

struct ListRow: View {

    @State var line: Int
    @Binding var selection: Int?

    var body: some View {

        HStack(alignment: .center, spacing: 2){

            Image(systemName: line == self.selection ? "checkmark.square" : "square")
                .padding(.horizontal, 3)
            Text(String(line))
            Spacer()

        }
        .onTapGesture {
            self.selection = self.selection == self.line ? nil : self.line
        }

    }

}

hope it'll be helpful.

Answer 2

In the new relase of SwiftUI for iOs 14 and MacOs Big Sur they added the ability to programmatically scroll to a specific cell using the new ScrollViewReader:

struct ContentView: View {
    let colors: [Color] = [.red, .green, .blue]

    var body: some View {
        ScrollView {
            ScrollViewReader { value in
                Button("Jump to #8") {
                    value.scrollTo(8)
                }

                ForEach(0..<10) { i in
                    Text("Example \(i)")
                        .frame(width: 300, height: 300)
                        .background(colors[i % colors.count])
                        .id(i)
                }
            }
        }
    }
}

Then you can use the method .scrollTo() like this

value.scrollTo(8, anchor: .top)

Credit: www.hackingwithswift.com