Programatically scroll to SwiftUI list position? [duplicate] - list

Question

It looks like in current tools/system, just released Xcode 11.4 / iOS 13.4, there will be no SwiftUI-native support for "scroll-to" feature in List. So even if they, Apple, will provide it in next major released, I will need backward support for iOS 13.x.
So how would I do it in most simple & light way?
scroll List to end
scroll List to top
and others
(I don't like wrapping full UITableView infrastructure into UIViewRepresentable/UIViewControllerRepresentable as was proposed earlier on SO).

Answer 1

SWIFTUI 2.0
Here is possible alternate solution in Xcode 12 / iOS 14 (SwiftUI 2.0) that can be used in same scenario when controls for scrolling is outside of scrolling area (because SwiftUI2 ScrollViewReader can be used only inside ScrollView)
Note: Row content design is out of consideration scope
Tested with Xcode 12b / iOS 14
class ScrollToModel: ObservableObject {
enum Action {
case end
case top
}
#Published var direction: Action? = nil
}
struct ContentView: View {
#StateObject var vm = ScrollToModel()
let items = (0..<200).map { $0 }
var body: some View {
VStack {
HStack {
Button(action: { vm.direction = .top }) { // < here
Image(systemName: "arrow.up.to.line")
.padding(.horizontal)
}
Button(action: { vm.direction = .end }) { // << here
Image(systemName: "arrow.down.to.line")
.padding(.horizontal)
}
}
Divider()
ScrollViewReader { sp in
ScrollView {
LazyVStack {
ForEach(items, id: \.self) { item in
VStack(alignment: .leading) {
Text("Item \(item)").id(item)
Divider()
}.frame(maxWidth: .infinity).padding(.horizontal)
}
}.onReceive(vm.$direction) { action in
guard !items.isEmpty else { return }
withAnimation {
switch action {
case .top:
sp.scrollTo(items.first!, anchor: .top)
case .end:
sp.scrollTo(items.last!, anchor: .bottom)
default:
return
}
}
}
}
}
}
}
}
SWIFTUI 1.0+
Here is simplified variant of approach that works, looks appropriate, and takes a couple of screens code.
Tested with Xcode 11.2+ / iOS 13.2+ (also with Xcode 12b / iOS 14)
Demo of usage:
struct ContentView: View {
private let scrollingProxy = ListScrollingProxy() // proxy helper
var body: some View {
VStack {
HStack {
Button(action: { self.scrollingProxy.scrollTo(.top) }) { // < here
Image(systemName: "arrow.up.to.line")
.padding(.horizontal)
}
Button(action: { self.scrollingProxy.scrollTo(.end) }) { // << here
Image(systemName: "arrow.down.to.line")
.padding(.horizontal)
}
}
Divider()
List {
ForEach(0 ..< 200) { i in
Text("Item \(i)")
.background(
ListScrollingHelper(proxy: self.scrollingProxy) // injection
)
}
}
}
}
}
Solution:
Light view representable being injected into List gives access to UIKit's view hierarchy. As List reuses rows there are no more values then fit rows into screen.
struct ListScrollingHelper: UIViewRepresentable {
let proxy: ListScrollingProxy // reference type
func makeUIView(context: Context) -> UIView {
return UIView() // managed by SwiftUI, no overloads
}
func updateUIView(_ uiView: UIView, context: Context) {
proxy.catchScrollView(for: uiView) // here UIView is in view hierarchy
}
}
Simple proxy that finds enclosing UIScrollView (needed to do once) and then redirects needed "scroll-to" actions to that stored scrollview
class ListScrollingProxy {
enum Action {
case end
case top
case point(point: CGPoint) // << bonus !!
}
private var scrollView: UIScrollView?
func catchScrollView(for view: UIView) {
if nil == scrollView {
scrollView = view.enclosingScrollView()
}
}
func scrollTo(_ action: Action) {
if let scroller = scrollView {
var rect = CGRect(origin: .zero, size: CGSize(width: 1, height: 1))
switch action {
case .end:
rect.origin.y = scroller.contentSize.height +
scroller.contentInset.bottom + scroller.contentInset.top - 1
case .point(let point):
rect.origin.y = point.y
default: {
// default goes to top
}()
}
scroller.scrollRectToVisible(rect, animated: true)
}
}
}
extension UIView {
func enclosingScrollView() -> UIScrollView? {
var next: UIView? = self
repeat {
next = next?.superview
if let scrollview = next as? UIScrollView {
return scrollview
}
} while next != nil
return nil
}
}

Answer 2

Just scroll to the id:
scrollView.scrollTo(ROW-ID)
Since SwiftUI structured designed Data-Driven, You should know all of your items IDs. So you can scroll to any id with ScrollViewReader from iOS 14 and with Xcode 12
struct ContentView: View {
let items = (1...100)
var body: some View {
ScrollViewReader { scrollProxy in
ScrollView {
ForEach(items, id: \.self) { Text("\($0)"); Divider() }
}
HStack {
Button("First!") { withAnimation { scrollProxy.scrollTo(items.first!) } }
Button("Any!") { withAnimation { scrollProxy.scrollTo(50) } }
Button("Last!") { withAnimation { scrollProxy.scrollTo(items.last!) } }
}
}
}
}
Note that ScrollViewReader should support all scrollable content, but now it only supports ScrollView
Preview

Answer 3

Preferred way
This answer is getting more attention, but I should state that the ScrollViewReader is the right way to do this. The introspect way is only if the reader/proxy doesn't work for you, because of a version restrictions.
ScrollViewReader { proxy in
ScrollView(.vertical) {
TopView().id("TopConstant")
...
MiddleView().id("MiddleConstant")
...
Button("Go to top") {
proxy.scrollTo("TopConstant", anchor: .top)
}
.id("BottomConstant")
}
.onAppear{
proxy.scrollTo("MiddleConstant")
}
.onChange(of: viewModel.someProperty) { _ in
proxy.scrollTo("BottomConstant")
}
}
The strings should be defined in one place, outside of the body property.
Legacy answer
Here is a simple solution that works on iOS13&14:
Using Introspect.
My case was for initial scroll position.
ScrollView(.vertical, showsIndicators: false, content: {
...
})
.introspectScrollView(customize: { scrollView in
scrollView.scrollRectToVisible(CGRect(x: 0, y: offset, width: 100, height: 300), animated: false)
})
If needed the height may be calculated from the screen size or the element itself.
This solution is for Vertical scroll. For horizontal you should specify x and leave y as 0

Answer 4

Thanks Asperi, great tip. I needed to have a List scroll up when new entries where added outside the view. Reworked to suit macOS.
I took the state/proxy variable to an environmental object and used this outside the view to force the scroll. I found I had to update it twice, the 2nd time with a .5sec delay to get the best result. The first update prevents the view from scrolling back to the top as the row is added. The 2nd update scrolls to the last row. I'm a novice and this is my first stackoverflow post :o
Updated for MacOS:
struct ListScrollingHelper: NSViewRepresentable {
let proxy: ListScrollingProxy // reference type
func makeNSView(context: Context) -> NSView {
return NSView() // managed by SwiftUI, no overloads
}
func updateNSView(_ nsView: NSView, context: Context) {
proxy.catchScrollView(for: nsView) // here NSView is in view hierarchy
}
}
class ListScrollingProxy {
//updated for mac osx
enum Action {
case end
case top
case point(point: CGPoint) // << bonus !!
}
private var scrollView: NSScrollView?
func catchScrollView(for view: NSView) {
//if nil == scrollView { //unB - seems to lose original view when list is emptied
scrollView = view.enclosingScrollView()
//}
}
func scrollTo(_ action: Action) {
if let scroller = scrollView {
var rect = CGRect(origin: .zero, size: CGSize(width: 1, height: 1))
switch action {
case .end:
rect.origin.y = scroller.contentView.frame.minY
if let documentHeight = scroller.documentView?.frame.height {
rect.origin.y = documentHeight - scroller.contentSize.height
}
case .point(let point):
rect.origin.y = point.y
default: {
// default goes to top
}()
}
//tried animations without success :(
scroller.contentView.scroll(to: NSPoint(x: rect.minX, y: rect.minY))
scroller.reflectScrolledClipView(scroller.contentView)
}
}
}
extension NSView {
func enclosingScrollView() -> NSScrollView? {
var next: NSView? = self
repeat {
next = next?.superview
if let scrollview = next as? NSScrollView {
return scrollview
}
} while next != nil
return nil
}
}

Answer 5

my two cents for deleting and repositioning list at any point based on other logic.. i.e. after delete/update, for example going to top.
(this is a ultra-reduced sample, I used this code after network call back to reposition: after network call I change previousIndex )
struct ContentView: View {
#State private var previousIndex : Int? = nil
#State private var items = Array(0...100)
func removeRows(at offsets: IndexSet) {
items.remove(atOffsets: offsets)
self.previousIndex = offsets.first
}
var body: some View {
ScrollViewReader { (proxy: ScrollViewProxy) in
List{
ForEach(items, id: \.self) { Text("\($0)")
}.onDelete(perform: removeRows)
}.onChange(of: previousIndex) { (e: Equatable) in
proxy.scrollTo(previousIndex!-4, anchor: .top)
//proxy.scrollTo(0, anchor: .top) // will display 1st cell
}
}
}
}

Answer 6

This can now be simplified with all new ScrollViewProxy in Xcode 12, like so:
struct ContentView: View {
let itemCount: Int = 100
var body: some View {
ScrollViewReader { value in
VStack {
Button("Scroll to top") {
value.scrollTo(0)
}
Button("Scroll to buttom") {
value.scrollTo(itemCount-1)
}
ScrollView {
LazyVStack {
ForEach(0 ..< itemCount) { i in
Text("Item \(i)")
.frame(height: 50)
.id(i)
}
}
}
}
}
}
}

Answer 7

MacOS 11: In case you need to scroll a list based on input outside the view hierarchy. I have followed the original scroll proxy pattern using the new scrollViewReader:
struct ScrollingHelperInjection: NSViewRepresentable {
let proxy: ScrollViewProxy
let helper: ScrollingHelper
func makeNSView(context: Context) -> NSView {
return NSView()
}
func updateNSView(_ nsView: NSView, context: Context) {
helper.catchProxy(for: proxy)
}
}
final class ScrollingHelper {
//updated for mac os v11
private var proxy: ScrollViewProxy?
func catchProxy(for proxy: ScrollViewProxy) {
self.proxy = proxy
}
func scrollTo(_ point: Int) {
if let scroller = proxy {
withAnimation() {
scroller.scrollTo(point)
}
} else {
//problem
}
}
}
Environmental object:
#Published var scrollingHelper = ScrollingHelper()
In the view: ScrollViewReader { reader in .....
Injection in the view:
.background(ScrollingHelperInjection(proxy: reader, helper: scrollingHelper)
Usage outside the view hierarchy: scrollingHelper.scrollTo(3)

Answer 8

As mentioned in #lachezar-todorov's answer Introspect is a nice library to access UIKit elements in SwiftUI. But be aware that the block you use for accessing UIKit elements are being called multiple times. This can really mess up your app state. In my cas CPU usage was going %100 and app was getting unresponsive. I had to use some pre conditions to avoid it.
ScrollView() {
...
}.introspectScrollView { scrollView in
if aPreCondition {
//Your scrolling logic
}
}

Answer 9

Another cool way is to just use namespace wrappers:
A dynamic property type that allows access to a namespace defined by the persistent identity of the object containing the property (e.g. a view).
struct ContentView: View {
#Namespace private var topID
#Namespace private var bottomID
let items = (0..<100).map { $0 }
var body: some View {
ScrollView {
ScrollViewReader { proxy in
Section {
LazyVStack {
ForEach(items.indices, id: \.self) { index in
Text("Item \(items[index])")
.foregroundColor(.black)
.frame(maxWidth: .infinity, alignment: .leading)
.padding()
.background(Color.green.cornerRadius(16))
}
}
} header: {
HStack {
Text("header")
Spacer()
Button(action: {
withAnimation {
proxy.scrollTo(bottomID)
}
}
) {
Image(systemName: "arrow.down.to.line")
.padding(.horizontal)
}
}
.padding(.vertical)
.id(topID)
} footer: {
HStack {
Text("Footer")
Spacer()
Button(action: {
withAnimation {
proxy.scrollTo(topID) }
}
) {
Image(systemName: "arrow.up.to.line")
.padding(.horizontal)
}
}
.padding(.vertical)
.id(bottomID)
}
.padding()
}
}
.foregroundColor(.white)
.background(.black)
}
}

Answer 10

Two parts:
Wrap the List (or ScrollView) with ScrollViewReader
Use the scrollViewProxy (that comes from ScrollViewReader) to scroll to an id of an element in the List. You can seemingly use EmptyView().
The example below uses a notification for simplicity (use a function if you can instead!).
ScrollViewReader { scrollViewProxy in
List {
EmptyView().id("top")
}
.onReceive(NotificationCenter.default.publisher(for: .ScrollToTop)) { _ in
// when using an anchor of `.top`, it failed to go all the way to the top
// so here we add an extra -50 so it goes to the top
scrollViewProxy.scrollTo("top", anchor: UnitPoint(x: 0, y: -50))
}
}
extension Notification.Name {
static let ScrollToTop = Notification.Name("ScrollToTop")
}
NotificationCenter.default.post(name: .ScrollToTop, object: nil)