Preserve Scroll Position in ScrollView when Adding New Items
I am currently struggling on my ChatView of my app. This is a sample code of what i want to achieve. It has functionality such as when the user is about to reach the top it fetches older messages and when bottom it fetches newer messages.
The Issue with this is when the user fetches newer messages the scrollview scrolls continuously to bottom and this happens recursively. This was the same issue when fetching older messages (it continuolsy scrolled to top) but i fixed it with flipped() modifier.But the bottom issue is still there.
struct ChatMessage: View {
let text: String
var body: some View {
HStack {
Text(text)
.foregroundStyle(.white)
.padding()
.background(.blue)
.clipShape(
RoundedRectangle(cornerRadius: 16)
)
.overlay(alignment: .bottomLeading) {
Image(systemName: "arrowtriangle.down.fill")
.font(.title)
.rotationEffect(.degrees(45))
.offset(x: -10, y: 10)
.foregroundStyle(.blue)
}
Spacer()
}
.padding(.horizontal)
}
}
struct Message : Identifiable,Equatable {
var id: Int
var text: String
}
struct GoodChatView: View {
@State var messages: [Message] = []
@State private var scrollViewProxy: ScrollViewProxy? // Store the proxy
@State var messageId: Int?
var body: some View {
ScrollViewReader { scrollView in
ScrollView {
LazyVStack {
ForEach(messages, id: \.id) { message in
ChatMessage(text: "\(message.text)")
.flippedUpsideDown()
.onAppear {
if message == messages.last {
print("old data")
loadMoreData()
}
if message == messages.first {
print("new data")
loadNewData()
}
}
}
}
.scrollTargetLayout()
}
.flippedUpsideDown()
.scrollPosition(id: $messageId)
.onAppear {
for i in 1...20 {
let message = Message(id: i, text: "\(i)")
messages.append(message)
}
messageId = messages.first?.id
}
}
}
func loadMoreData() {
DispatchQueue.main.asyncAfter(deadline: .now() + 1.0) {
let count = messages.count
var newMessages: [Message] = []
for i in count+1...count+20 {
let message = Message(id: i, text: "\(i)")
newMessages.append(message)
}
messages.append(contentsOf: newMessages)
}
}
func loadNewData() {
DispatchQueue.main.asyncAfter(deadline: .now() + 1.0) {
let count = messages.count
var newMessages: [Message] = []
for i in count+1...count+20 {
let message = Message(id: i, text: "\(i)")
newMessages.append(message)
}
newMessages = newMessages.reversed()
messages.insert(contentsOf: newMessages, at: 0)
}
}
}
struct FlippedUpsideDown: ViewModifier {
func body(content: Content) -> some View {
content
.rotationEffect(.radians(Double.pi))
.scaleEffect(x: -1, y: 1, anchor: .center)
}
}
extension View {
func flippedUpsideDown() -> some View {
modifier(FlippedUpsideDown())
}
}
Any Help is appreciated. If there are another ways to achieve this please let me know 🙂
Your example code is setting the scroll position by using .scrollPosition on the ScrollView, together with .scrollTargetLayout on the LazyVStack. The trouble is, .scrollPosition can only have one anchor. If the anchor is .top then it is difficult to set the position to the last entry at the bottom and the same goes for the first entry at the top when the anchor is .bottom. So:
A
ScrollViewReaderis perhaps a better way to set the scroll position. This allows different anchors to be supplied toScrollViewProxy.scrollTo. You had aScrollViewReaderin your example already, but it wasn't being used.The use of
.scrollTargetLayoutand.scrollPositioncan be removed.
Other notes:
I think you can remove all the flipping. I don't think it is necessary and it just complicates everything. Use a reversed array in the
ForEachinstead.To prevent a load of data from triggering another load immediately, I would suggest using a simple flag
isLoading. This should be set before loading. When a load completes, it can be reset after a short delay. This ensures that loading remains blocked while scroll adjustments are happening.When "new" messages are added, they appear at the bottom of the
ScrollView. It is not necessary to performscrollToafter adding the new entries because theScrollViewretains its position anyway.Adding "old" messages to the top is the harder part. If the
ScrollViewis being actively scrolled (that is, if the user has their finger in action) thenscrollTois ignored and the highest entry is shown. As a workaround, aGeometryReadercan be placed behind each entry to detect the relative scroll offset. A load should only be triggered when the offset is (almost) exactly 0. This might still happen when the user is scrolling manually, but it is much less likely. Usually, a load is only triggered when theScrollViewcomes to rest.I tried to update the code to use
Taskandasyncfunctions instead ofDispatchQueue. However, I have to admit, I'm a beginner in this area, as anyone with more experience will probably spot at a glance. Happy to make corrections if any suggestions are made. In particular, I am not sure if it is necessary for@MainActorto be specified so much.On intial load, the scroll position needs to be set to the bottom after the first load of data has been added. I tried using
Task.yield()to let the updates happen, but this wasn't always reliable. So it is now sleeping for 10ms instead. I expect there may be a better way of doing this too.
Here is the updated example:
struct GoodChatView: View {
@State private var messages: [Message] = []
@State private var isLoading = true
var body: some View {
ScrollViewReader { scrollView in
ScrollView {
LazyVStack {
ForEach(messages.reversed(), id: \.id) { message in
ChatMessage(text: "\(message.text)")
.background {
GeometryReader { proxy in
let minY = proxy.frame(in: .scrollView).minY
let isReadyForLoad = abs(minY) <= 0.01 && message == messages.last
Color.clear
.onChange(of: isReadyForLoad) { oldVal, newVal in
if newVal && !isLoading {
isLoading = true
Task { @MainActor in
await loadMoreData()
await Task.yield()
scrollView.scrollTo(message.id, anchor: .top)
await resetLoadingState()
}
}
}
}
}
.onAppear {
if !isLoading && message == messages.first {
isLoading = true
Task {
await loadNewData()
// When new data is appended, the scroll position is
// retained - no need to set it again
await resetLoadingState()
}
}
}
}
}
}
.task { @MainActor in
await loadNewData()
if let firstMessageId = messages.first?.id {
try? await Task.sleep(for: .milliseconds(10))
scrollView.scrollTo(firstMessageId, anchor: .bottom)
}
await resetLoadingState()
}
}
}
@MainActor
func loadMoreData() async {
let lastId = messages.last?.id ?? 0
print("old data > \(lastId)")
var oldMessages: [Message] = []
for i in lastId+1...lastId+20 {
let message = Message(id: i, text: "\(i)")
oldMessages.append(message)
}
messages += oldMessages
}
@MainActor
func loadNewData() async {
let firstId = messages.first?.id ?? 21
print("new data < \(firstId)")
var newMessages: [Message] = []
for i in firstId-20...firstId-1 {
let message = Message(id: i, text: "\(i)")
newMessages.append(message)
}
messages.insert(contentsOf: newMessages, at: 0)
}
@MainActor
private func resetLoadingState() async {
try? await Task.sleep(for: .milliseconds(500))
isLoading = false
}
}
