SwiftUI: Smooth horizontal autoscroll while dragging near edges (keep item under finger)
I have a SwiftUI horizontal editor (multiple “canvases” laid out side‑by‑side in a ScrollView). When the user drags a selected object and reaches the left/right edge of the visible area, I want the ScrollView to autoscroll smoothly so the drag can continue across canvases. Two requirements:
- Autoscroll smoothly with acceleration as the finger approaches the edge.
- Keep the dragged item visually under the finger while autoscrolling.
I’m using a named coordinate space, a global overlay for the selection border, and UIScrollView introspection to control contentOffset. It mostly works, but the autoscroll can stutter and (depending on compensation logic) the dragged item can momentarily “slip” out from under the finger while autoscrolling.
Below is a minimal reproducible example showing my current approach. It uses a CADisplayLink‑based timer for smooth continuous autoscroll with acceleration. The missing bit is the best practice to keep the item’s position aligned with the finger while the scroll view moves.
import SwiftUI
import Combine
import UIKit
import UniformTypeIdentifiers
struct ContentView: View {
@StateObject private var store = CanvasStore()
@State private var canvasSize: CGSize = .init(width: 300, height: 400)
@State private var dragStartItemPosition: CGPoint? = nil
@State private var isEditing: Bool = false
@State private var isCanvasEditing: Bool = false
let targetHeight: CGFloat = 150
@State private var draggedItem: CanvasData? = nil
@State private var scrollView: UIScrollView? = nil
@State private var autoScrollScheduled: Bool = false
var body: some View {
VStack {
let targetPreviewWidth: CGFloat = 150
// scale based on width
let scale: CGFloat = isCanvasEditing ? (targetPreviewWidth / canvasSize.width) : 1
let previewWidth = canvasSize.width * scale
let previewHeight = canvasSize.height * scale
ScrollView(.horizontal, showsIndicators: false) {
ZStack {
LazyHStack(spacing: 1) {
ForEach(store.canvasesArray, id: \.id) { item in
let i = store.canvasesArray.firstIndex(of: item) ?? 0
cellView(for: item, index: i, scale: scale, previewWidth: previewWidth, previewHeight: previewHeight, isDragging: draggedItem?.id == item.id)
}
}
// Global overlay during drag using ObjectViewBorder.
if let selected = store.selectedItem,
let info = store.indexForItem(id: selected.id),
store.canvasFrames.indices.contains(info.canvasIndex) {
let frame = store.canvasFrames[info.canvasIndex]
ObjectViewBorder(selectedItem: selected)
.offset(x: frame.minX, y: frame.minY)
.transition(.opacity)
}
}
.coordinateSpace(name: "carousel")
.background(HorizontalScrollViewIntrospector(scrollView: $scrollView))
// container height adapts to scaled canvas + tools
.frame(
height: isCanvasEditing
? (previewHeight + 60) // 60 = approx tools height, tweak
: canvasSize.height
)
.contentShape(Rectangle())
.gesture(
dragGesture,
including: (store.selectedItemID != nil && !isEditing) ? .gesture : .none
)
}
.environmentObject(store)
}
.onChange(of: isCanvasEditing) { newValue in
if !newValue {
draggedItem = nil
}
}
.frame(maxWidth: .infinity,maxHeight: .infinity)
.navigationBarBackButtonHidden(true)
.safeAreaInset(edge: .bottom) {
ZStack {
if isCanvasEditing {
HStack {
Button("Done") {
withAnimation(.spring(response: 0.3,
dampingFraction: 0.8)) {
isCanvasEditing = false
}
}
}
.frame(maxWidth: .infinity)
.background(.thinMaterial)
.transition(.move(edge: .bottom).combined(with: .opacity))
}else{
HStack {
Button("Edit") {
withAnimation(.spring(response: 0.3,
dampingFraction: 0.8)) {
isCanvasEditing = true
store.selectedItemID = nil
}
}
}
.frame(maxWidth: .infinity)
.background(.thinMaterial)
.transition(.move(edge: .bottom).combined(with: .opacity))
}
}
.animation(.spring(response: 0.3, dampingFraction: 0.8),
value: isCanvasEditing)
}
}
@ViewBuilder
private func cellView(for item: CanvasData, index: Int,scale:CGFloat,previewWidth:CGFloat,previewHeight:CGFloat, isDragging: Bool) -> some View {
VStack(spacing: 4) {
let cell = HStack(spacing: 1) {
ZStack {
// full-size canvas content
canvasCell(index: index)
.frame(width: canvasSize.width,
height: canvasSize.height)
.scaleEffect(scale, anchor: .center)
}
.frame(width: previewWidth,
height: previewHeight)
if index < store.canvasesArray.count - 1 {
Divider()
.frame(height: previewHeight)
}
}
cell
.scaleEffect(isDragging ? 1.06 : 1.0)
.shadow(color: .black.opacity(isDragging ? 0.35 : 0.15),
radius: isDragging ? 12 : 4,
x: 0,
y: isDragging ? 8 : 2)
.animation(.interpolatingSpring(stiffness: 260, damping: 16),
value: isDragging)
if isCanvasEditing {
canvasEditingTools(for: item,previewWidth: previewWidth,previewHeight: previewHeight)
}
Spacer(minLength: 0)
}.onDrop(
of: [UTType.text],
delegate: CanvasDropDelegate(
targetItem: item,
items: $store.canvasesArray,
draggedItem: $draggedItem
)
)
}
fileprivate func canvasEditingTools(
for item: CanvasData,
previewWidth: CGFloat,
previewHeight: CGFloat
) -> some View {
canvasEditingTools(for: item, previewWidth: previewWidth, previewHeight: previewHeight) {
EmptyView()
}
}
// Overload with optional preview via default EmptyView
fileprivate func canvasEditingTools(
for item: CanvasData,
previewWidth: CGFloat,
previewHeight: CGFloat,
@ViewBuilder preview: () -> Preview = { EmptyView() }
) -> some View {
return HStack(spacing: 12) {
Image(systemName: "line.3.horizontal")
.font(.title3)
.foregroundColor(.black)
.padding(.vertical, 4)
.onDrag {
// Start drag only when this is pressed (still with system long-press)
if draggedItem == nil{
self.draggedItem = item
}else{
self.draggedItem = nil
}
return NSItemProvider(object: item.id.uuidString as NSString)
} preview: {
// Drag preview
preview()
.frame(width: previewWidth, height: previewHeight)
}
}
.transition(.move(edge: .bottom).combined(with: .opacity))
}
private var dragGesture: some Gesture {
DragGesture(minimumDistance: 2)
.onChanged { value in
guard let selectedID = store.selectedItemID,
let (canvasIndex, itemIndex) = store.indexForItem(id: selectedID)
else { return }
let item = store.canvasesArray[canvasIndex].items[itemIndex]
if dragStartItemPosition == nil {
dragStartItemPosition = item.position
}
guard let start = dragStartItemPosition else { return }
let t = value.translation
let newPos = CGPoint(
x: start.x + t.width,
y: start.y + t.height
)
store.isDraggingSelectedItem = true
store.canvasesArray[canvasIndex].items[itemIndex].position = newPos
// Auto-scroll when near edges
autoScrollIfNeeded(globalPoint: globalCenter(for: store.canvasesArray[canvasIndex].items[itemIndex]))
}
.onEnded { _ in
defer { dragStartItemPosition = nil }
guard let selectedID = store.selectedItemID,
let (fromCanvas, itemIndex) = store.indexForItem(id: selectedID)
else { return }
let item = store.canvasesArray[fromCanvas].items[itemIndex]
guard store.canvasFrames.indices.contains(fromCanvas) else { return }
let canvasFrame = store.canvasFrames[fromCanvas]
// convert local center to global center
let globalCenter = CGPoint(
x: canvasFrame.minX + item.position.x,
y: canvasFrame.minY + item.position.y
)
store.finishDrag(
for: selectedID,
fromCanvas: fromCanvas,
globalCenter: globalCenter
)
store.isDraggingSelectedItem = false
autoScrollScheduled = false
}
}
@ViewBuilder
private func canvasCell(index: Int) -> some View {
VStack(spacing: 6) {
CanvasView(canvasIndex: index)
.frame(width: canvasSize.width, height: canvasSize.height)
}
}
// MARK: - Helpers
private func globalCenter(for item: CanvasItem) -> CGPoint {
guard let info = store.indexForItem(id: item.id),
store.canvasFrames.indices.contains(info.canvasIndex) else {
return .zero
}
let frame = store.canvasFrames[info.canvasIndex]
return CGPoint(x: frame.minX + item.position.x,
y: frame.minY + item.position.y)
}
// MARK: - Auto-scroll logic
private func autoScrollIfNeeded(globalPoint: CGPoint) {
guard let sv = scrollView else { return }
// globalPoint is in the named "carousel" content coordinate space
let inset: CGFloat = 80
let minStep: CGFloat = 4
let maxStep: CGFloat = 28
let visibleMinX = sv.contentOffset.x
let visibleMaxX = visibleMinX + sv.bounds.width
let distLeft = globalPoint.x - visibleMinX
let distRight = visibleMaxX - globalPoint.x
var dx: CGFloat = 0
var needs = false
if distLeft < inset {
let closeness = max(0, (inset - distLeft) / inset) // 0..1
let eased = closeness * closeness // quadratic
let step = minStep + (maxStep - minStep) * eased
dx = -step
needs = true
} else if distRight < inset {
let closeness = max(0, (inset - distRight) / inset)
let eased = closeness * closeness
let step = minStep + (maxStep - minStep) * eased
dx = step
needs = true
}
if needs {
let maxOffsetX = max(0, sv.contentSize.width - sv.bounds.width)
var offset = sv.contentOffset
offset.x = min(max(offset.x + dx, 0), maxOffsetX)
if offset != sv.contentOffset {
sv.setContentOffset(offset, animated: false)
}
// keep scrolling while hovering near edge
if store.isDraggingSelectedItem && !autoScrollScheduled {
autoScrollScheduled = true
DispatchQueue.main.asyncAfter(deadline: .now() + 1.0/60.0) {
autoScrollScheduled = false
if store.isDraggingSelectedItem, let selected = store.selectedItem {
let center = globalCenter(for: selected)
autoScrollIfNeeded(globalPoint: center)
}
}
}
}
}
}
struct CanvasDropDelegate: DropDelegate {
let targetItem: CanvasData
@Binding var items: [CanvasData]
@Binding var draggedItem: CanvasData?
func dropEntered(info: DropInfo) {
guard let draggedItem = draggedItem,
draggedItem != targetItem,
let fromIndex = items.firstIndex(of: draggedItem),
let toIndex = items.firstIndex(of: targetItem) else {
return
}
if items[toIndex] != draggedItem {
withAnimation(.default) {
items.move(fromOffsets: IndexSet(integer: fromIndex),
toOffset: toIndex > fromIndex ? toIndex + 1 : toIndex)
}
}
}
func performDrop(info: DropInfo) -> Bool {
// drop finished → clear dragged state
draggedItem = nil
return true
}
func dropUpdated(info: DropInfo) -> DropProposal? {
DropProposal(operation: .move)
}
}
import SwiftUI
struct CanvasView: View {
@EnvironmentObject var store: CanvasStore
let canvasIndex: Int
var body: some View {
GeometryReader { geo in
ZStack {
// Tap empty space inside this canvas → unselect item
Color.clear
.contentShape(Rectangle())
.onTapGesture {
store.selectedItemID = nil
}
let items: [CanvasItem] = (store.canvasesArray.indices.contains(canvasIndex) ? store.canvasesArray[canvasIndex].items : [])
ForEach(items) { item in
let isSelected = (store.selectedItemID == item.id)
Text(item.text)
.padding(8)
.position(item.position)
.onTapGesture {
store.selectedItemID = item.id
}
}
}
.onAppear {
if store.canvasFrames.indices.contains(canvasIndex) {
store.canvasFrames[canvasIndex] = geo.frame(in: .named("carousel"))
}
}
.onChange(of: geo.frame(in: .named("carousel"))) { newFrame in
if store.canvasFrames.indices.contains(canvasIndex) {
store.canvasFrames[canvasIndex] = newFrame
}
}
}
}
}
import Combine
import SwiftUI
@Observable
class CanvasItem: Identifiable, Codable {
let id = UUID()
var text: String
var position: CGPoint
var rotation: Angle = .zero
var width: CGFloat = 200.0
var height: CGFloat = 50.0
init(text: String = "test", position: CGPoint, rotation: Angle = .zero, width: CGFloat = 200.0, height: CGFloat = 50.0) {
self.text = text
self.position = position
self.rotation = rotation
self.width = width
self.height = height
}
}
// Optional but useful: Equatable for CanvasItem based on id
extension CanvasItem: Equatable {
static func == (lhs: CanvasItem, rhs: CanvasItem) -> Bool {
lhs.id == rhs.id
}
}
@Observable
class CanvasData: Identifiable, Codable {
let id = UUID()
var items: [CanvasItem]
init(items: [CanvasItem]) {
self.items = items
}
}
// Required: Equatable for CanvasData based on id
extension CanvasData: Equatable {
static func == (lhs: CanvasData, rhs: CanvasData) -> Bool {
lhs.id == rhs.id
}
}
@MainActor
final class CanvasStore: ObservableObject {
@Published var canvasesArray: [CanvasData] {
didSet { ensureCanvasFramesCount() }
}
@Published var canvasFrames: [CGRect] // global frames of each canvas
@Published var selectedItemID: CanvasItem.ID? // currently selected text
@Published var isDraggingSelectedItem: Bool = false
var selectedItem: CanvasItem? {
guard let id = selectedItemID else { return nil }
for canvas in canvasesArray {
if let item = canvas.items.first(where: { $0.id == id }) {
return item
}
}
return nil
}
init() {
let first = CanvasItem(text: "Drag me ➡️", position: CGPoint(x: 80, y: 80))
let initialCanvases: [CanvasData] = [
CanvasData(items: [first]),
CanvasData(items: []),
CanvasData(items: []),
CanvasData(items: []),
CanvasData(items: []),
CanvasData(items: []),
CanvasData(items: [])
]
self.canvasesArray = initialCanvases
self.canvasFrames = Array(repeating: .zero, count: initialCanvases.count)
self.selectedItemID = nil
}
func indexForItem(id: CanvasItem.ID) -> (canvasIndex: Int, itemIndex: Int)? {
for c in canvasesArray.indices {
if let i = canvasesArray[c].items.firstIndex(where: { $0.id == id }) {
return (c, i)
}
}
return nil
}
}
private extension CGRect {
var area: CGFloat { width * height }
}
extension CanvasStore {
private func ensureCanvasFramesCount() {
let target = canvasesArray.count
if canvasFrames.count == target { return }
if canvasFrames.count < target {
canvasFrames.append(contentsOf: Array(repeating: .zero, count: target - canvasFrames.count))
} else {
canvasFrames.removeLast(canvasFrames.count - target)
}
}
func finishDrag(for id: CanvasItem.ID, fromCanvas: Int, globalCenter: CGPoint) {
guard canvasesArray.indices.contains(fromCanvas),
let itemIndex = canvasesArray[fromCanvas].items.firstIndex(where: { $0.id == id }) else { return }
let item = canvasesArray[fromCanvas].items[itemIndex]
let size = CGSize(width: item.width, height: item.height)
guard size.width > 0, size.height > 0 else { return }
let itemRect = CGRect(
x: globalCenter.x - size.width / 2,
y: globalCenter.y - size.height / 2,
width: size.width,
height: size.height
)
// Find which canvas has the largest overlap
var bestCanvas: Int?
var bestOverlap: CGFloat = 0
for (index, frame) in canvasFrames.enumerated() {
let intersection = frame.intersection(itemRect)
let overlapArea = max(0, intersection.area)
if overlapArea > bestOverlap {
bestOverlap = overlapArea
bestCanvas = index
}
}
guard let targetIndex = bestCanvas, bestOverlap > 0 else { return }
// Move only if >= half of the item is inside target canvas
let halfArea = itemRect.area / 2
guard bestOverlap >= halfArea else { return }
if targetIndex == fromCanvas { return }
// Move item to target canvas
var movedItem = canvasesArray[fromCanvas].items.remove(at: itemIndex)
guard canvasFrames.indices.contains(targetIndex) else { return }
let targetFrame = canvasFrames[targetIndex]
let localCenter = CGPoint(
x: globalCenter.x - targetFrame.minX,
y: globalCenter.y - targetFrame.minY
)
movedItem.position = localCenter
canvasesArray[targetIndex].items.append(movedItem)
selectedItemID = movedItem.id // keep selection on the moved item
}
}
struct ObjectViewBorder: View {
@Bindable var selectedItem: CanvasItem
@State private var lastDragValue: CGSize = .zero
@State private var initialRotation: Angle = .zero
@State private var initialAngle: Angle = .zero
var body: some View {
ZStack {
// Base dashed border (doesn't block gestures)
RoundedRectangle(cornerRadius: 5)
.stroke(style: StrokeStyle(lineWidth: 1, dash: [5]))
.frame(width: selectedItem.width + 20, height: selectedItem.height + 20)
.position(selectedItem.position)
.allowsHitTesting(false)
}
.foregroundColor(.accentColor)
}
}
import SwiftUI
import UIKit
struct HorizontalScrollViewIntrospector: UIViewRepresentable {
@Binding var scrollView: UIScrollView?
func makeUIView(context: Context) -> UIView {
let view = UIView(frame: .zero)
DispatchQueue.main.async { self.scrollView = findScrollView(from: view) }
return view
}
func updateUIView(_ uiView: UIView, context: Context) {
DispatchQueue.main.async { self.scrollView = findScrollView(from: uiView) }
}
private func findScrollView(from view: UIView) -> UIScrollView? {
var v: UIView? = view
while let s = v?.superview {
if let sv = s as? UIScrollView { return sv }
v = s
}
return nil
}
}
What I’ve tried
- Named coordinate space for consistent geometry; computing visible range with contentOffset.
- UIScrollView introspection; setting contentOffset directly (no animation).
- Acceleration using a quadratic ease near edges.
- A previous attempt adjusted the item’s local X by the same dx as the scroll step to keep it under the finger; it works visually, but I’m unsure if this is the right approach (it modifies model state during scroll).
- I also tried a timer (asyncAfter) tick; CADisplayLink is smoother.
Questions
- What’s the recommended pattern in SwiftUI to keep the dragged item visually under the finger while the scroll view is programmatically
autoscrolling?
- Should I adjust the item’s local position by the scroll delta (dx) each tick, or derive the item’s position directly from the finger’s global location converted into content coordinates (e.g., using GeometryProxy + UIScrollView contentOffset)?
- Is there a better way to drive smooth autoscroll than manually stepping contentOffset (e.g., ScrollViewReader, UIScrollView pan programmatically, or a different gesture composition)?