SwiftUI | GeometryReader: Smooth resizable Header when scrolling through List - swift

Question

I am new to SwiftUI and I want to recreate the Contact-Card View from the Contacts App.
I am struggling to resize the Image on the top smoothly when scrolling in the List below.
I have tried using GeometryReader, but ran into issues there.
When scrolling up for example, the picture size just jumps abruptly to the minimumPictureSize I have specified. The opposite happens when scrolling up: It stops resizing abruptly when I stop scrolling.
Wanted behaviour: https://gifyu.com/image/Ai04
Current behaviour: https://gifyu.com/image/AjIc
struct SwiftUIView: View {
#State var startOffset: CGFloat = 0
#State var offset: CGFloat = 0
var minPictureSize: CGFloat = 100
var maxPictureSize: CGFloat = 200
var body: some View {
VStack {
Image("person")
.resizable()
.frame(width: max(minPictureSize, min(maxPictureSize, minPictureSize + offset)),
height: max(minPictureSize, min(maxPictureSize, minPictureSize + offset)))
.mask(Circle())
Text("startOffset: \(startOffset)")
Text("offset: \(offset)")
List {
Section {
Text("Top Section")
}.overlay(
GeometryReader(){ geometry -> Color in
let rect = geometry.frame(in: .global)
if startOffset == 0 {
DispatchQueue.main.async {
startOffset = rect.minY
}
}
DispatchQueue.main.async {
offset = rect.minY - startOffset
}
return Color.clear
}
)
ForEach((0..<10)) { row in
Section {
Text("\(row)")
}
}
}.listStyle(InsetGroupedListStyle())
}.navigationBarHidden(true)
}
}

Answer 1

Not a perfect solution, but you could separate the header and List into 2 layers in a ZStack:
struct SwiftUIView: View {
#State var startOffset: CGFloat!
#State var offset: CGFloat = 0
let minPictureSize: CGFloat = 100
let maxPictureSize: CGFloat = 200
var body: some View {
ZStack(alignment: .top) {
if startOffset != nil {
List {
Section {
Text("Top Section")
} header: {
// Leave extra space for `List` so it won't clip its content
Color.clear.frame(height: 100)
}
.overlay {
GeometryReader { geometry -> Color in
DispatchQueue.main.async {
let frame = geometry.frame(in: .global)
offset = frame.minY - startOffset
}
return Color.clear
}
}
ForEach((0..<10)) { row in
Section {
Text("\(row)")
}
}
}
.listStyle(InsetGroupedListStyle())
.padding(.top, startOffset-100) // Make up extra space
}
VStack {
Circle().fill(.secondary)
.frame(width: max(minPictureSize, min(maxPictureSize, minPictureSize + offset)),
height: max(minPictureSize, min(maxPictureSize, minPictureSize + offset)))
Text("startOffset: \(startOffset ?? -1)")
Text("offset: \(offset)")
}
.frame(maxWidth: .infinity)
.padding(.bottom, 20)
.background(Color(uiColor: UIColor.systemBackground))
.overlay {
if startOffset == nil {
GeometryReader { geometry -> Color in
DispatchQueue.main.async {
let frame = geometry.frame(in: .global)
startOffset = frame.maxY + // Original small one
maxPictureSize - minPictureSize -
frame.minY // Top safe area height
}
return Color.clear
}
}
}
}
.navigationBarHidden(true)
}
}
Notice that Color.clear.frame(height: 100) and .padding(.top, startOffset-100) are intended to leave extra space for List to avoid being clipped, which will cause the scroll bar get clipped. Alternatively, UIScrollView.appearance().clipsToBounds = true will work. However, it'll make element which moves outside the bounds of List disappear. Don't know if it's a bug.