SwiftUI - Custom Snap Scroll With Offset

Question

I want to create a custom scroll that snaps views to middle But I can't figure out how to set the offset correctly.

This is the sample of the scroll:

struct contentView: View {

    @State var startOffset: CGFloat = 0
    @State var translationWidth: CGFloat = 0
    @State var offset: CGFloat = 0
    @State var index: Int = 0


    var body: some View  {
        GeometryReader { geo in
            HStack {

                ForEach(0..<5, id: \.self) { num in
                    Button(action: {
                    // some action
                    }) {
                        Circle()
                    }
                    .frame(width: geo.size.width)
                    .offset(x: -self.offset*CGFloat(self.index) + self.translationWidth)

                }

            }.frame(width: geo.size.width, height: 200)
                .offset(x: self.startOffset )
                .highPriorityGesture (
                    DragGesture(minimumDistance: 0.1, coordinateSpace: .global)
                    .onChanged({ (value) in
                         self.translationWidth = value.translation.width
                     })

                     .onEnded({ (value) in
                         if self.translationWidth > 40 {
                                self.index -= 1

                        } else if self.translationWidth < -40 {
                            self.index += 1
                        }
                        self.translationWidth = 0
                    })
            )

            .onAppear {
                withAnimation(.none) {
                    let itemsWidth: CGFloat = CGFloat(geo.size.width*5) - (geo.size.width)
                    self.offset = geo.size.width
                    self.startOffset = itemsWidth/2
                }
            }
        }
    }
}

It works but it is slightly off the middle, it doesn't scroll exactly in the middle and I don't understand why.

The problem occurs with the onAppear closure:

.onAppear {
    withAnimation(.none) {
        let itemsWidth: CGFloat = CGFloat(geo.size.width*5) - (geo.size.width)
        self.offset = geo.size.width
        self.startOffset = itemsWidth/2
    }
}

I might be missing some small pixels in my calculations.

UPDATE

So Apparently the HStack has a default value for spacing between views. so to fix it you should remove it like so:

 HStack(spacing: 0) {
    ....
 }

My bad, I meant scroll that snaps its view to middle, I updated the post — Kevin
– Kevin, Commented Jun 17, 2020 at 18:18

Marcy · Accepted Answer · 2023-05-20 20:02:34Z

2

TabView's PageTabViewStyle() is another option for scrolling views in SwiftUI that snap to the middle:

struct ContentView: View {
    
    var body: some View {
        TabView {
            ForEach(0..<5) { _ in
                Button(action: {
                    // some action
                }) {
                    Circle().frame(width: 200, height: 200)
                }
            }
        }
        .tabViewStyle(PageTabViewStyle())
    }
}

The result looks like this:

answered May 20, 2023 at 20:02

Marcy

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2 Comments

Lennart Wisbar Over a year ago

This works great, is simple and feels completely native. The only problem I have is that I need my items to scroll in groups, e.g. 3 at a time. Let's say there are 7 items, then the desired (UIKit like) behavior would be to show the following groups of items when swiping repeatedly: 123, 456, 567, the last swipe swiping just far enough to make item 7 visible. I'm afraid that's impossible with this approach. :-( On iOS 17, this behavior is built into ScrollView with scrollTargetBehavior(.paging). But I need to support iOS 16, too.

Lennart Wisbar Over a year ago

Oh, I just realized, the second problem is you can't make the previous and next item peek in, which is also a requirement for me. :-(

Kevin · Accepted Answer · 2020-06-27 07:11:03Z

1

So after a bit of experimenting, I realized the HStack is using a default spacing which is not equal to zero. It means that in my calculations I didn't include the spacing between items

so to fix this all you need to add is:

 HStack(spacing: 0) {
    ...
 }

answered Jun 27, 2020 at 7:11

Kevin

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