TabView and Swift Charts giving inconsistent behaviour when swiping between pages

Ok. I have paired it all the way back to a single page of code.

You can see the text at the top of the graph is the Y coords and it doesn't match the chart. If I had included interactivity then tapping on the graph would make it show the correct chart.

Code:

import SwiftUI
import Charts

struct Point: Identifiable {
    var x: Int
    var y: Int
    let id = UUID()
}

struct PointData: Identifiable {
    var points: [Point]
    let id = UUID()
}

struct TabSelectorView: View {
       
    var pointDataArray: [PointData] = []
    
    init() {
        self.pointDataArray = initData()
    }
      
    var body: some View {
        let data = pointDataArray
        VStack {
            TabView  {
                ForEach((0..<data.count), id: \.self) { index in
                    VStack {
                        let graphData = pointDataArray[index]
                        Text (graphData.points.map { String($0.y) }.joined(separator: ","))
                        Chart(graphData.points) { item in
                            LineMark(
                                x: .value("X", item.x),
                                y: .value("Y", item.y)
                            )
                            .symbol {
                                Circle()
                                    .fill(Color.orange)
                                    .frame(width: 10)
                            }
                        }
                    }
                    .tag((index))
                }
            }
            .tabViewStyle(.page)
            .indexViewStyle(.page(backgroundDisplayMode: .always))
        }
    }
    
    func initData() -> [PointData] {
        var ret: [PointData] = []
        for chartNum in 0...6 {
            var pointArray: [Point] = []
            for point in 0...5 {
                pointArray.append(Point(x: point, y: point + chartNum))
            }
            ret.append(PointData(points: pointArray))
        }
        return ret
    }
}

#Preview {
    return TabSelectorView()
        .frame(height: 400)
        .padding(50)
}

Am I doing something obviously wrong?!

Note that some tab pages are correct and others are incorrect, and it changes as you swipe through.

This time with the correct screenshot!

This seems to be a serious bug of SwiftUIs Charts API. In case there are several Chart-Views generated inside a ForEach-Loop the Chart-Views sometimes do not match the data of the relevant loop-run anymore. This bug appears especially as soon as the view was updated by the OS (e.g. switching tabs, scrolling, ...), not so often when the view was generated initially.

This bug appeared with one of the latest OS Updates (iOS, iPadOS, macOS). I can't see any misuse here. I also couldn't find a workaround. I recommend reporting this to Apple.

Maybe, it's not a bug and I found the solution: Use .id() with Charts, e.g. Chart {...}.id(someHashableObject)

Pseudocode example:

ForEach (identifiableObjects) { identifiableObject in 
       Chart {
            // some Marks...
       }
       .id(UUID())
}

Seems to work in my case.

For some reason I didn't get notified of these posts. Apple's feedback was to raise a bug which I did. Thank you so much for the workaround / correction. I have tested it quickly in the test app and it works for me also. It will take me some time to look at my live app as I am travelling. Thanks again. :-)

Ok.....so I made the change to my live app to add the ID to the chart with a UUID. Good news is that the swiping bug is now cleared. Bad news is now interactivity is broken!

Without the .id(UUID()) modifier to the chart then when I interact with the chart it behaves normally insofar as when I tap the chart then up comes the annotation view that I have coded . When I scroll with my finger then the annotation updates to the closest point in the chart. When I remove my finger then the annotation disappears.

However if I add the .id(UUID()) modifier to the chart then interactions go awry. I cannot keep my finger down and scroll through the points and, worse, the annotation does not remove itself after I have lifted my finger. Therefore there doesn't seem to be a way to remove the annotation.

I have updated my MVP code to illustrate. If you keep the '.id' line in then the swiping works but the interaction doesn't work. If you comment out the '.id' line then the original swiping problem comes back but the interaction works as expected.

I have updated this in the feedback item I raised.

Updated code:

import SwiftUI
import Charts

struct Point: Identifiable {
    var x: Int
    var y: Int
    let id = UUID()
}

struct PointData: Identifiable {
    var points: [Point]
    let id = UUID()
}

struct TabSelectorView: View {
       
    @State private var selectedIndex: Int?     // NEW
    var pointDataArray: [PointData] = []
    
    init() {
        self.pointDataArray = initData()
    }
      
    var body: some View {
        VStack {
            TabView  {
                ForEach((0..<pointDataArray.count), id: \.self) { index in
                    VStack {
                        let graphData = pointDataArray[index]
                        Text ("Selected index: \(selectedIndex == nil ? "None" : String(selectedIndex!) )")        // NEW
                        Text (graphData.points.map { String($0.y) }.joined(separator: ","))
                        Chart(graphData.points) { item in
                            LineMark(
                                x: .value("X", item.x),
                                y: .value("Y", item.y)
                            )
                            .symbol {
                                Circle()
                                    .fill(Color.orange)
                                    .frame(width: 10)
                            }
                        }
                        .id(UUID())       // NEW. Comment out to fix interaction but break swiping
                        .chartXSelection(value: $selectedIndex)          // NEW
                    }
                    .tag((index))
                }
            }
            .tabViewStyle(.page)
            .indexViewStyle(.page(backgroundDisplayMode: .always))
        }
    }
    
    func initData() -> [PointData] {
        var ret: [PointData] = []
        for chartNum in 0...6 {
            var pointArray: [Point] = []
            for point in 0...5 {
                pointArray.append(Point(x: point, y: point + chartNum))
            }
            ret.append(PointData(points: pointArray))
        }
        return ret
    }
}

#Preview {
    return TabSelectorView()
        .frame(height: 400)
        .padding(50)
}

Unfortunately this bug still exists in iOS18. I am coming to the conclusion that my app is the only app in the App Store using a combo of Interactive Swift Charts and TabView.

That can't be true :-)

Any workaround suggestions very welcome!

@Greigarious You are not the only one now, I've just hit this same problem!

Could you please let me know your feedback number so that I can file a duplicate? Did you already include a sample project in that bug report? If not, I will do so.

Thank you for discussing the workaround, I will be making use of it for now.

@__bb Thanks. It was FB13686014