Grid.swift

@available(iOS 13.0, OSX 10.15, *)
struct Grid<Data, Content>: View where Data: RandomAccessCollection, Content: View, Data.Index == Int, Data.Element: Identifiable {

    private let data: [GridData<Data>]
    private let column: Int
    private let content: (Data.Index, Data.Element) -> Content

    init(_ data: Data, column: Int = 2, content: @escaping (Data.Index, Data.Element) -> Content) {
        self.column = column
        self.content = content

        // 最適なやりかたではないかも
        self.data = data
            .reduce(into: [], { result, element in
                guard let last = result.last else {
                    result.append([element])
                    return
                }

                if last.count == column {
                    result.append([element])
                } else {
                    result.removeLast()
                    result.append(last + [element])
                }
            })
            .enumerated()
            .map { index, column in
                GridData(id: index, data: column)
            }
    }

    public var body: some View {
        List(zip(data.indices, data).map { $0 }, id: \.1.id) { listIndex, gridData in
            HStack {
                ForEach(gridData.data.indices) { columnIndex in
                    content(listIndex * column + columnIndex, gridData.data[columnIndex])
                }
            }
        }
    }
}

struct GridData<Data>: Identifiable where Data: RandomAccessCollection, Data.Element: Identifiable {
    let id: Int
    let data: [Data.Element]
}