Grouping Data with Lazy Stack Views

Overview 概要

LazyHStack and LazyVStack views are both able to display groups of views organized into logical sections, arranging their children in lines that grow horizontally and vertically, respectively. These stacks are “lazy” in that the stack views don’t create items until they need to be rendered onscreen. Like stack views, lazy stacks don’t include any inherent support for scrolling, and you should wrap lazy stack views in ScrollView containers.

To group content or data inside a lazy stack view, use Section instances as containers for collections of grouped views. Section views don’t have any visual representation themselves but can contain header and footer views that can either scroll with the stack’s content or that you can pin to the top or bottom of the ScrollView.

The code samples in this article build a user interface for visualizing shades of primary colors. Each section in the stack represents a primary color, containing five subviews, each showing a different variation of the color.

Prepare Your Data

As with views contained within a stack, each Section must be uniquely identifiable when iterated by ForEach. In this example, ColorData instances represent the sections, and ShadeData instances represent the shades of each color inside a section. Both ColorData and ShadeData conform to the Identifiable protocol.

struct ColorData: Identifiable {
    let id = UUID()
    let name: String
    let color: Color
    let variations: [ShadeData]


    struct ShadeData: Identifiable {
        let id = UUID()
        var brightness: Double
    }


    init(color: Color, name: String) {
        self.name = name
        self.color = color
        self.variations = stride(from: 0.0, to: 0.5, by: 0.1)
            .map { ShadeData(brightness: $0) }
    }
}

Display Sections with Headers and Footers

The ColorSelectionView below sets up an array containing ColorData instances for each primary color. The LazyVStack iterates over the array of color data to create sections, then iterates over the variations to create views from the shades.

struct ColorSelectionView: View {
    let sections = [
        ColorData(color: .red, name: "Reds"),
        ColorData(color: .green, name: "Greens"),
        ColorData(color: .blue, name: "Blues")
    ]


    var body: some View {
        ScrollView {
            LazyVStack(spacing: 1) {
                ForEach(sections) { section in
                    Section(header: SectionHeaderView(colorData: section)) {
                        ForEach(section.variations) { variation in
                            section.color
                                .brightness(variation.brightness)
                                .frame(height: 20)
                        }
                    }
                }
            }
        }
    }
}

Group data with Section views and pass in a header or footer view with the header and footer properties. This example implements a SectionHeaderView as a header view, containing a semi-transparent stack view and the name of the section’s color in a Text label.

struct SectionHeaderView: View {
    var colorData: ColorData


    var body: some View {
        HStack {
            Text(colorData.name)
                .font(.headline)
                .foregroundColor(colorData.color)
            Spacer()
        }
        .padding()
        .background(Color.primary
                        .colorInvert()
                        .opacity(0.75))
    }
}

For more information on using ForEach to repeat views inside a stack, see Creating Performant Scrollable Stacks.

Keep Important Information Visible

By default, section header and footer views will scroll in sync with section content. If you want header and footer views to always remain visible, regardless of whether the top or bottom of the section is visible, then specify a set of PinnedScrollableViews for the pinnedViews property of the lazy stack view.

LazyVStack(spacing: 1, pinnedViews: [.sectionHeaders]) {
    // ...
}

In LazyVStack containers, headers attach to the top and footers to the bottom. In LazyHStack containers, headers attach to the leading edge and footers to the trailing edge.

With this change, section headers are pinned to the top of the view as the user begins to scroll.