Infinite Scrollable Bar Chart using Swift Charts

README.md

Infinite Scrollable Bar Chart using Swift Charts

Just like charts in the official Health App. You can:

• Slow swipe by day
• Fast swipe by page

Check the video demo in the comment.

InfiniteScrollChart.swift

//
//  InfiniteScrollChart.swift
//  ChartsGallery
//
//  Created by beader on 2022/11/3.
//

import SwiftUI
import Charts

struct YAxisWidthPreferenceyKey: PreferenceKey {
    static var defaultValue: CGFloat = .zero
    static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
        value = max(value, nextValue())
    }
}

struct YAxisWidthModifier: ViewModifier {
    func body(content: Content) -> some View {
        content.background(
            GeometryReader { geometry in
                Color.clear.preference(key: YAxisWidthPreferenceyKey.self, value: geometry.size.width)
            }
        )
    }
}

struct BarChart: View {
    @Binding var unitOffset: Int
    @State var upperBound: Double?
    
    init(unitOffset: Binding<Int>) {
        self._unitOffset = unitOffset
    }
    
    private let calendar: Calendar = {
        Calendar.init(identifier: .gregorian)
    }()
    
    private var initDate: Date {
        calendar.startOfDay(for: Date().addingTimeInterval(TimeInterval(unitOffset * 24 * 3600)))
    }
    
    private var data: [(date: Date, value: Double)] {
        return (-7..<14).map { i in
            (date: initDate.addingTimeInterval(Double(i) * 24 * 3600), value: abs(Double(i + unitOffset) * 10))
        }
    }
    
    var body: some View {
        Chart {
            ForEach(data, id: \.date) { item in
                BarMark(
                    x: .value("Day", item.date, unit: .weekday),
                    y: .value("Value", min(item.value, upperBound ?? item.value))
                )
            }
        }
        .onAppear {
            upperBound = data[7..<14].map(\.value).max()
        }
        .onChange(of: unitOffset) { newValue in
            withAnimation(.spring()) {
                upperBound = data[7..<14].map(\.value).max()
            }
        }
    }
}

struct InfiniteScrollChart: View {
    private let height: CGFloat = 250
    private let numBins: Int = 7
    private let pagingAnimationDuration: CGFloat = 0.2
    
    @GestureState private var translation: CGFloat = .zero
    @State private var offset: CGFloat = .zero
    
    // Width of the visible plot area
    @State private var chartContentContainerWidth: CGFloat = .zero
    // Width of the yAxis of chart
    @State private var yAxisWidth: CGFloat = .zero
    // Each bar represents a unit duration along xAxis
    @State private var currentUnitOffset: Int = .zero
    
    @Environment(\.locale) var locale
    
    private var drag: some Gesture {
        DragGesture(minimumDistance: 0)
            .updating($translation) { value, state, _ in
                state = value.translation.width
            }
            .onEnded { value in
                offset = offset + value.translation.width

                let unitWidth = chartContentContainerWidth / Double(numBins)
                let unitOffset = (value.translation.width / unitWidth).rounded(.toNearestOrAwayFromZero)
                var predictedUnitOffset = (value.predictedEndTranslation.width / unitWidth).rounded(.toNearestOrAwayFromZero)

                // If swipe carefully, change to the nearest time unit
                // If swipe fast enough, change to the next page
                predictedUnitOffset = max(-Double(numBins), predictedUnitOffset)
                predictedUnitOffset = min(Double(numBins), predictedUnitOffset)
                withAnimation(.easeOut(duration: pagingAnimationDuration)) {
                    if abs(predictedUnitOffset) >= Double(numBins) {
                        offset = predictedUnitOffset * unitWidth
                    } else {
                        offset = unitOffset * unitWidth
                    }
                }
                DispatchQueue.main.asyncAfter(deadline: .now() + pagingAnimationDuration) {
                    currentUnitOffset = currentUnitOffset - Int(offset / unitWidth)
                    offset = 0
                }
            }
    }
    
    
    var body: some View {
        GeometryReader { geometry in
            HStack(alignment: .top, spacing: 0) {
                VStack(spacing: 0) {
                    chartContent
                        // The actual width of the plot area is three times of page width
                        .frame(width: chartContentContainerWidth * 3, height: height)
                        .offset(x: translation)
                        .offset(x: offset)
                        .gesture(drag)
                    // This is a magic component to avoid some weird UI behavior
                    Text("")
                }
                .frame(width: chartContentContainerWidth)
                .clipped()
                
                chartYAxis
                    .modifier(YAxisWidthModifier())
                    .onPreferenceChange(YAxisWidthPreferenceyKey.self) { newValue in
                        yAxisWidth = newValue
                        chartContentContainerWidth = geometry.size.width - yAxisWidth
                    }
            }
        }
        .frame(height: height)
    }
    
    var chart: some View {
        BarChart(unitOffset: $currentUnitOffset)
    }
    
    var chartContent: some View {
        chart
            .chartXAxis {
                AxisMarks(
                    format: .dateTime.weekday().locale(locale),
                    preset: .extended,
                    values: .stride(by: .day)
                )
            }
            .chartYAxis {
                AxisMarks(position: .trailing, values: .automatic(desiredCount: 4)) {
                    AxisGridLine()
                }
            }
    }
    
    var chartYAxis: some View {
        chart
            .foregroundStyle(.clear)
            .chartYAxis {
                AxisMarks(position: .trailing, values: .automatic(desiredCount: 4))
            }
            .chartPlotStyle { plot in
                plot.frame(width: 0)
            }
    }
}

struct InfinityScrollChart_Previews: PreviewProvider {
    static var previews: some View {
        InfiniteScrollChart()
            .padding(.horizontal, 4)
            .environment(\.locale, .init(identifier: "zh"))
    }
}

In WWDC23, scrollable charts are officially supported in iOS 17.
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