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"))
    }
}

@beader Ok, I understand what your saying. But why does it initially show the middle part of the big chart and not the first 7 elements? Is it something Swift does automatically?

To make it left-scrollable.

What happens if the data you have is current, i.e. the chart can't be scrolled left and right, and you would like to be able to scroll left going backwards in time? The offset doesn't seem to work...

@iamgp
I added a spring-like pull once you reach the right-most part of the graph.

Some other changes in this code you may want to parse out:

• Easier to flick to next week (harder to slow drag)
• When you flick it goes to an offset of 7
  (ex. For a Monday start week. If you slow drag back to see Sunday, then flick back a week, you end up on another Monday not 7 days before the Sunday you looked at. I believe this is the behavior in Apples Health App)

DragGesture(minimumDistance: 0)
        .updating($translation) { value, state, _ in
            state = calculateTranslationFromDrag(dragWidth: value.translation.width)
        }
        .onEnded { value in
            let realTranslation = calculateTranslationFromDrag(dragWidth: value.translation.width)

            offset = offset + realTranslation
            
            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
            
            if unitOffset != predictedUnitOffset {
                predictedUnitOffset = copysign(Double(numBins), predictedUnitOffset)
            }
            
            if abs(predictedUnitOffset) >= Double(numBins) {
                predictedUnitOffset = copysign(Double(numBins), predictedUnitOffset)
            
                let expectedOffset = currentUnitOffset - Int(predictedUnitOffset)
                let nearestNewPage = Double(numBins) * (Double(expectedOffset) / Double(numBins)).rounded(.toNearestOrAwayFromZero)
                predictedUnitOffset = Double(currentUnitOffset) - nearestNewPage
            }
            
            if currentUnitOffset - Int(predictedUnitOffset) > 0 {
                predictedUnitOffset = (CGFloat)(currentUnitOffset)
            }
            
            withAnimation(.easeOut(duration: pagingAnimationDuration)) {
                offset = predictedUnitOffset * unitWidth
            }
            DispatchQueue.main.asyncAfter(deadline: .now() + pagingAnimationDuration) {
                currentUnitOffset = currentUnitOffset - Int(offset / unitWidth)
                offset = 0
            }
        }


private func calculateTranslationFromDrag(dragWidth: CGFloat) -> CGFloat {
      let unitWidth = chartContentContainerWidth / Double(numBins)
        let currentOffset = CGFloat(currentUnitOffset)
        if currentOffset - (dragWidth/unitWidth) > 0 {
            let backToZeroWidth = currentOffset*unitWidth
            let extraWidth = currentOffset - dragWidth/unitWidth
            let springPullWidth = (extraWidth + 1).squareRoot() - 1
            
            return backToZeroWidth - (springPullWidth * unitWidth)
        }
        else {
            return dragWidth
        }
    }

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