refreshable-scrollview-implementation.swift

// Authoer: The SwiftUI Lab
// Full article: https://swiftui-lab.com/scrollview-pull-to-refresh/

import SwiftUI

struct RefreshableScrollView<Content: View>: View {
    @State private var previousScrollOffset: CGFloat = 0
    @State private var scrollOffset: CGFloat = 0
    @State private var frozen: Bool = false
    @State private var rotation: Angle = .degrees(0)
    
    var threshold: CGFloat = 80
    @Binding var refreshing: Bool
    let content: Content

    init(height: CGFloat = 80, refreshing: Binding<Bool>, @ViewBuilder content: () -> Content) {
        self.threshold = height
        self._refreshing = refreshing
        self.content = content()

    }
    
    var body: some View {
        return VStack {
            ScrollView {
                ZStack(alignment: .top) {
                    MovingView()
                    
                    VStack { self.content }.alignmentGuide(.top, computeValue: { d in (self.refreshing && self.frozen) ? -self.threshold : 0.0 })
                    
                    SymbolView(height: self.threshold, loading: self.refreshing, frozen: self.frozen, rotation: self.rotation)
                }
            }
            .background(FixedView())
            .onPreferenceChange(RefreshableKeyTypes.PrefKey.self) { values in
                self.refreshLogic(values: values)
            }
        }
    }
    
    func refreshLogic(values: [RefreshableKeyTypes.PrefData]) {
        DispatchQueue.main.async {
            // Calculate scroll offset
            let movingBounds = values.first { $0.vType == .movingView }?.bounds ?? .zero
            let fixedBounds = values.first { $0.vType == .fixedView }?.bounds ?? .zero
            
            self.scrollOffset  = movingBounds.minY - fixedBounds.minY
            
            self.rotation = self.symbolRotation(self.scrollOffset)
            
            // Crossing the threshold on the way down, we start the refresh process
            if !self.refreshing && (self.scrollOffset > self.threshold && self.previousScrollOffset <= self.threshold) {
                self.refreshing = true
            }
            
            if self.refreshing {
                // Crossing the threshold on the way up, we add a space at the top of the scrollview
                if self.previousScrollOffset > self.threshold && self.scrollOffset <= self.threshold {
                    self.frozen = true

                }
            } else {
                // remove the sapce at the top of the scroll view
                self.frozen = false
            }
            
            // Update last scroll offset
            self.previousScrollOffset = self.scrollOffset
        }
    }
    
    func symbolRotation(_ scrollOffset: CGFloat) -> Angle {
        
        // We will begin rotation, only after we have passed
        // 60% of the way of reaching the threshold.
        if scrollOffset < self.threshold * 0.60 {
            return .degrees(0)
        } else {
            // Calculate rotation, based on the amount of scroll offset
            let h = Double(self.threshold)
            let d = Double(scrollOffset)
            let v = max(min(d - (h * 0.6), h * 0.4), 0)
            return .degrees(180 * v / (h * 0.4))
        }
    }
    
    struct SymbolView: View {
        var height: CGFloat
        var loading: Bool
        var frozen: Bool
        var rotation: Angle
        
        
        var body: some View {
            Group {
                if self.loading { // If loading, show the activity control
                    VStack {
                        Spacer()
                        ActivityRep()
                        Spacer()
                    }.frame(height: height).fixedSize()
                        .offset(y: -height + (self.loading && self.frozen ? height : 0.0))
                } else {
                    Image(systemName: "arrow.down") // If not loading, show the arrow
                        .resizable()
                        .aspectRatio(contentMode: .fit)
                        .frame(width: height * 0.25, height: height * 0.25).fixedSize()
                        .padding(height * 0.375)
                        .rotationEffect(rotation)
                        .offset(y: -height + (loading && frozen ? +height : 0.0))
                }
            }
        }
    }
    
    struct MovingView: View {
        var body: some View {
            GeometryReader { proxy in
                Color.clear.preference(key: RefreshableKeyTypes.PrefKey.self, value: [RefreshableKeyTypes.PrefData(vType: .movingView, bounds: proxy.frame(in: .global))])
            }.frame(height: 0)
        }
    }
    
    struct FixedView: View {
        var body: some View {
            GeometryReader { proxy in
                Color.clear.preference(key: RefreshableKeyTypes.PrefKey.self, value: [RefreshableKeyTypes.PrefData(vType: .fixedView, bounds: proxy.frame(in: .global))])
            }
        }
    }
}

struct RefreshableKeyTypes {
    enum ViewType: Int {
        case movingView
        case fixedView
    }

    struct PrefData: Equatable {
        let vType: ViewType
        let bounds: CGRect
    }

    struct PrefKey: PreferenceKey {
        static var defaultValue: [PrefData] = []

        static func reduce(value: inout [PrefData], nextValue: () -> [PrefData]) {
            value.append(contentsOf: nextValue())
        }

        typealias Value = [PrefData]
    }
}

struct ActivityRep: UIViewRepresentable {
    func makeUIView(context: UIViewRepresentableContext<ActivityRep>) -> UIActivityIndicatorView {
        return UIActivityIndicatorView()
    }
    
    func updateUIView(_ uiView: UIActivityIndicatorView, context: UIViewRepresentableContext<ActivityRep>) {
        uiView.startAnimating()
    }
}

Hey, please don't use this code. If you keep on refreshing the view hierarchy like this it will cause you performance issues.

Use Swift introspect instead + the underlying UIScrollView. Much better performance.

These are the limitations of SwiftUI we have to live with at the moment. Peace.

Hey, please don't use this code. If you keep on refreshing the view hierarchy like this it will cause you performance issues.

Use Swift introspect instead + the underlying UIScrollView. Much better performance.

These are the limitations of SwiftUI we have to live with at the moment. Peace.

Hey, you're right ! My app lag very much... Do you have a github repo where i can see how you do for "the pull to refresh" feature please please thanks !

Hey, please don't use this code. If you keep on refreshing the view hierarchy like this it will cause you performance issues.

Use Swift introspect instead + the underlying UIScrollView. Much better performance.

These are the limitations of SwiftUI we have to live with at the moment. Peace.

But you can't use LayVStack, LazyHStack with UIScrollView. That's why I can't go back to UIScrollView.

Hi, I have an issue, when I push scroll and left it before data are completely loaded, I have the small bounce of the scroll view content and the loader is showing under the content, all the time when data are coming from the ajax call?

I got it to work just fine when I commented out these lines of code:

if self.previousScrollOffset > self.threshold && self.scrollOffset <= self.threshold {
    self.frozen = true
}

So I am not adding extra space because data are coming faster than space is created and I have a small bounce when the loader is shown.

Do you have any suggestions?

Thank you.

Does not work well with NavigationView's large navigation title