Animate changes to Array - SwiftUI ForEach

Question

I'm new to SwiftUI coming from UIKit. I'm trying to achieve the following:

Given an array of mutable states in a view model that represents current progress to be displayed on a Circle using trim(from:to:)
Loop over each item to render a circle with the current progress and animate changes from the old to the new position
A view model will periodically update the array that is a @Published value in an ObservableObject.

I setup a demo view that looks like this


@StateObject var demoViewModel: DemoViewModel = DemoViewModel()

var body: some View {
            ForEach(demoViewModel.progress, id: \.self) { progress in
                Circle()
                    .trim(from: 0, to: progress.progress)
                     .stroke(style: StrokeStyle(lineWidth: 15, lineCap: .round))
                     .foregroundColor(.red)
                     .frame(width: 100)
                     .rotationEffect(.degrees(-90))
                     .animation(.linear(duration: 1), value: demoViewModel.progress)
            }
        }

and a test view model like this

class DemoViewModel: ObservableObject {

    @Published var progress: [Demo] = [.init(progress: 0)]

    struct Demo: Hashable {
        let progress: Double
    }

    init() {
        Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { timer in
            let current = self.progress[0]

            if current.progress < 1 {
                self.progress[0] = .init(progress: current.progress + 0.1)
            }

        }
    }
}

For the sake of the demo the array only has one item. What I expected was on each iteration a new value is written to the item in the array which triggers the animation to animate to that value. What actually happens is the progress updates, but it jumps to each new value for no animation.

workingdog support Ukraine · Accepted Answer · 2022-11-14 23:44:50Z

2

Try this approach, where the self.demos[0].progress in init() is updated rather than creating a new Demo(...) at each time tick.

Note, modified a few other things as well, such as the .animation(...value:..) and in particular the ForEach loop with the added Demo unique id.

struct ContentView: View {
    @StateObject var demoViewModel: DemoViewModel = DemoViewModel()
    
    var body: some View {
        VStack {
            ForEach(demoViewModel.demos) { demo in  // <-- here
                Circle()
                    .trim(from: 0, to: demo.progress)
                    .stroke(style: StrokeStyle(lineWidth: 15, lineCap: .round))
                    .foregroundColor(.red)
                    .frame(width: 100)
                    .rotationEffect(.degrees(-90))
                    .animation(.linear(duration: 1), value: demo.progress) // <-- here
            }
        }
    }
}

class DemoViewModel: ObservableObject {
    @Published var demos:[Demo] = [Demo(progress: 0)]
    
    struct Demo: Identifiable, Hashable {  // <-- here
        let id = UUID()  // <-- here
        var progress: Double  // <-- here
    }
    
    init() {
        Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { timer in
            if self.demos[0].progress < 1 {
                self.demos[0].progress = self.demos[0].progress + 0.1 // <-- here
            }
        }
    }
   
}

edited Nov 14, 2022 at 23:44

answered Nov 14, 2022 at 23:29

workingdog support Ukraine

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1 Comment

Nick Over a year ago

This worked great. Seems like I need to ready a bit more about identity in SwiftUI. Thanks!

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