How to draw part of a solid shape using trim()

Paul Hudson @twostraws July 1st 2021

Updated for Xcode 13.0

SwiftUI lets us draw only part of a stroke or fill for a shape using its trim() modifier, which takes two parameters: a start value and an end value, both stored as a CGFloat between 0 and 1.

For example, if you wanted a semicircle you would write this:

Circle()
    .trim(from: 0, to: 0.5)
    .frame(width: 200, height: 200)

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The bottom half of a circle.

SwiftUI draws its shapes so that 0 degrees is directly to the right, so if you want to change that so 0 degrees is directly up you should apply a rotationEffect() modifier.

For example, this uses a timer to adjust the values being passed into trim() so that a rectangle’s stroke grows over time, like a progress indicator:

struct ContentView: View {
    @State private var completionAmount: CGFloat = 0.0
    let timer = Timer.publish(every: 1, on: .main, in: .common).autoconnect()

    var body: some View {
        Rectangle()
            .trim(from: 0, to: completionAmount)
            .stroke(Color.red, lineWidth: 20)
            .frame(width: 200, height: 200)
            .rotationEffect(.degrees(-90))
            .onReceive(timer) { _ in
                withAnimation {
                    if completionAmount == 1 { 
                        completionAmount = 0
                    } else {
                        completionAmount += 0.2
                    }
                }
            }
    }
}

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