GooeyView.swift

import SwiftUI

struct GooeyView: View {
    @State
    var yPos: Double = 0.75

    @State
    var threshold: Double = 0.5

    @State
    var radius: Double = 15

    var body: some View {
        VStack(alignment: .leading) {
            Text("Preview")
            Color.pink.mask {
                Canvas { ctx, size in
                    ctx.addFilter(.alphaThreshold(min: threshold))

                    ctx.addFilter(.blur(radius: radius))

                    // This drawing operation needs to happen on a separate layer
                    // for the effect to work.
                    ctx.drawLayer { ctx in
                        var rect = CGRect(x: 10, y: 50, width: 150, height: 150)

                        ctx.fill(Circle().path(in: rect), with: .color(.black))

                        rect.origin.x += yPos * (size.width - rect.width - 20.0)

                        ctx.fill(Circle().path(in: rect), with: .color(.black))
                    }
                }
            }

            Group {
                Text("y-Position")
                Slider(value: $yPos)

                Divider()

                Text("Threshold")
                Slider(value: $threshold, in: 0.01 ... 0.99)

                Divider()

                Text("Radius")
                Slider(value: $radius, in: 0 ... 30)
            }

        }
        .padding()
        .font(.caption2)
    }
}

struct GooeyView_Previews: PreviewProvider {
    static var previews: some View {
        GooeyView()
    }
}

Super clever!

^^^^^