最好是自己制作符合Shape的HalfCircle，而不是试图将Circle拼凑成您需要的形状。让我们更进一步，使其符合InsettableShape，以防您想要使用它来描边。

import SwiftUI
struct HalfCircle: InsettableShape {
    var _inset: CGFloat = 0
    func inset(by amount: CGFloat) -> Self {
        var copy = self
        copy._inset += amount
        return copy
    }
    func path(in rect: CGRect) -> Path {
        var path = Path()
        // This is a half-circle centered at the origin with radius 1.
        path.addArc(
            center: .zero,
            radius: 1,
            startAngle: .zero,
            endAngle: .radians(.pi),
            clockwise: false
        )
        path.closeSubpath()
        // Since it's the bottom half of a circle, we only want
        // to inset the left, right, and bottom edges of rect.
        let rect = rect
            .insetBy(dx: _inset, dy: 0.5 * _inset)
            .offsetBy(dx: 0, dy: -(0.5 * _inset))
        // This transforms bounding box of the path to fill rect.
        let transform = CGAffineTransform.identity
            .translatedBy(x: rect.origin.x + 0.5 * rect.size.width, y: 0)
            .scaledBy(x: rect.width / 2, y: rect.height)
        return path.applying(transform)
    }
}

我们可以用它来画这个：

使用这个Playground：

import PlaygroundSupport
PlaygroundPage.current.setLiveView(HalfCircle()
    .inset(by: 20)
    .fill(.black)
    .background(.gray)
    .aspectRatio(2, contentMode: .fit)
    .frame(width: 200)
    .padding()
)

您可以使用GeometryReader以不同方式对其进行掩码：

GeometryReader { proxy in
    Circle()
        .fill(.blue)
        .offset(y: -proxy.size.height) // <- Shows bottom part
        .frame(height: proxy.size.width) // <- Makes the circle match frame
        .background(.red) // <- Just for testing
}
.aspectRatio(2, contentMode: .fit) // <- Makes the frame ration 2 : 1 (landscape rectangle)
.clipped() // <- Removes out of bound drawings