How to scale text to fit parent view with SwiftUI?

One possible "hack" is to use a big font size and a small scale factor so it will shrink itself:

ZStack {
    Circle().strokeBorder(Color.red, lineWidth: 30)

    Text("Text")
        .padding(40)
        .font(.system(size: 500))
        .minimumScaleFactor(0.01)
     }
}

One can use GeometryReader in order to make it also work in landscape mode.

It first checks if the width or the height is smaller and then adjusts the font size according to the smaller of these.

GeometryReader{g in
    ZStack {
        Circle().strokeBorder(Color.red, lineWidth: 30)
        Text("Text")
            .font(.system(size: g.size.height > g.size.width ? g.size.width * 0.4: g.size.height * 0.4))
    }
}