How to make width of view equal to superview in SwiftUI

You can use GeometryReader: https://developer.apple.com/documentation/swiftui/geometryreader

According to Apple:

This view returns a flexible preferred size to its parent layout.

It is a flexible solution, as it changes according to the parent layout changes.

Simple adding following to your code will make the button extend edge to edge. (You can add padding as well if you want)

.frame(minWidth: 0, maxWidth: .infinity)

The entire Button code will look like this:

struct ContentView : View {
    var body: some View {
        HStack {
            Button(action: {}) {
                Text("MyButton")
                    .color(.white)
                    .font(Font.system(size: 17))
            }
            .frame(minWidth: 0, maxWidth: .infinity)
            .padding(.top, 8)
            .padding(.bottom, 8)
            .background(Color.red, cornerRadius: 0)
        }
    }
}

You need to use .frame(minWidth: 0, maxWidth: .infinity) modifier

Add the next code

        Button(action: tap) {
            Text("Button")
                .frame(minWidth: 0, maxWidth: .infinity)
                .background(Color.red)
        }
        .padding(.horizontal, 20)

Padding modifiers will allow you to have some space from the edge.

Keep in mind that the order of modifiers is essential. Because modifiers are functions that are wrapping the view below (they do not change properties of views)

Here's a pure SwiftUI solution where you want to fill the width of the parent but constrain the height to an arbitrary aspect ratio (say you want square images):

        Image("myImage")
            .resizable()
            .scaledToFill()
            .frame(maxWidth: .infinity, maxHeight: .infinity)
            .aspectRatio(16.0 / 9.0, contentMode: .fit)

Just replace 16.0 / 9.0 with whatever aspect ratio you want.

I spent about a day trying to figure this out because I didn't want to use GeometryReader or UIScreen.main.bounds (which isn't cross-platform)

Edit: Found an even simpler way.