Making Button span across VStack

@d.felber's answer is almost complete, but you'd need a Spacer() on each side to center:

Button(action: {
    // TODO: ...
}) {
    HStack {
        Spacer()
        Text("Sign In")
        Spacer()
    }
}

The best way to do this is via .frame(maxWidth: .infinity) https://developer.apple.com/documentation/swiftui/view-layout

If you want the button not to be centered you need to specify alignment.
e.g.: .frame(maxWidth: .infinity, alignment: .leading)

Button(action: handleSignInAction) {
    Text("Sign In")
}
.frame(maxWidth: .infinity)
.background(Color.green)

Old answer from 2019:

You could use a HStack with a Text and Spacer to get a Button that fills the width of its parent:

Button(action: handleSignInAction) {
    HStack {
        Spacer()
        Text("Sign In")
        Spacer()
    }
}.background(Color.green)

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