Full width button w/ flex-box in react-native

I came here looking for the same question. Having experimented further, I’ve found the simplest way is to use alignSelf: 'stretch'. This forces the individual element to take up the full width available e.g.

 button: {
    alignSelf: 'stretch'
 }

Nader’s answer does work of course, but this would seem to be the correct way using Flexbox.