[flutter] How to add a border to a widget in Flutter?

As stated in the documentation, flutter prefer composition over parameters. Most of the time what you're looking for is not a property, but instead a wrapper (and sometimes a few helpers/"builder")

For borders what you want is DecoratedBox, which has a decoration property that defines borders ; but also background images or shadows.

Alternatively like @Aziza said, you can use Container. Which is the combination of DecoratedBox, SizedBox and a few other useful widgets.