How can I add a border to a widget in Flutter?

You can add the Text as a child to a Container that has a BoxDecoration with border property:

Container(
  margin: const EdgeInsets.all(15.0),
  padding: const EdgeInsets.all(3.0),
  decoration: BoxDecoration(
    border: Border.all(color: Colors.blueAccent)
  ),
  child: Text('My Awesome Border'),
)

Here is an expanded answer. A DecoratedBox is what you need to add a border, but I am using a Container for the convenience of adding margin and padding.

Here is the general setup.

Widget myWidget() {
  return Container(
    margin: const EdgeInsets.all(30.0),
    padding: const EdgeInsets.all(10.0),
    decoration: myBoxDecoration(), //             <--- BoxDecoration here
    child: Text(
      "text",
      style: TextStyle(fontSize: 30.0),
    ),
  );
}

where the BoxDecoration is

BoxDecoration myBoxDecoration() {
  return BoxDecoration(
    border: Border.all(),
  );
}

Border width

These have a border width of 1, 3, and 10 respectively.

BoxDecoration myBoxDecoration() {
  return BoxDecoration(
    border: Border.all(
      width: 1, //                   <--- border width here
    ),
  );
}

Border color

These have a border color of

• Colors.red
• Colors.blue
• Colors.green

Code

BoxDecoration myBoxDecoration() {
  return BoxDecoration(
    border: Border.all(
      color: Colors.red, //                   <--- border color
      width: 5.0,
    ),
  );
}

Border side

These have a border side of

• left (3.0), top (3.0)
• bottom (13.0)
• left (blue[100], 15.0), top (blue[300], 10.0), right (blue[500], 5.0), bottom (blue[800], 3.0)

Code

BoxDecoration myBoxDecoration() {
  return BoxDecoration(
    border: Border(
      left: BorderSide( //                   <--- left side
        color: Colors.black,
        width: 3.0,
      ),
      top: BorderSide( //                    <--- top side
        color: Colors.black,
        width: 3.0,
      ),
    ),
  );
}

Border radius

These have border radii of 5, 10, and 30 respectively.

BoxDecoration myBoxDecoration() {
  return BoxDecoration(
    border: Border.all(
      width: 3.0
    ),
    borderRadius: BorderRadius.all(
        Radius.circular(5.0) //                 <--- border radius here
    ),
  );
}

Going on

DecoratedBox/BoxDecoration are very flexible. Read Flutter — BoxDecoration Cheat Sheet for many more ideas.

The best way is using BoxDecoration()

Advantage

• You can set the border of a widget
• You can set the border Color or Width
• You can set a Rounded corner of a border
• You can add a Shadow of a widget

Disadvantage

• BoxDecoration only use with Container widget, so you want to wrap your widget in Container()

Example

    Container(
      margin: EdgeInsets.all(10),
      padding: EdgeInsets.all(10),
      alignment: Alignment.center,
      decoration: BoxDecoration(
        color: Colors.orange,
        border: Border.all(
            color: Colors.pink[800], // Set border color
            width: 3.0),   // Set border width
        borderRadius: BorderRadius.all(
            Radius.circular(10.0)), // Set rounded corner radius
        boxShadow: [BoxShadow(blurRadius: 10,color: Colors.black,offset: Offset(1,3))] // Make rounded corner of border
      ),
      child: Text("My demo styling"),
    )

As stated in the documentation, Flutter prefers composition over parameters.

Most of the time you're not looking for a property, but instead a wrapper (and sometimes a few helpers/"builder").

For borders, you want 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.