Flutter: Outline input border

For others coming here who just want a TextField with a border all around it:

TextField(
  decoration: InputDecoration(
    border: OutlineInputBorder(),
  ),
),

What your looking for is - enabledBorder property of InputDecoration.

If you want to Change Border on focus use - focusedBorder

    TextField(
        decoration: new InputDecoration(
            focusedBorder: OutlineInputBorder(
                borderSide: BorderSide(color: Colors.greenAccent, width: 5.0),
            ),
            enabledBorder: OutlineInputBorder(
                borderSide: BorderSide(color: Colors.red, width: 5.0),
            ),
            hintText: 'Mobile Number',
        ),
    ),