contour_gradient

A Flutter ShapeBorder that paints a gradient along the length of a border, for any OutlinedBorder shape.

Gradient borders on a star, rounded rectangle, circle, button and chip

A SweepGradient or a gradient background behind a padded child colors a border by angle or by position in the box, so the colors bunch up on long sides and stretch on short ones. ContourGradientBorder places colors by distance along the border instead: a color at 0.5 sits exactly halfway around, whatever the shape.

Usage

ContourGradientBorder is an OutlinedBorder, so it works anywhere Flutter accepts a shape. The outline comes from shape, and the width and alignment come from side.

Container(
  width: 200,
  height: 80,
  decoration: const ShapeDecoration(
    shape: ContourGradientBorder(
      colors: [Colors.purple, Colors.blue, Colors.red],
      shape: RoundedRectangleBorder(
        borderRadius: BorderRadius.all(Radius.circular(16)),
      ),
      side: BorderSide(width: 3),
    ),
  ),
)

Buttons and chips

OutlinedButton and Chip apply their own side to the shape, so set the width there:

OutlinedButton(
  style: OutlinedButton.styleFrom(
    shape: const ContourGradientBorder(
      colors: [Colors.purple, Colors.blue, Colors.red],
      shape: StadiumBorder(),
    ),
    side: const BorderSide(width: 2),
  ),
  onPressed: () {},
  child: const Text('Button'),
)

Other buttons, Card and FloatingActionButton use the border's own side.

Text fields

InputDecoration takes an InputBorder, so text fields use ContourGradientInputBorder. It takes the same colors, stops, startOffset and shape, and leaves a gap in the top of the border for a floating label, like OutlineInputBorder:

TextField(
  decoration: InputDecoration(
    labelText: 'Name',
    border: ContourGradientInputBorder(
      colors: [Colors.purple, Colors.blue, Colors.red],
      shape: RoundedRectangleBorder(
        borderRadius: BorderRadius.all(Radius.circular(16)),
      ),
    ),
  ),
)

The field applies its own side, so the border gets thicker when the field has focus. The side's color is ignored, so an error does not change the border's colors; set errorBorder and focusedErrorBorder to show errors. The border also animates to and from an OutlineInputBorder, so you can use one for some states and the other for the rest.

Animation

startOffset moves the gradient along the border, as a fraction of its length. Animating it from 0.0 to 1.0 moves the gradient once around, and because 0.0 and 1.0 look the same, a repeating animation has no visible jump. The example below repeats the first color at the end, so no hard edge travels around the border either (see How colors are placed).

class Spinning extends StatefulWidget {
  const Spinning({super.key});

  @override
  State<Spinning> createState() => _SpinningState();
}

class _SpinningState extends State<Spinning>
    with SingleTickerProviderStateMixin {
  late final AnimationController _controller = AnimationController(
    vsync: this,
    duration: const Duration(seconds: 3),
  )..repeat();

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return AnimatedBuilder(
      animation: _controller,
      builder: (context, child) => Container(
        width: 120,
        height: 120,
        decoration: ShapeDecoration(
          shape: ContourGradientBorder(
            colors: const [
              Colors.purple,
              Colors.blue,
              Colors.red,
              Colors.purple,
            ],
            startOffset: _controller.value,
            shape: const StarBorder(),
            side: const BorderSide(width: 4),
          ),
        ),
      ),
    );
  }
}

The border also interpolates, so AnimatedContainer and other implicit animations can change its shape, width, colors and offset. It interpolates with plain OutlinedBorders too, which it treats as a single color.

How colors are placed

Position 0.0 is the point of the outline nearest the top-left corner of the box, and the gradient runs clockwise from there. On rounded rectangles it starts halfway around the top-left corner.
The gradient runs from the first color to the last and does not blend back into the first, so where the end of the gradient meets its start the colors change in a hard edge. To blend back into the first color instead, repeat it at the end:
```
ContourGradientBorder(
  colors: [Colors.purple, Colors.blue, Colors.red, Colors.purple],
)
```
Without stops, the colors are spaced evenly from 0.0 to 1.0. On a closed border the repeated first color joins up with itself across position 0.0, so it gets the same share of the border as each of the others.
With stops, each color sits at its stop. Before the first stop the border is the first color, and after the last stop it is the last color. Two equal stops make a hard edge:
```
ContourGradientBorder(
  colors: [Colors.red, Colors.red, Colors.blue, Colors.blue],
  stops: [0.0, 0.5, 0.5, 1.0],
)
```
A LinearBorder runs the gradient along only the edges it draws. Edges that meet at a corner form one line, which starts at its end nearest the top-left corner of the box: an underline runs left to right, and a start and bottom edge run down and then right. If all four edges meet, they form a loop like a rectangle. Separate lines, such as a top and a bottom edge, share the gradient one after the other.
On an open line, such as an underline, the gradient runs from the first color at one end to the last color at the other. When startOffset moves it along, the colors that pass one end come back in at the other, so repeat the first color at the end here too to avoid a hard edge.
A single color paints a plain border.

Supported shapes

Any OutlinedBorder works, including RoundedRectangleBorder, StadiumBorder, CircleBorder, OvalBorder, StarBorder, RoundedSuperellipseBorder, BeveledRectangleBorder, ContinuousRectangleBorder, LinearBorder and your own subclasses.

Performance

Every border is painted by masking a colored band with the shape's own border, so its edges match Flutter's exactly. That costs two Canvas.saveLayer calls per paint.

For RoundedRectangleBorder, StadiumBorder and circular CircleBorder, the band is quick to build and costs well under a millisecond of CPU time, even when animated. Other shapes take about 1–2 ms per paint on a desktop machine. That is fine for static borders, which are only painted when their area repaints, and for a few animated ones. Animating many such borders at once can drop frames on slower phones.

Limitations

Keep the border narrow compared to the shape's detail. When a border is so wide that it meets itself, as across the arms of a StarBorder or inside the curves of a rounded one, each part takes the color of the nearest part of the outline. The colors then meet at sharp seams, and a few pixels along them can be left uncolored.

Example

The example app shows every supported shape, buttons, cards, chips, text fields, animation, morphing and stops.