RadialGradient class

A 2D radial gradient.

This class is used by BoxDecoration to represent radial gradients. This abstracts out the arguments to the ui.Gradient.radial constructor from the dart:ui library.

A normal radial gradient has a center and a radius. The center point corresponds to 0.0, and the ring at radius from the center corresponds to 1.0. These lengths are expressed in fractions, so that the same gradient can be reused with varying sized boxes without changing the parameters. (This contrasts with ui.Gradient.radial, whose arguments are expressed in logical pixels.)

It is also possible to create a two-point (or focal pointed) radial gradient (which is sometimes referred to as a two point conic gradient, but is not the same as a CSS conic gradient which corresponds to a SweepGradient). A focal point and focalRadius can be specified similarly to center and radius, which will make the rendered gradient appear to be pointed or directed in the direction of the focal point. This is only important if focal and center are not equal or focalRadius > 0.0 (as this case is visually identical to a normal radial gradient). One important case to avoid is having focal and center both resolve to Offset.zero when focalRadius > 0.0. In such a case, a valid shader cannot be created by the framework.

The colors are described by a list of Color objects. There must be at least two colors. The stops list, if specified, must have the same length as colors. It specifies fractions of the radius between 0.0 and 1.0, giving concentric rings for each color stop. If it is null, a uniform distribution is assumed.

The region of the canvas beyond radius from the center is colored according to tileMode.

Typically this class is used with BoxDecoration, which does the painting. To use a RadialGradient to paint on a canvas directly, see createShader.

void paintSky(Canvas canvas, Rect rect) {
  const RadialGradient gradient = RadialGradient(
    center: Alignment(0.7, -0.6), // near the top right
    radius: 0.2,
    colors: <Color>[
      Color(0xFFFFFF00), // yellow sun
      Color(0xFF0099FF), // blue sky
    ],
    stops: <double>[0.4, 1.0],
  );
  // rect is the area we are painting over
  final Paint paint = Paint()
    ..shader = gradient.createShader(rect);
  canvas.drawRect(rect, paint);
}

See also:

• LinearGradient, which displays a gradient in parallel lines, and has an example which shows a different way to use Gradient objects.
• SweepGradient, which displays a gradient in a sweeping arc around a center point.
• BoxDecoration, which can take a RadialGradient in its BoxDecoration.gradient property.
• CustomPainter, which shows how to use the above sample code in a custom painter.