lerp method Null safety

BoxDecoration? lerp(
  1. BoxDecoration? a,
  2. BoxDecoration? b,
  3. double t
)
override

Linearly interpolate between two box decorations.

Interpolates each parameter of the box decoration separately.

The shape is not interpolated. To interpolate the shape, consider using a ShapeDecoration with different border shapes.

If both values are null, this returns null. Otherwise, it returns a non-null value. If one of the values is null, then the result is obtained by applying scale to the other value. If neither value is null and t == 0.0, then a is returned unmodified; if t == 1.0 then b is returned unmodified. Otherwise, the values are computed by interpolating the properties appropriately.

The t argument represents position on the timeline, with 0.0 meaning that the interpolation has not started, returning a (or something equivalent to a), 1.0 meaning that the interpolation has finished, returning b (or something equivalent to b), and values in between meaning that the interpolation is at the relevant point on the timeline between a and b. The interpolation can be extrapolated beyond 0.0 and 1.0, so negative values and values greater than 1.0 are valid (and can easily be generated by curves such as Curves.elasticInOut).

Values for t are usually obtained from an Animation<double>, such as an AnimationController.

See also:

Implementation

static BoxDecoration? lerp(BoxDecoration? a, BoxDecoration? b, double t) {
  assert(t != null);
  if (a == null && b == null) {
    return null;
  }
  if (a == null) {
    return b!.scale(t);
  }
  if (b == null) {
    return a.scale(1.0 - t);
  }
  if (t == 0.0) {
    return a;
  }
  if (t == 1.0) {
    return b;
  }
  return BoxDecoration(
    color: Color.lerp(a.color, b.color, t),
    image: t < 0.5 ? a.image : b.image, // TODO(ianh): cross-fade the image
    border: BoxBorder.lerp(a.border, b.border, t),
    borderRadius: BorderRadiusGeometry.lerp(a.borderRadius, b.borderRadius, t),
    boxShadow: BoxShadow.lerpList(a.boxShadow, b.boxShadow, t),
    gradient: Gradient.lerp(a.gradient, b.gradient, t),
    shape: t < 0.5 ? a.shape : b.shape,
  );
}