# lerp method Null safety

- ShapeDecoration? a,
- ShapeDecoration? b,
- double t

Linearly interpolate between two shapes.

Interpolates each parameter of the decoration separately.

If both values are null, this returns null. Otherwise, it returns a non-null value, with null arguments treated like a ShapeDecoration whose fields are all null (including the shape, which cannot normally be null).

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:

- Decoration.lerp, which can interpolate between any two types of Decorations, not just ShapeDecorations.
- lerpFrom and lerpTo, which are used to implement Decoration.lerp and which use ShapeDecoration.lerp when interpolating two ShapeDecorations or a ShapeDecoration to or from null.

## Implementation

```
static ShapeDecoration? lerp(ShapeDecoration? a, ShapeDecoration? b, double t) {
assert(t != null);
if (a == null && b == null)
return null;
if (a != null && b != null) {
if (t == 0.0)
return a;
if (t == 1.0)
return b;
}
return ShapeDecoration(
color: Color.lerp(a?.color, b?.color, t),
gradient: Gradient.lerp(a?.gradient, b?.gradient, t),
image: t < 0.5 ? a!.image : b!.image, // TODO(ianh): cross-fade the image
shadows: BoxShadow.lerpList(a?.shadows, b?.shadows, t),
shape: ShapeBorder.lerp(a?.shape, b?.shape, t)!,
);
}
```