# lerpFrom method

- ShapeBorder? a,
- double t

Linearly interpolates from another ShapeBorder (possibly of another
class) to `this`

.

When implementing this method in subclasses, return null if this class
cannot interpolate from `a`

. In that case, lerp will try `a`

's lerpTo
method instead. If `a`

is null, this must not return null.

The base class implementation handles the case of `a`

being null by
deferring to scale.

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 `this`

(or something equivalent to `this`

), and values in
between meaning that the interpolation is at the relevant point on the
timeline between `a`

and `this`

. 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.

Instead of calling this directly, use ShapeBorder.lerp.

## Implementation

```
@override
ShapeBorder? lerpFrom(ShapeBorder? a, double t) {
if (a is StadiumBorder) {
return StadiumBorder(side: BorderSide.lerp(a.side, side, t));
}
if (a is CircleBorder) {
return _StadiumToCircleBorder(
side: BorderSide.lerp(a.side, side, t),
circularity: 1.0 - t,
eccentricity: a.eccentricity,
);
}
if (a is RoundedRectangleBorder) {
return _StadiumToRoundedRectangleBorder(
side: BorderSide.lerp(a.side, side, t),
borderRadius: a.borderRadius,
rectilinearity: 1.0 - t,
);
}
return super.lerpFrom(a, t);
}
```