# lerp method Null safety

lerp(
)

Linearly interpolate between two BoxConstraints.

If either is null, this function interpolates from a BoxConstraints object whose fields are all set to 0.0.

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.

## Implementation

``````static BoxConstraints? lerp(BoxConstraints? a, BoxConstraints? b, double t) {
assert(t != null);
if (a == null && b == null)
return null;
if (a == null)
return b! * t;
if (b == null)
return a * (1.0 - t);
assert(a.debugAssertIsValid());
assert(b.debugAssertIsValid());
assert((a.minWidth.isFinite && b.minWidth.isFinite) || (a.minWidth == double.infinity && b.minWidth == double.infinity), 'Cannot interpolate between finite constraints and unbounded constraints.');
assert((a.maxWidth.isFinite && b.maxWidth.isFinite) || (a.maxWidth == double.infinity && b.maxWidth == double.infinity), 'Cannot interpolate between finite constraints and unbounded constraints.');
assert((a.minHeight.isFinite && b.minHeight.isFinite) || (a.minHeight == double.infinity && b.minHeight == double.infinity), 'Cannot interpolate between finite constraints and unbounded constraints.');
assert((a.maxHeight.isFinite && b.maxHeight.isFinite) || (a.maxHeight == double.infinity && b.maxHeight == double.infinity), 'Cannot interpolate between finite constraints and unbounded constraints.');
return BoxConstraints(
minWidth: a.minWidth.isFinite ? ui.lerpDouble(a.minWidth, b.minWidth, t)! : double.infinity,
maxWidth: a.maxWidth.isFinite ? ui.lerpDouble(a.maxWidth, b.maxWidth, t)! : double.infinity,
minHeight: a.minHeight.isFinite ? ui.lerpDouble(a.minHeight, b.minHeight, t)! : double.infinity,
maxHeight: a.maxHeight.isFinite ? ui.lerpDouble(a.maxHeight, b.maxHeight, t)! : double.infinity,
);
}``````