lerpWhere<T> static method

FWidgetStateMap<T> lerpWhere<T>(
  1. FWidgetStateMap<T> a,
  2. FWidgetStateMap<T> b,
  3. double t,
  4. T? lerp(
    1. T?,
    2. T?,
    3. double
    ),
)

Linearly interpolate between two FWidgetStateMaps using the provided lerp function.

Consider using lerpBoxDecoration, lerpColor, lerpIconThemeData, or lerpTextStyle.

Constraint operand order matters

TL;DR: Always make sure that constraint operands are exactly the same across a and b.

WidgetStatesConstraints are not commutative. Reversed constraint operands (like .hovered & .pressed vs .pressed & .hovered) are treated as distinct keys.

For each constraint in either map, lerp is called with the values from both maps, using null for missing values.

WidgetState.pressed && Widget
final a = FWidgetStateMap<double>({
  WidgetState.pressed: 1.0,
  WidgetState.hovered & Widget.focused: 0.8,
});

final b = FWidgetStateMap<double>({
  WidgetState.pressed: 0.5,
  WidgetState.focused & Widget.focused: 0.9,
});

final result = FWidgetStateMap.lerpWhere(a, b, 0.5, lerpDouble);
// Result contains:
// - .pressed: 0.75 (lerp between 1.0 and 0.5)
// - .hovered & .focused: 0.4 (lerp between 0.8 and null)
// - .focused & .hovered: 0.45 (lerp between null and 0.9)

As resolve treats state combinations as equivalent regardless of order, {.hovered & .focused} will always return 0.4, the first matching constraint, and never 0.45, the last matching constraint.

Implementation

static FWidgetStateMap<T> lerpWhere<T>(
  FWidgetStateMap<T> a,
  FWidgetStateMap<T> b,
  double t,
  T? Function(T?, T?, double) lerp,
) {
  final visited = <WidgetStatesConstraint>{};
  final constraints = <WidgetStatesConstraint, T>{};

  for (final MapEntry(key: constraint, :value) in a._constraints.entries) {
    visited.add(constraint);
    if (lerp(value, b._constraints[constraint], t) case final lerped?) {
      constraints[constraint] = lerped;
    }
  }

  for (final MapEntry(key: constraint, :value) in b._constraints.entries) {
    if (!visited.contains(constraint)) {
      if (lerp(a._constraints[constraint], value, t) case final lerped?) {
        constraints[constraint] = lerped;
      }
    }
  }

  return FWidgetStateMap(constraints);
}