computeWidth method
计算终值
Implementation
// double computeWidth(Size size) => literalGetter != null ? pxUnitDoubleGetter(literalGetter!.call(size))(size) + percentGetter(size) * size.width : percentGetter(size) * size.width;
// double computeHeight(Size size) => literalGetter != null ? pxUnitDoubleGetter(literalGetter!.call(size))(size) + percentGetter(size) * size.height : percentGetter(size) * size.height;
double computeWidth(Size viewport) => expression.computeWidth(viewport);