where method Null safety

ValueListenable<T> where(
  1. bool selector(
    1. T
    )
)

where allows you to set a filter on a ValueListenable so that an installed handler function is only called if the passed selector function returns true. Because the selector function is called on every new value you can change the filter during runtime.

ATTENTION: Due to the nature of ValueListeners that they always have to have a value the filter can't work on the initial value. Therefore it's not advised to use where inside the Widget tree if you use setState because that will recreate the underlying WhereValueNotifier again passing through the lates value of the this even if it doesn't fulfill the selector condition. Therefore it's better not to use it directly in the Widget tree but in your state objects

example: lets only print even values

final sourceListenable = ValueNotifier<int>(0);
final subscription = sourceListenable.where( (x)=>x.isEven )
   .listen( (s,_) => print(x) );

Implementation

ValueListenable<T> where(bool Function(T) selector) {
  return WhereValueNotifier(this.value, this, selector);
}