combineLatest<TIn2, TOut> method Null safety

ValueListenable<TOut> combineLatest<TIn2, TOut>(
  1. ValueListenable<TIn2> combineWith,
  2. CombiningFunction2<T, TIn2, TOut> combiner

Imagine having two ValueNotifier in you model and you want to update a certain region of the screen with their values every time one of them get updated. combineLatest combines two ValueListenable in that way that it returns a new ValueNotifier that changes its value of TOut whenever one of the input listenables this or combineWith updates its value. This new value is built by the combiner function that is called on any value change of the input listenables.


   class StringIntWrapper {
     final String s;
     final int i;

     StringIntWrapper(this.s, this.i);

     String toString() {
       return '$s:$i';

   final listenable1 = ValueNotifier<int>(0);
   final listenable2 = ValueNotifier<String>('Start');

   final destValues = <StringIntWrapper>[];
   final subscription = listenable1
       .combineLatest<String, StringIntWrapper>(
           listenable2, (i, s) => StringIntWrapper(s, i))
       .listen((x, _) {

   listenable1.value = 42;
   listenable1.value = 43;
   listenable2.value = 'First';
   listenable1.value = 45;

   expect(destValues[0].toString(), 'Start:42');
   expect(destValues[1].toString(), 'Start:43');
   expect(destValues[2].toString(), 'First:43');
   expect(destValues[3].toString(), 'First:45');


ValueListenable<TOut> combineLatest<TIn2, TOut>(
  ValueListenable<TIn2> combineWith,
  CombiningFunction2<T, TIn2, TOut> combiner,
) {
  return CombiningValueNotifier<T, TIn2, TOut>(
    combiner(this.value, combineWith.value),