How can you return null from orElse within Iterable.firstWhere with null-safety enabled? - flutter

Question

Prior to null-safe dart, the following was valid syntax:
final list = [1, 2, 3];
final x = list.firstWhere((element) => element > 3, orElse: () => null);
if (x == null) {
// do stuff...
}
Now, firstWhere requires orElse to return an int, opposed to an int?, therefore I cannot return null.
How can I return null from orElse?

Answer 1

A handy function, firstWhereOrNull, solves this exact problem.
Import package:collection which includes extension methods on Iterable.
import 'package:collection/collection.dart';
final list = [1, 2, 3];
final x = list.firstWhereOrNull((element) => element > 3);
if (x == null) {
// do stuff...
}

Answer 2

You don't need external package for this instead you can use try/catch
int? x;
try {
x = list.firstWhere((element) => element > 3);
} catch(e) {
x = null;
}

Answer 3

A little bit late but i came up with this:
typedef FirstWhereClosure = bool Function(dynamic);
extension FirstWhere on List {
dynamic frstWhere(FirstWhereClosure closure) {
int index = this.indexWhere(closure);
if (index != -1) {
return this[index];
}
return null;
}
}
Example use:
class Test{
String name;
int code;
Test(code, this.name);
}
Test? test = list.frstWhere(t)=> t.code==123);

Answer 4

An alternative is that you set a nullable type to the list.
Instead of just [1, 2, 3], you write <int?>[1, 2, 3], allowing it to be nullable.
void main() {
final list = <int?>[1, 2, 3];
final x = list.firstWhere(
(element) => element != null ? (element > 3) : false,
orElse: () => null);
print(x);
}

Answer 5

This should work, and it's a better solution:
extension IterableExtensions<T> on Iterable<T> {
T? firstWhereOrNull(bool Function(T element) comparator) {
try {
return firstWhere(comparator);
} on StateError catch (_) {
return null;
}
}
}

Answer 6

To add to #Alex Hartfords answer, and for anyone who doesn't want to import a full package just for this functionality, this is the actual implementation for firstWhereOrNull from the collection package that you can add to your app.
extension FirstWhereExt<T> on List<T> {
/// The first element satisfying [test], or `null` if there are none.
T? firstWhereOrNull(bool Function(T element) test) {
for (final element in this) {
if (test(element)) return element;
}
return null;
}
}