Future execution leads to unhandled exception - dart

Question

import 'dart:async';
void main() {
divide(1, 0).then((result) => print('1 / 0 = $result'))
.catchError((error) => print('Error occured during division: $error'));
}
Future<double> divide(int a, b) {
if (b == 0) {
throw new Exception('Division by zero');
}
return new Future.value(a/b);
}
Currently I am learning how to work with futures in Dart and I got stucked on such an easy example. My future throws exception when user tries to perform division by zero. However, .catchError doesn't handle my exception. I got unhandled exception with a stack trace instead. I'm pretty sure that I am missing something obvious but can't understand what exactly.
As I understand, there is another way to handle error:
divide(1, 0).then((result) => print('1 / 0 = $result'),
onError: (error) => print('Error occured during division: $error'));
to use named optional argument - onError. Doing like this still leads to unhandled exception.
I would like to clarify one more thing. Am I right? - The only difference between these 2 approaches is that .catchError() also handles errors thrown by inner futures (futures that are called inside then() method of the outer future) while onError only catches errors thrown by the outer future?
Dmitry
Thank you.

Answer 1

Your error handling didn't work because the error is thrown in the synchronous part of your code. Just because the method returns a future doesn't mean everything in this method is async.
void main() {
try {
divide(1, 0)
.then((result) => print('1 / 0 = $result'));
} catch (error) {
print('Error occured during division: $error');
}
}
If you change your divide function like
Future<double> divide(int a, b) {
return new Future(() {
if (b == 0) {
throw new Exception('Division by zero');
}
return new Future.value(a / b);
});
}
you get an async error and your async error handling works.
An easier way is to use the new async/await
main() async {
try {
await divide(1, 0);
} catch(error){
print('Error occured during division: $error');
}
}
try at DartPad
and another advantage is this works in both cases.