I'm learning dart. I can't figure it out why the following dart code can't handle an exception. What wrong am I doing below?:

void main() {
  print(1);
  try {
    File file = File('random_path.txt');
    print(2);
    var contents = file.readAsString();
    print(3);
    contents.then((value) => print(value));
    print(4);
  } on FileSystemException catch (e) {
    print(5);
    print(e);
  } catch (e) {
    print(6);
    print(e);
  } finally {
    print(7);
  }

  print(8);
}

The file random_path.txt doesn't exist. I was expecting the on FileSystemException catch(e) to catch and print e. But that's not a case. Here is the output:

1
2
3
4
7
8
Unhandled exception:
FileSystemException: Cannot open file, path = 'random_path.txt' (OS Error: No such file or directory, errno = 2)
#0      _File.open.<anonymous closure> (dart:io/file_impl.dart:356:9)
<asynchronous suspension>

CodePudding user response：

You need to handle the error that is reported by the contents future, and forwarded to the future returned by then.

That means either adding an error handler yourself:

  contents.then(print).catchError((e, s) {
    print("Future error: $e\n$s");
  }); // Catches errors from `contents` or thrown by `print`.

// or

  contents.then(print, one rror: (e, s) {
    print("Future error: $e\n$s");
  }); // Won't catch error thrown by `print`.

Or using an async function to handle the future using await, which is the recommended approach:

void main() async { // <- marked as async so you can use `await`.
  print(1);
  try {
    File file = File('random_path.txt');
    print(2);
    Future<String> contents = file.readAsString();
    print(3);
    var value = await contents; // <- await gets value, or rethrows error
    print(value);
    print(4);
  } on FileSystemException catch (e) {
    print(5);
    print(e);
  } catch (e) {
    print(6);
    print(e);
  } finally {
    print(7);
  }
  print(8);
}

In general, always use async functions and await to deal with futures. Using the .then API is mainly for low-level code which integrates different asynchronous computations, and has to wait for more than one thing at a time. It's what's being used internally to build a good Future based API that you can just await on. And you should just await when you can.