feature request: optionally cancelling adding to the stack

[Xananax]

Sometimes, a user does an operation that triggers a function, but the operation turns out to be invalid, and state is unchanged (at least, as far as a user can tell).

For example, in my application, items can be added in arbitrary positions on a canvas, but they are only valid depending on specific rules. Because I need to loop over a prohibitively large amount of objects, I am reluctant to do a verification pass, then an update pass; I do both at the same time. So the function is what goes in Undo's execute, whether it leads to a meaningful change or not.

In those cases, enabling the undo button is confusing, and pressing it does nothing that the user can see or care about.

if the line https://github.com/rodydavis/undo/blob/master/lib/src/undo_stack.dart#L20 allowed the following:

  void add<T>(Change<T> change) {
    final bool? response = change.execute();
    if(response == false){ 
      return;
     }
    _history.addLast([change]);
    _moveForward();
  }

Then one could optionally return false from the execute function, which would solve my problem.

Line https://github.com/rodydavis/undo/blob/master/lib/src/undo_stack.dart#L88 would need to be changed thusly:

final bool? Function() _execute;

None of those changes break any backwards compatibility, and they make things more convenient.

Passing lambdas works, but typed named void functions don't, so it is backward compatibility breaking. dynamic works, I'm not acquainted with Dart enough to know if something else works better.

Then, one could write:

changes.add(Change(
  data, 
  (){
     try{
      attemptInsertData();
      return true;
     }catch(e){
      return false;
    }
  }, 
  (old) => data = old
  ));

If you like this change, I can create a PR.

[Xananax]

My workaround at the moment is the following:

changes.add(Change(
  data, 
  (){
     try{
      attemptInsertData();
     }catch(e){
       await Future.delayed(const Duration(milliseconds: 1), () {
         changes.undo();
       });
    }
  }, 
  (old) => data = old
  ));

Not the most elegant, but it works

[rodydavis]

I am going to add the try/catch method!

[rodydavis]

Fixed on v1.5.0

[rodydavis]

I implemented it by throwing an exception and the application can handle the error but it will not insert.

If there was and error you could undo if needed

[Xananax]

Thank you!