Question

Let’s say I have a GUI with multiple types of viewers of user objects. For example, a tree view, a list view and a diagram view. The three views show the same objects. If a user deletes an object from one view, I would like to fire off an event to notify the other two views. I currently do this by exposing an event on the object itself. So if the object is deleted from View 1, View 1 will call delete on the object, which will then fire an event to the subscribers (all 3 views). Each subscriber has the chance to cancel the deletion.

There are a few problems as I see it. If a subscriber cancels a deletion after another subscriber has already approved of the deletion, then I have to instruct those subscribers to undo the deletion.

Are there any good patterns to implement this kind of common scenario?

Answer 1

If an object is to be deleted from all views, or no view at all

1. Ask every subscriber if it’s ok to delete the item; if yes:
2. Issue a “delete item” call to remove the object from the source, perform a soft delete or whatever you’d like
3. Update each view. This would be the observer part, listen for a “object deleted” call and take appropriate actions, for example manually remove the now deleted object from each view

If you always want the user to be able to delete the object from its own view:

1. Step 2. from above, with the addition that it’s only been deleted either for 1) the user; or 2) that user in that view
2. Step 1. from above, and continue.. (might be skipped, depending on how much you’d like the views to be coherent)