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    <p>The twist here is that each subscriber has the chance to cancel the deletion. Normally, when you use the words "view" and "subscribe", it means that you are being passive and just reacting to what you see.</p> <p>That doesn't mean that what you're trying to do is impossible, but it's definitely tricky. For example, you could try to do a sort of two-phase commit, where you mark the object is deleted and then wait for all of the viewers to acknowledge the deletion before really removing the object. (This is basically the "ask every subscriber if it's OK to delete the item" approach that chelmertz suggests.) However, this means you need to know exactly how many viewers there are, and all viewers will need to respond before you can complete the deletion. Do you <em>always</em> have three viewers? Are there ever only two? What if there is an error in one of the viewers - Should the delete fail, or do you want to go ahead and delete the object anyway?</p> <p>The nice thing about an event-driven system is that you don't normally have to worry about these sorts of questions: You just make your change to the model (in this case, delete the object) and fire a change event. You don't need to know anything about your viewers.</p> <p>So, if this were my system, I would try to figure out a way to make model changes cancelable only <em>before</em> they are applied to the model, rather than trying to apply changes to other views through the model and then trying to roll back those changes later.</p>
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    1. USJoe Carnahan
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