16: Understanding Undo in Java

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16: Understanding Undo
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legend says Undo 5 actions. The fact that the designers had to add that text legend tells me that, regardless of how the programmers constructed it, the users were applying a different mental model. The users imagined that they could go down the list and select a single action from the past to Undo. The program didn t offer that option, so the signs were posted. This is like a door with a pull handle that has been pasted with Push signs which everybody still pulls on anyway. While multiple Undo is certainly a very useful mechanism, there s no reason not to finish the job and use our ample computing resources to allow users to Undo just the undesirable actions, instead of everything that has happened since them.
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Other Models for Undo-Like Behavior
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The manifest model of Undo in its simplest form single Undo conforms to the user s mental model: I just did something I now wish I hadn t done. I want to click a button and Undo that last thing I did. Unfortunately, this manifest model rapidly diverges from the user s mental model as the complexity of the situation grows. In this section, we discuss models of Undo-like behavior that work a bit differently from the more standard Undo and Redo idioms.
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Besides providing robust support for the terminally indecisive, the paired UndoRedo function is a convenient comparison tool. Say that you d like to compare the visual effect of ragged-right margins against justified right margins. Beginning with ragged-right, you invoke Justification. Now you click Undo to see ragged-right and now you press Redo to see justified margins again. In effect, toggling between Undo and Redo implements a comparison or what-if function; it just happens to be represented in the form of its implementation model. If this same function were to be added to the interface following a user s mental model, it might be manifested as a comparison control. This function would let you repeatedly take one step forward or backward to compare two states. Some TV remote controls include a function labeled Jump, which switches between the current channel and the previous channel very convenient for viewing two programs concurrently. The jump function provides the same utility as the UndoRedo function pair with a single command a 50% reduction in excise for the same functionality. When used as comparison functions, Undo and Redo are really one function and not two. One says Apply this change, and the other says Don t apply this change. A single Compare button might more accurately represent the action to users.
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Part III: Designing Interaction Details
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Although we have been describing this tool in the context of a text-oriented word processing program, a compare function might be most useful in a graphic manipulation or drawing program, where users are applying successive visual transformations on images. The ability to see the image with the transformation and quickly and easily compare it to the image without the transformation would be a great help to the digital artist. Many products address this with small thumbnail preview images. Doubtlessly, the Compare function would remain an advanced function. Just as the jump function is probably not used by a majority of TV users, the Compare button would remain one of those niceties for frequent users. This shouldn t detract from its usefulness, however, because drawing programs tend to be used very frequently by those who use them. For programs like this, catering to the frequent user is a reasonable design choice.
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Category-specific Undo
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The Backspace key is really an Undo function, albeit a special one. When a user mistypes, the Backspace key undoes the erroneous characters. If a user mistypes something, then enters an unrelated function such as paragraph reformatting, then presses the Backspace key repeatedly, the mistyped characters are erased and the reformatting operation is ignored. Depending on how you look at it, this can be a great flexible advantage giving users the ability to Undo discontiguously at any selected location. You could also see it as a trap for users because they can move the cursor and inadvertently backspace away characters that were not the last ones keyed in. Logic says that this latter case is a problem. Empirical observation says that it is rarely a problem for users. Such discontiguous, incremental Undo so hard to explain in words is so natural and easy to use because everything is visible: Users can clearly see what will be backspaced away. Backspace is a classic example of an incremental Undo, reversing only some data while ignoring other, intervening actions. Yet if you imagined an Undo facility that had a pointer that could be moved and that could Undo the last function that occurred where the pointer points, you d probably think that such a feature would be patently unmanageable and would confuse a typical user. Experience tells us that Backspace does nothing of the sort. It works as well as it does because its behavior is consistent with a user s mental model of the cursor: Because it is the source of added characters, it can also reasonably be the locus of deleted characters.
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