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Re: [platform-ui-dev] Undo/Redo Proposal

See attached with my comments.

(See attached file: undo_redo.html)

                    "Randy Giffen/OTT/OTI"                                                              
                    <Randy_Giffen@xxxxxxx>         To:     platform-ui-dev@xxxxxxxxxxx                  
                    Sent by:                       cc:                                                  
                    platform-ui-dev-admin@e        Subject:     [platform-ui-dev] Undo/Redo Proposal    
                    11/20/01 09:59 AM                                                                   
                    Please respond to                                                                   

Attached are proposed changes to Undo/Redo to implement a global Undo/Redo
stack for the workbench.

Title: Undo Redo Proposal

See DS>

Undo Redo Proposal

Topic: Undo Redo support in the Eclipse workbench
Created: 15 Nov 2001 by Randy Giffen
Modified:  20 Nov 2001 by Randy with comments from Dirk


Currently, the Eclipse workbench includes global actions for Undo and Redo. They appear as the first two items in the edit menu. As global actions, they are targeted to the current active part in the current perspective. If the active part does not supply a global action handler for them, they are disabled. This means that the action that will be performed by Undo or Redo is dependent on the active part and thus the behavior of these actions is modal. In many cases the user must go back to the appropriate part before being able to undo an action performed there.

The Details


Global actions are actions defined by the Eclipse workbench UI which delegate their enablement and action to a global action handler supplied but the active workbench part. Global actions were introduced to provide a degree of consistency in the UI. The idea was that common functions like undo/redo, cut/copy/paste, and delete should have a consistent location in the UI and consistent menu accelerators. An additional benefit is that views (which cannot contribute to the window menu and thus cannot define menu accelerators) are able to respond to the menu accelerators defined for global actions.

Thus, in a way, the name global is a bit misleading in the undo/redo case. The actions do not support a global undo/redo stack. They support a targeted undo/redo directed at, and implemented by, the active part. This is typically not what most users expect. In the case of an IDE, they are using the UI to manipulate (make changes to) a model. They expect that undo/redo will allow them to back out of or redo these changes regardless of what part of the ui they are currently working.

This architecture requires that parts maintain their own undo/redo stack (although theoretically some parts may decide to implement a shred stack among themselves).

Another problem is that actions that don't belong to a view (e.g. like the "Refactor" menu which gets retargeted to the active view) can't access the undo/redo actions in the edit menu.

Possible Options

Let us assume for the moment that a workbench undo/redo stack is a good idea and consider its implementation (this is based on discussions and PR 1887). We would define an IUndoManager and add a method to IWorkbench to obtain such a manger. The manager would have api for adding and removing a IUndoActions from the manager.

DS> I like this design in general.  As you say, the current design is modal in nature, so it is impossible to undo an action, which you may have just performed, unless the target part is active.  This makes it very easy to lose the undo feature.

IUndoAction would have the following definition

public interface IUndoAction {
 String getLabel();
 void undo();
 void redo();
 boolean isUndoValid();
 boolean isRedoValid();
 void dispose();

DS> I am concerned about the use of the term "Action".  IUndoAction is not a subtype of IAction, and has none of the standard methods like run, getImage, etc.  I think this object is actually a "record of an action" which can be undone, rather than the action itself.  So I might call it IUndoFrame or IUndoRecord.

Comment: Should we consider having an abstract UndoAction rather than an IUndoAction? In some cases this would allow us to modify API in a non-breaking way but it means that clients will not be able to use an existing type to implement IUndoAction (probably a rare case).

DS> From a purely Java standpoint, there is no reason for an abstract superclass unless you can provide some default implementation.  Given the nature of the methods in IUndoAction, I don't think you can actually provide much implementation.  So, the ONLY reason to create UndoAction is for interface evolution.  If the public API for the IUndoStack is simple, like just one method for adding items ( addFrame(IUndoAction) ), and we do not expose IUndoAction ourselves in any getter methods, I think this can evolve the interface if needed by adding methods like addFrame(IUndoAction2).  Does anybody but the workbench really care if there is anything on the undo stack?  If not, there is no need to expose anything else within the API.

The manager will ensure that the top undo action and top redo action are valid by calling the isUndoValid() and isRedoValid() api methods. If the undo/redo action is no longer valid it will be punted and the next one moved to the top.

DS> Will we update the enablement of the menu item whenever the edit menu is about to show?  If the user invokes the Undo accelerator (Ctrl+Z) will we check the enablement of the action at that point, and return silently if disabled?

The valid check is required as an undo action cannot validate itself simply by listening to model changes. It would also somehow need additional information if the received delta will have an undo which will be put onto the undo stack. Consider the following case: A Java refactoring renames file to Then the user goes to the navigator and renames to C.txt. If the navigator pushes an undo action onto the stack, the refactoring undo is still valid since the user must first perform the undo for "rename to C.txt" before he can execute the undo for "rename to". If the refactoring kernel listened to model changes only, without knowing that there is an undo for the renaming of to C.txt, the kernel would invalidate the undo for rename to

Note that we are only going to validate the top undo/redo actions. Validating others on the stack would be too complicated. Consider the case where one renames a compilation unit. In this case we have two changes, one that renames the top level type and one that actual renames the file. To be able to figure out if the undo for this refactoring is still valid, we have to check if both changes are still valid. To do so we have to simulate the undo of the rename of the file to be able to check if we still can undo the renaming of the top level type. This is necessary since the rename of the top level type doesn't know anything about the rename of the file. For example:

Rename to

  1. change top level type in file to B. The undo is change top level type in file to A
  2. change filename from to The undo is change to
To figure out if this refactoring can still be undone we have to simulate the rename form to Otherwise we can't check the undo "change top level type in file to A" since doesn't exist.

We would add tool bar buttons for undo and redo with a menu button to the right like the "new" button. The menu button would show the current undo or redo stack and let you select at any level in the stack. Again it is important that IUndoActions be able to validate themselves since this will determine the enablement of these buttons. The undo and redo actions in the Edit menu would show the label for the action currently at the top of their stacks.

DS> The "arrow" idea seems incompatable with many of the previous statements.  In particular, "we are only going to validate the top undo/redo actions".  If it is only possible to accurately determine if the top actions are valid, how can you possibly expose the entire undo stack and let the user go back n steps?  This may be an invalid operation.

Once the new api is available, it is likely than many more actions would become undoable (ex. resource rename, move etc.).

DS> I disagree.  These actions are theoretically undoable now.  The only reason you can't undo them is because the Navigator has absolutely no support for undo / redo, and core has absolutely no support for undo.  If these conditions were met, the undo / redo actions would work for resource rename, move, etc.

Problems with a Global Undo/Redo Stack

1) Since undo and redo are part of the IWorkbenchActionConstants.GLOBAL_ACTIONS, it would be a breaking API change to no longer support undo and redo as global (retargetable) actions. Parts that are currently maintaining their own undo/redo stack would have to change to add IUndoActions to the workbench stack. Until then the Edit menu actions and their accelerators would no longer trigger an action in the part. This is probably an acceptable breaking change.

DS> If an action is performed, but no IUndoAction is added to the IUndoStack, what will appear on the IUndoStack and in the undo menu?  Will it show the text for an older, possibly obsolete action?  How will you ensure consistent use of the IUndoStack throughout the product?

DS> I am not convinced that this is an acceptable breaking change.  For those parts which do not expose undo / redo within their context menu, undo / redo will be completely broken, and the part functionally incomplete.  If the part has a global action handler for undo / redo, it might be better to delegate to those handlers temporarily, while the part is active.  Perhaps some indication of the local nature of undo could be given in the menus.  If this approach is taken, at least existing parts will continue to work.

If you make a breaking change, every editor on the planet not derived from AbstractTextEditor will be broken.  If we're willing to accept this, let's give up on API compatability.

2) The text widget implements its own undo. For example create a new task and edit its description note that there is an undo item in the widgets popup menu. This undo is local to the widget. Currently Undo is not enabled under edit in this case since the tasklist does not supply a global action handler for this purpose (There is no api on Text to allow programmatic undo). However if we have a global stack it is likely that Edit>Undo would be enabled but it would perform something different than undo on the popup.
Thus should we:
    1) We could ask SWT for undo API on text in this case.
    2) Continue to allow Undo/Redo to be global actions for which we simply define a default behavior (which will not be retargeted by 95% of the parts).

3) It is not clear that an IDE built using the workbench has only a single model. For example, suppose I
i) Edit some .java file in the Java perspective
ii) Switch to the Team perspective and create something in the Repositories view
iii) Switch back to the Java perspective, decide I don't like my change and press Undo.
A global stack will cause the item I created in the team perspective to be removed (this will perhaps happen silently).
But this is exactly what some would expect. They find it confusing to have more than one undo stack in an application.

DS> You have a number of choices. There could be one undo stack for each page, each window, or the entire app.  What would I expect as a user?  Given that a page == a window, there are only 2 choices: window and app.

If I'm in an editor, and I make a change, I would be surprised if an undo action for that editor appeared in an entirely different window.  If we say that each window is a separate task, good task separation should be maintained, and each window should have a separate undo stack.

If you adopt a window approach perhaps the method to get the undo stack should be on the window, instead.  For convenience within a part there should also be a method on IWorkbenchPartSite to get the undo stack, too, so there is no need to traverse upwards.

4) How do we define what can go on the undo redo stack? Do we limit it to model changes? For example opening an editor could have an undo that would allow the user to close the editor and return to the previously active part.

DS> In most applications, the undo stack contains model actions, but does not contain UI actions.  For instance, if I move a toolbar in Netscape Composer, an undo action does not appear for the action.  The Undo stack only contains document centric items.  I suspect this represents a general belief that the user is more interested in interaction with the document, not the user interface, so you don't clutter the undo stack with ui gestures.   UI interaction is also unconscious for the user, whereas doc interaction is conscious (the focus of attention).

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