| Re: [platform-ui-dev] Undo/Redo Proposal |
Attached is the proposal containing my comments. Simon :-) ------------------ (See attached file: undo_redo.html)Title: Undo Redo Proposal
[SA: My comments below]
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.
[SA: I believe the undo/redo manager is at a too high level on IWorkbench. It should be on a IWorkbenchPage level instead. For example, let say you open two "type hierarchy perspectives" in separate page (whether these pages are in separate windows or both in the same window matters not), you make some changes in the 1st page, then switch to the 2nd page. A user would not expect to be able to undo changes done in the 1st page from the 2nd page - the user would not even be able to see these changes as they are undone.]
[SA: How about the interface name IChangeManager instead of IUndoManager and IChangeRecord instead of IUndoAction. Having the word "Action" is misleading to me.]
IUndoAction would have the following definition
public interface IUndoAction {
String getLabel();
void undo();
void redo();
boolean isUndoValid();
boolean isRedoValid();
void dispose();
}
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).
[SA: Leave it as an interface. Of the method listed above, I see none of them that the abstract class could provide a default implementation. However, just like we do with other interfaces, it might be nice to provide an abstract class in addition to the interface if it can provide useful helper methods...we'll know this when we start implementing undo/redo for our views and editors.]
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.
[SA: Do other applications do this (e.g. remove changes on that stack that cannot be undone/redone)? I wonder if the user will be surprised that something was skipped when hitting undo? Maybe not. I would assume the typical user can probably only remember the last 2-3 changes anyway. Something to check with the usuability guys maybe?]
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 A.java to B.java. Then the user goes to the navigator and renames B.java 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 B.java to C.txt" before he can execute the undo for "rename A.java to B.java". If the refactoring kernel listened to model changes only, without knowing that there is an undo for the renaming of B.java to C.txt, the kernel would invalidate the undo for rename A.java to B.java.
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 A.java to B.java.
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.
[SA: I find the undo/redo stack list an advanced user option. It should not be visible by default. Most apps to not provide such a list (I usually only see this lsit in graphic software). If we provide the list, how does it work? If I select in the middle of the list does it mean undo everything on the stack unto the one I selected? Or simply undo the one selected? You mentionned above that only the top undo action will be validated - how can you populate the list if you do not know if the other undo actions are valid?]
Once the new api is available, it is likely than many more actions would become undoable (ex. resource rename, move etc.).
[SA: If the only way to keep things consistent for the user requires that the API be broken, so be it. However, if there is a way to be somewhat backward compatible without confusing the user, then bonus! Focus on what is best for the user, not the plug-in developer.]
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).
[SA: I'm not sure what the second point means...could you provide more details?]
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.
[SA: See my first comment about this. The user will be focus on a workbench page and therefore will want the undo/redo to be limited to that page (even more so now with the work being done for the lost of context problems)]
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.
[SA: Most undo/redo I've worked with stick to
model changes only. We'll need to write some guidelines - there is a guideline
doc being written as part of the lost of context work that it should
go into.]