Kate Internals: The Undo/Redo System

The Kate Editor Component (also called KatePart) has its own undo/redo system. It did not change much since KDE2 and basically it is very simple. Meanwhile there are classes for undo/redo support in Qt as well. In fact both systems are very similar. This article focuses on Kate Part’s system.

Text Operations

First we have to take a look at what actions need to be saved. In Kate Part this basically comes down to

  • insert text or line
  • remove text or line
  • selection changes
  • (and a few others like wrapping a line)

When typing text, each keystroke inserts a character. This is exactly one undo/redo item. As example, typing a character ‘x’ creates a new undo item:

  • the content is ‘x’
  • the type is ‘insert text

Undo in this case means ‘remove x‘. Redo means ‘insert x (again)‘. Hence, the undo/redo history is just a list (more like a stack to be precise) of simple edit actions.

KateUndo Items

In KatePart, each edit primitive in the undo history is based on the class KateUndo:

class KateUndo {
  KateUndo(KateDocument* document);
  virtual ~KateUndo();

  enum UndoType {

  virtual bool mergeWith(const KateUndo* undo);

  virtual void undo() = 0;
  virtual void redo() = 0;

  virtual KateUndo::UndoType type() const = 0;

For each of the edit primitives above exists a subclass of KateUndo, for instance the class for inserting text looks like this:

class KateEditInsertTextUndo : public KateUndo
    KateEditInsertTextUndo (KateDocument *document,
                            int line, int col, const QString &text);

    virtual void undo();
    virtual void redo();

    bool mergeWith (const KateUndo *undo);

    KateUndo::UndoType type() const { return KateUndo::editInsertText; }

    const int m_line;
    const int m_col;
    QString m_text;

Item Merging

Note the function KateUndo::mergeWith(const KateUndo* undo); This functions merges two undo items of the same type if possible. For instance, typing ‘hello world’ inserts one undo item for every character, i.e. 11 undo items of type ‘insert text’. Kate merges those 11 items into only 1 item with the string ‘hello world’. Merging leads to less KateUndo items (less memory) and faster undo/redo replaying.

Item Grouping

What’s still missing is the possibility to group several undo items together. Imagine you have selected the text ‘hello world’ and paste the text ‘cheers’ from the clipboard. What happens is this

  1. remove selected text
  2. insert text from clipboard

So there are two undo items of different type. They cannot be merged into only one KateUndo item. Though, we want to support undoing both items in one go, that’s why we add several undo items into undo groups. In KatePart, this is done by the class KateUndoGroup:

class KateUndoGroup {
  explicit KateUndoGroup (KateUndoManager *manager,
                          const KTextEditor::Cursor &cursorPosition,
                          const KTextEditor::Range &selectionRange);

  void undo(KTextEditor::View *view);
  void redo(KTextEditor::View *view);

  enum UndoType { ... };

  void addItem (KateUndoGroup::UndoType type, uint line,
                uint col, uint len, const QString &text);
  void setUndoSelection (const KTextEditor::Range &selection);
  void setRedoSelection (const KTextEditor::Range &selection);
  void setUndoCursor(const KTextEditor::Cursor &cursor);
  void setRedoCursor(const KTextEditor::Cursor &cursor);
  bool merge(KateUndoGroup* newGroup,bool complex);
  void safePoint (bool safePoint=true);

Every KateUndo item belongs to one KateUndoGroup. A KateUndoGroup can have an arbitrary count of KateUndo items. In the example above we want to group ‘remove selected text‘ and ‘insert text‘ together. Grouping can be explicitely done in the code as follows (simplified version):

void KateDocument::paste( KateView* view, QClipboard::Mode mode )
  QString s = QApplication::clipboard()->text(mode);
  insertText(pos, s, view->blockSelectionMode());

Grouping: editStart()/editEnd()

The call of editStart() tells the document that an edit operation is running. All text operations are added to the current KateUndoGroup, until editEnd() is called. editStart() and editEnd() do reference counting, i.e. editStart() can be called nested as long as for each call of editStart() there is (finally) a call of editEnd().

Grouping: Cursors and Selections

Undoing the paste-action above should restore the selection if there was one previously. Redo (i.e. paste again) should remove the selection again. So there are two different types of selections: one before the undo group, and one after. That’s why each undo group has the functions setUndoSelection() and setRedoSelection(). The same applies for the cursor position: We have to store two different cursor positions, one for undo and one for redo.
For instance, imagine we removed the text ‘world’. Undo (i.e. insert ‘hello‘) should set the cursor position to the end of ‘hello’. Redo (i.e. remove ‘hello‘) should set the cursor position to the start of it.

Luckily a programmer does not have to set the undo/redo cursor positions and text selections manually. undoStart() is called the first time editStart() is called. The closing editEnd() finally calls undoEnd(). So undoStart() sets the undo cursor position and undo text selection, while undoEnd() sets the redo cursor position and redo text selection.

Group Merging

The careful reader might have noticed KateUndoGroup::merge(). So merging of two groups is also supported. Whether text operations should be merged into an existing undo group can be controlled with KateDocument::setUndoDontMerge(). Pasting text for example set’s this flag.

Undo and Redo

Every document in KatePart has two lists: An undo list, and a redo list. Suppose we have 10 KateUndoGroups in the undo list and the user invokes undo 4 times. Then the undo list only contains 6 items and the redo list 4. Now it is also possible to redo. However, typing text clears the redo list.

Document Modified Flag

KateDocument::updateModifed() is called to update the modified flag of a file. This update also relies on the current active undo group. Saving the file saves a pointer to the current undo group, and later we simply can check whether the current undo group is the active one. Pretty simple mechanism.

Akademy 08: Kate Flashback

This Akademy’s Kate changes include

  • fix: drag & drop of text
  • code completion: only show group header if the group name is not empty
  • reintroduction of buffer blocks in Kate’s document buffer (one buffer contains up to 4096 lines). The blocks build a linked list. Editing a 500 MB file kind of works now again. It’s still rather slow, though.
  • more speedup in Kate’s document buffer
  • Kate is using KEncodingProber instead of KEncodingDetector now
  • generate internal version of KatePart automatically, so developers don’t have to adapt it manually each release
  • python encoding detection plugin that warns if encoding is not correct while saving
  • new plugin: Backtrace browser, mainly for developers
  • find in files: several speed optimizations
  • find in files: progress indicator while search is active
  • find in files: redesign of workflow. Search happens with non-modal dialog and results are shown in toolviews.
  • lots of vi mode changes
  • lots of bugs closed, mainly old ones
  • some real bug fixes…
  • things I forgot

Kate: Fast backtrace navigation

I’ve added a new plugin to kdesdk/kate/plugin: a backtrace browser. It’s meant for developers and probably of no use for users. What does it do? It shows a backtrace delivered by gdb in a listview in a Kate toolview. Clicking on an item opens the selected file and jumps to the correct line number. It works for backtraces generated on your own machine, but it will also work for backtraces from other people, i.e. with /home/dummy/qt-copy/…/qwidget.cpp will still be found on other machines. For that to work, you have to index the directories where the source code is located.
Sometimes there are several files with the same name, e.g.

  • trunk/kdegraphics/okular/generators/dvi/config.h
  • trunk/kdepim/mimelib/mimelib/config.h

To pick the right choice, the plugin picks the last two parts of the url, in this case this would be

  • dvi/config.h
  • mimelib/config.h

and then usually finds the correct one. Indexing trunk/KDE and branches/KDE/4.1 of course will lead to a clash, now way to fix it. Maybe I could present a list of valid files to the user and let the user pick the right one. I don’t think that’s necessary though for now.

How to configure

  1. Enable the plugin: go to Settings > Configure Kate > Application Plugins and enable ‘Kate Backtrace Browser’
  2. A config page appeared, so click on it and add the directories containing the source code
  3. Clicking OK will start indexing. It will take some time (the index of kdesupport + kdelibs + kdepimlibs + kdebase + kdesdk + playground/plasma + plasma-addons + kdevplatform + kdegraphics is about 6MB)

When indexing is finished, open the toolview “Backtrace Browser”. Now you can load a backtrace from the clipboard (e.g. when you clicked “Copy to Clipboard” in Dr. Konqi) or from a file.

Hope it’s useful :)

Do you understand the word HTML?

During the Kate developer meeting we also thought about simplifying KWrite and how to make the decision whether KWrite should be launched in full featured mode or in a stripped version. …well, and we found a really funny idea:

Note, that this would even work, the question would be rather annoying, though :) The solution right now is to always start KWrite in a simple mode. Mostly only actions are hidden in the menus (@distributors: kdelibs/kate/data/katepartsimpleui.rc), but you can also change c++ code at Kate part level, as there are some functions:

  • bool KateDocument::simpleMode() (kate part internal), and
  • bool KTextEditor::Editor::simpleMode() (along with setSimpleMode())

This way config dialogs can be adapted depending on the mode as well. Ah, and if you want the normal mode back, go into the KWrite settings and enable [x] Enable Developer Mode. Then restart KWrite.

PS: Tackat, we came up with this image before our phone call. That’s why it was really funny when you said HTML is something that should not be removed. hehe… :)