Understanding QUndoView: The Undo/Redo Interface for Your Qt Widgets Application

Purpose

QUndoView is a widget in Qt that displays the list of commands (undo/redo actions) stored in a QUndoStack or managed by a QUndoGroup. It essentially provides a visual representation of the undo/redo history for your application.

Constructors

QUndoView offers two constructors to create an instance:

1. • Creates an empty QUndoView without an associated undo stack or group.
   • You'll need to set the stack or group later using setStack() or setGroup().

2. QUndoView(QUndoStack *stack, QWidget *parent = nullptr)

   • Creates a QUndoView that's directly connected to the provided stack.
   • The view will automatically display the commands in the stack and update itself whenever the stack changes.

Functionality

• Customization
  
  • cleanIcon: This property controls the icon used to represent the clean state (usually the state when the document was saved). You can set it using setCleanIcon().
  • emptyLabel: This property defines the text displayed for the empty state (before any commands were pushed on the stack). You can modify it using setEmptyLabel().
• Selection
  
  • The most recently executed command is always selected by default.
  • Selecting a different command in the list triggers the QUndoStack::setIndex() function, effectively rolling back or forward the application's state to that specific command.
• List View
  QUndoView inherits from QListView, so it displays the undo/redo commands as a list.

Example Usage

#include <QtWidgets>
#include <QUndoStack>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    // Create a document editing widget (replace with your actual widget)
    QTextEdit *textEdit = new QTextEdit;

    // Create an undo stack
    QUndoStack *undoStack = new QUndoStack;

    // Create a QUndoView and connect it to the undo stack
    QUndoView *undoView = new QUndoView(undoStack);

    // Layout and show widgets
    QHBoxLayout *layout = new QHBoxLayout;
    layout->addWidget(textEdit);
    layout->addWidget(undoView);

    QWidget *window = new QWidget;
    window->setLayout(layout);
    window->show();

    // ... (connect your text edit signals to undo stack commands)

    return app.exec();
}

In this example:

• Selecting an item in the undoView will undo or redo the corresponding action performed on the text editor.
• The QUndoView is connected to the undoStack, so it will display the undo history of the textEdit widget.

Key Points

• Customize the appearance with cleanIcon and emptyLabel.
• Connect it to a QUndoStack or manage it through a QUndoGroup.
• Use QUndoView to provide a user-friendly undo/redo interface for your Qt applications.

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Example 1: Text Editing with Undo/Redo (Enhanced)

This improved example builds upon the previous one, addressing potential areas for enhancement:

#include <QtWidgets>
#include <QUndoStack>

class TextDocument : public QTextEdit {
    Q_OBJECT

public:
    TextDocument(QWidget *parent = nullptr) : QTextEdit(parent) {}

signals:
    void textChangedWithUndo(const QString &text);
};

class TextCommand : public QUndoCommand {
    Q_OBJECT

public:
    TextCommand(TextDocument *document, const QString &oldText, const QString &newText)
        : QUndoCommand(document), document(document), oldText(oldText), newText(newText) {}

    virtual void redo() override { document->setText(newText); }
    virtual void undo() override { document->setText(oldText); }

private:
    TextDocument *document;
    QString oldText, newText;
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    TextDocument *textEdit = new TextDocument;
    QUndoStack *undoStack = new QUndoStack;
    QUndoView *undoView = new QUndoView(undoStack);

    // Connect text edits to create undo commands
    connect(textEdit, &TextDocument::textChangedWithUndo, [undoStack, textEdit](const QString &text) {
        QUndoCommand *command = new TextCommand(textEdit, textEdit->toPlainText(), text);
        undoStack->push(command);
    });

    // Layout and show widgets
    QHBoxLayout *layout = new QHBoxLayout;
    layout->addWidget(textEdit);
    layout->addWidget(undoView);

    QWidget *window = new QWidget;
    window->setLayout(layout);
    window->show();

    return app.exec();
}

Improvements

• Clarity
  Descriptive variable names enhance code readability.
• Connected Signal
  Ensures undo commands are created whenever the document text changes.
• TextCommand Class
  Inherits from QUndoCommand to handle undo/redo for text edits.
• Custom TextDocument Class
  Encapsulates text editing logic and emits a textChangedWithUndo signal when content changes.

Example 2: Undo/Redo for Drawing Operations

This example demonstrates QUndoView with custom drawing commands:

#include <QtWidgets>
#include <QUndoStack>
#include <QPainter>

class DrawingWidget : public QWidget {
    Q_OBJECT

public:
    DrawingWidget(QWidget *parent = nullptr) : QWidget(parent) {}

protected:
    void paintEvent(QPaintEvent *event) override {
        QPainter painter(this);
        // ... (paint your drawing based on commands)
    }

signals:
    void drawingChangedWithUndo(const QList<QUndoCommand*> &commands);
};

class DrawLineCommand : public QUndoCommand {
    Q_OBJECT

public:
    DrawLineCommand(DrawingWidget *widget, const QPoint &start, const QPoint &end, const QPen &pen)
        : QUndoCommand(widget), widget(widget), start(start), end(end), oldPen(widget->pen())
    {}

    virtual void redo() override { widget->setPen(pen); widget->update(); }
    virtual void undo() override { widget->setPen(oldPen); widget->update(); }

private:
    DrawingWidget *widget;
    QPoint start, end;
    QPen pen, oldPen;
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    DrawingWidget *drawingWidget = new DrawingWidget;
    QUndoStack *undoStack = new QUndoStack;
    QUndoView *undoView = new QUndoView(undoStack);

    // Connect drawing to create undo commands (replace with your drawing logic)
    // ...

    // Layout and show widgets
    QHBoxLayout *layout = new QHBoxLayout;
    layout->addWidget(drawingWidget);
    layout->addWidget(undoView);

    QWidget *window = new QWidget;
    window->setLayout(layout);
    window->show();

    return app.exec();
}

• **
• DrawingWidget Class
  Custom widget for drawing and emitting a drawingChangedWithUndo signal.

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Custom List View

• However, it requires more effort to replicate the features of QUndoView.
• This approach offers more control over the appearance and behavior of the undo/redo list.
• Implement manual selection handling and connect it to your undo stack/group for functionality.
• Create a QListView and populate it with text or icons representing undo/redo actions.

Third-Party Libraries

• Evaluate the library's complexity and licensing terms before adopting it.
• These libraries might offer additional features like branching undo history or transaction management.

Disable Undo/Redo

• This simplifies your application's design but might limit user experience.
• If undo/redo functionality isn't crucial, consider disabling it altogether.

Choosing the Right Alternative

The best alternative depends on your application's requirements:

• Disable undo/redo if it doesn't add significant value to your application.
• If you need advanced undo/redo features or a well-maintained library, consider third-party options.
• For basic undo/redo with a standard list view, a custom list might suffice.