Mastering Undo/Redo in Your Qt App: When to Choose QUndoView and Alternatives

Purpose

• The QUndoView then displays the undo/redo history (list of commands) managed by the QUndoGroup.
• Associates a QUndoView object with a specific QUndoGroup instance.

Key Points

• By linking a QUndoView to a QUndoGroup, you establish a visual representation of the undo/redo history for the functionalities associated with that group.
• QUndoGroup acts as a central hub for managing undo/redo stacks in your Qt application.
• QUndoView inherits from QListView, making it suitable for presenting the undo/redo commands as a list.

Function Breakdown

void QUndoView::setGroup(QUndoGroup *group);

• group: A pointer to a QUndoGroup object.

  • If group is not null (nullptr), the QUndoView displays the commands within the provided QUndoGroup.
  • If group is null, the QUndoView becomes empty, indicating no undo/redo history is available.

• setGroup(): The member function that establishes the association.

• QUndoView: The class representing the undo/redo view widget.

Benefits of Using setGroup()

• Automatic Updates
  When the active undo stack within the group changes, the QUndoView automatically reflects the updated history, ensuring the view remains synchronized.
• Centralized Undo/Redo Management
  Groups multiple undo stacks under a single QUndoGroup, allowing coordinated undo/redo operations across different parts of your application.

Example Usage

#include <QApplication>
#include <QUndoStack>
#include <QUndoGroup>
#include <QUndoView>
#include <QPushButton>

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

    // Create undo stacks and a group
    QUndoStack stack1;
    QUndoStack stack2;
    QUndoGroup group;
    group.addStack(&stack1);
    group.addStack(&stack2);

    // Create undo/redo commands (replace with your specific commands)
    // ...

    // Create a QUndoView and associate it with the group
    QUndoView view;
    view.setGroup(&group);
    view.show();

    // Create buttons (optional) to trigger undo/redo actions
    QPushButton undoButton("Undo");
    connect(&undoButton, &QPushButton::clicked, &group, QUndoStack::undo);

    QPushButton redoButton("Redo");
    connect(&redoButton, &QPushButton::clicked, &group, QUndoStack::redo);

    // ... (rest of your application logic)

    return app.exec();
}

In this example:

1. Two QUndoStack objects are created to hold undo/redo commands for separate functionalities.
2. A QUndoGroup is created and the stacks are added to it.
3. A QUndoView is instantiated and linked to the QUndoGroup using setGroup().
4. The QUndoView is displayed, showing the combined undo/redo history from both stacks.
5. (Optional) Buttons are created to trigger undo/redo actions on the group.

---

#include <QApplication>
#include <QUndoStack>
#include <QUndoGroup>
#include <QUndoView>
#include <QPushButton>
#include <QTextEdit>

class TextChangeCommand : public QUndoCommand {
    Q_OBJECT

public:
    TextChangeCommand(QTextEdit *textEdit, const QString &oldText, const QString &newText):
        QUndoCommand(textEdit->windowTitle()), textEdit_(textEdit), oldText_(oldText), newText_(newText) {}

    virtual void redo() override {
        textEdit_->setPlainText(newText_);
    }

    virtual void undo() override {
        textEdit_->setPlainText(oldText_);
    }

private:
    QTextEdit *textEdit_;
    QString oldText_;
    QString newText_;
};

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

    // Create a QTextEdit for text editing
    QTextEdit *textEdit = new QTextEdit;
    textEdit->setPlainText("Initial text");
    textEdit->show();

    // Create undo stacks and a group
    QUndoStack stack1;
    QUndoStack stack2;
    QUndoGroup group;
    group.addStack(&stack1);
    group.addStack(&stack2);

    // Create undo/redo commands for text changes
    connect(textEdit, &QTextEdit::textChanged, [&](const QString &text) {
        QUndoCommand *command = new TextChangeCommand(textEdit, textEdit->toPlainText(), text);
        stack1.push(command);
    });

    // Create a QUndoView and associate it with the group
    QUndoView view;
    view.setGroup(&group);
    view.show();

    // Create buttons to trigger undo/redo actions
    QPushButton undoButton("Undo");
    connect(&undoButton, &QPushButton::clicked, &group, QUndoStack::undo);

    QPushButton redoButton("Redo");
    connect(&redoButton, &QPushButton::clicked, &group, QUndoStack::redo);

    return app.exec();
}

In this enhanced example:

1. A custom TextChangeCommand class is created that inherits from QUndoCommand.
2. The TextChangeCommand constructor stores the QTextEdit object, the original text, and the new text.
3. The redo() and undo() methods of TextChangeCommand set the text editor's content to the new and old text, respectively.
4. Inside the main() function, a signal-slot connection is established between the textChanged signal of the QTextEdit and a lambda function.
5. The lambda function creates a TextChangeCommand object whenever the text in the editor changes, capturing the old and new text for undo/redo purposes.
6. The command object is pushed onto the stack1 (assuming you want text changes to be part of Stack 1).

---

1. • Some third-party libraries might offer alternative undo/redo management solutions with different features or functionalities.
   • It's important to evaluate the specific needs of your application and choose a library that integrates well with your existing codebase.
   • Carefully review the documentation and examples provided by the library to understand how it handles undo/redo history display.

Choosing the Right Approach

• Third-party libraries might be a viable option only if they provide significant advantages or specific features you require and are well-suited for your project.
• If you need a highly customized display or have specific requirements not met by the default QUndoView, consider implementing a custom undo/redo display widget.
• For most scenarios, QUndoView::setGroup() is a simple and effective way to display undo/redo history in Qt applications.