QUndoCommand Class Reference
[QtGui module]

Detailed Description

The QUndoCommand class is the base class of all commands stored on a QUndoStack.

For an overview of Qt's Undo Framework, see the overview document.

A QUndoCommand represents a single editing action on a document; for example, inserting or deleting a block of text in a text editor. QUndoCommand can apply a change to the document with redo() and undo the change with undo(). The implementations for these functions must be provided in a derived class.

 class AppendText : public QUndoCommand
 {
 public:
     AppendText(QString *doc, const QString &text)
         : m_document(doc), m_text(text) { setText("append text"); }
     virtual void undo()
         { m_document->chop(m_text.length()); }
     virtual void redo()
         { m_document->append(m_text); }
 private:
     QString *m_document;
     QString m_text;
 };

A QUndoCommand has an associated text(). This is a short string describing what the command does. It is used to update the text properties of the stack's undo and redo actions; see QUndoStack::createUndoAction() and QUndoStack::createRedoAction().

QUndoCommand objects are owned by the stack they were pushed on. QUndoStack deletes a command if it has been undone and a new command is pushed. For example:

 MyCommand *command1 = new MyCommand();
 stack->push(command1);
 MyCommand *command2 = new MyCommand();
 stack->push(command2);

 stack->undo();

 MyCommand *command3 = new MyCommand();
 stack->push(command3); // command2 gets deleted

In effect, when a command is pushed, it becomes the top-most command on the stack.

To support command compression, QUndoCommand has an id() and the virtual function mergeWith(). These functions are used by QUndoStack::push().

To support command macros, a QUndoCommand object can have any number of child commands. Undoing or redoing the parent command will cause the child commands to be undone or redone. A command can be assigned to a parent explicitly in the constructor. In this case, the command will be owned by the parent.

The parent in this case is usually an empty command, in that it doesn't provide its own implementation of undo() and redo(). Instead, it uses the base implementations of these functions, which simply call undo() or redo() on all its children. The parent should, however, have a meaningful text().

 QUndoCommand *insertRed = new QUndoCommand(); // an empty command
 insertRed->setText("insert red text");

 new InsertText(document, idx, text, insertRed); // becomes child of insertRed
 new SetColor(document, idx, text.length(), Qt::red, insertRed);

 stack.push(insertRed);

Another way to create macros is to use the convenience functions QUndoStack::beginMacro() and QUndoStack::endMacro().

See also QUndoStack.

Member Function Documentation

 bool AppendText::mergeWith(const QUndoCommand *other)
 {
     if (other->id() != id()) // make sure other is also an AppendText command
         return false;
     m_text += static_cast<const AppendText*>(other)->m_text;
     return true;
 }