CopperSpice API  1.7.2
QUndoCommand Class Reference

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

Public Methods

 QUndoCommand (const QString &text, QUndoCommand *parent=nullptr)
 
 QUndoCommand (QUndoCommand *parent=nullptr)
 
virtual ~QUndoCommand ()
 
QString actionText () const
 
const QUndoCommand * child (int index) const
 
int childCount () const
 
virtual int id () const
 
virtual bool mergeWith (const QUndoCommand *other)
 
virtual void redo ()
 
void setText (const QString &text)
 
QString text () const
 
virtual void undo ()
 

Friends

class QUndoStack
 

Detailed Description

The QUndoCommand class is the base class of all commands stored on a QUndoStack. For an overview of the CopperSpice refer to the Undo System documentation.

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

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 methods 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 does not 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

Constructor & Destructor Documentation

QUndoCommand::QUndoCommand ( QUndoCommand *  parent = nullptr)
explicit

Constructs a QUndoCommand object with the given parent. If parent is not a nullptr this command is appended to the parent's child list. The parent command then owns this command and will delete it in the destructor.

See also
~QUndoCommand()
QUndoCommand::QUndoCommand ( const QString text,
QUndoCommand *  parent = nullptr 
)
explicit

Constructs a QUndoCommand object with the given parent and text. If parent is not a nullptr this command is appended to parent's child list. The parent command then owns this command and will delete it in the destructor.

See also
~QUndoCommand()
QUndoCommand::~QUndoCommand ( )
virtual

Destroys the QUndoCommand object and all child commands.

See also
QUndoCommand()

Method Documentation

QString QUndoCommand::actionText ( ) const

Returns a short text string describing what this command does; for example, "insert text".

The text is used when the text properties of the stack's undo and redo actions are updated.

See also
text(), setText(), QUndoStack::createUndoAction(), QUndoStack::createRedoAction()
const QUndoCommand * QUndoCommand::child ( int  index) const

Returns the child command at index.

See also
childCount(), QUndoStack::command()
int QUndoCommand::childCount ( ) const

Returns the number of child commands in this command.

See also
child()
int QUndoCommand::id ( ) const
virtual

Returns the ID of this command. A command ID is used in command compression. It must be an integer unique to this command's class, or -1 if the command does not support compression. If the command supports compression this method must be overridden in the derived class to return the correct ID. The base implementation returns -1.

QUndoStack::push() will only try to merge two commands if they have the same ID, and the ID is not -1.

See also
mergeWith(), QUndoStack::push()
bool QUndoCommand::mergeWith ( const QUndoCommand *  other)
virtual

Attempts to merge this QUndoCommand with other. Returns true on success, otherwise returns false.

If this method returns true, calling redo() must have the same effect as redoing both this QUndoCommand and the passed command. QUndoStack will only try to merge two commands if they have the same id and the id is not -1.

The default implementation returns false.

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;
}
See also
id(), QUndoStack::push()
void QUndoCommand::redo ( )
virtual

Applies a change to the document. This method must be implemented in the derived class. Calling QUndoStack::push(), QUndoStack::undo() or QUndoStack::redo() from this method leads to undefined behavior.

The default implementation calls redo() on all child commands.

See also
undo()
void QUndoCommand::setText ( const QString text)

Sets the command's text to be the text specified. The specified text should be a short user-readable string describing what this command does.

If you need to have two different strings for text() and actionText(), separate them with "\n" and pass into this function. Even if you do not use this feature for English strings during development, you can still let translators use two different strings in order to match specific languages' needs.

See also
text(), actionText(), QUndoStack::createUndoAction(), QUndoStack::createRedoAction()
QString QUndoCommand::text ( ) const

Returns a short text string describing what this command does. For example, "insert text". The text is used for names of items in QUndoView.

See also
actionText(), setText(), QUndoStack::createUndoAction(), QUndoStack::createRedoAction()
void QUndoCommand::undo ( )
virtual

Reverts a change to the document. After undo() is called, the state of the document should be the same as before redo() was called. This function must be implemented in the derived class. Calling QUndoStack::push(), QUndoStack::undo() or QUndoStack::redo() from this function leads to undefined behavior.

The default implementation calls undo() on all child commands in reverse order.

See also
redo()