com.trolltech.qt.gui
Class QUndoCommand

java.lang.Object
  com.trolltech.qt.internal.QSignalEmitterInternal
      com.trolltech.qt.QSignalEmitter
          com.trolltech.qt.QtJambiObject
              com.trolltech.qt.gui.QUndoCommand

All Implemented Interfaces:

QtJambiInterface

public class QUndoCommand
extends QtJambiObject

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.

    public class AppendText extends QUndoCommand {

        private String m_document;
        private String m_text;

        public AppendText(String doc, String text) {
            m_document = doc;
            m_text= text;
            setText("append text");
        }

        public void undo() {
            m_document = m_document.substring(0,
                    m_document.length() -m_text.length());
        }

        public void redo() {
            m_document += 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.GlobalColor.red, insertRed);

        stack.push(insertRed);

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

See also:

QUndoStack.

Nested Class Summary

Nested classes/interfaces inherited from class com.trolltech.qt.QSignalEmitter

QSignalEmitter.AbstractSignal, QSignalEmitter.PrivateSignal0, QSignalEmitter.PrivateSignal1, QSignalEmitter.PrivateSignal2, QSignalEmitter.PrivateSignal3, QSignalEmitter.PrivateSignal4, QSignalEmitter.PrivateSignal5, QSignalEmitter.PrivateSignal6, QSignalEmitter.PrivateSignal7, QSignalEmitter.PrivateSignal8, QSignalEmitter.PrivateSignal9, QSignalEmitter.Signal0, QSignalEmitter.Signal1, QSignalEmitter.Signal2, QSignalEmitter.Signal3, QSignalEmitter.Signal4, QSignalEmitter.Signal5, QSignalEmitter.Signal6, QSignalEmitter.Signal7, QSignalEmitter.Signal8, QSignalEmitter.Signal9

Nested classes/interfaces inherited from class com.trolltech.qt.internal.QSignalEmitterInternal

com.trolltech.qt.internal.QSignalEmitterInternal.AbstractSignalInternal

Field Summary

Fields inherited from class com.trolltech.qt.internal.QSignalEmitterInternal

currentSender

Constructor Summary

QUndoCommand()
Constructs a QUndoCommand object with parent parent.

QUndoCommand(QUndoCommand parent)
Constructs a QUndoCommand object with parent parent.

QUndoCommand(java.lang.String text)
Constructs a QUndoCommand object with the given parent and text.

QUndoCommand(java.lang.String text, QUndoCommand parent)
Constructs a QUndoCommand object with the given parent and text.

Method Summary

QUndoCommand	child(int index) Returns the child command at index.
int	childCount() Returns the number of child commands in this command.
int	id() Returns the ID of this command.
boolean	mergeWith(QUndoCommand other) Attempts to merge this command with command.
void	redo() Applies a change to the document.
void	setText(java.lang.String text) Sets the command's text to be the text specified.
java.lang.String	text() Returns a short text string describing what this command does; for example, "insert text".
void	undo() Reverts a change to the document.

Methods inherited from class com.trolltech.qt.QtJambiObject

dispose, disposed, equals, finalize, reassignNativeResources, tr, tr, tr

Methods inherited from class com.trolltech.qt.QSignalEmitter

blockSignals, disconnect, disconnect, signalsBlocked, signalSender, thread

Methods inherited from class com.trolltech.qt.internal.QSignalEmitterInternal

__qt_signalInitialization

Methods inherited from class java.lang.Object

clone, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface com.trolltech.qt.QtJambiInterface

disableGarbageCollection, nativeId, nativePointer, reenableGarbageCollection, setJavaOwnership

Constructor Detail

QUndoCommand

public QUndoCommand()

Constructs a QUndoCommand object with parent parent.

If parent is not 0, this command is appended to parent's child list. The parent command then owns this command and will delete it in its destructor.

See also:

~QUndoCommand().

QUndoCommand

public QUndoCommand(QUndoCommand parent)

Constructs a QUndoCommand object with parent parent.

If parent is not 0, this command is appended to parent's child list. The parent command then owns this command and will delete it in its destructor.

See also:

~QUndoCommand().

QUndoCommand

public QUndoCommand(java.lang.String text)

Constructs a QUndoCommand object with the given parent and text.

If parent is not 0, this command is appended to parent's child list. The parent command then owns this command and will delete it in its destructor.

See also:

~QUndoCommand().

QUndoCommand

public QUndoCommand(java.lang.String text,
                    QUndoCommand parent)

Constructs a QUndoCommand object with the given parent and text.

If parent is not 0, this command is appended to parent's child list. The parent command then owns this command and will delete it in its destructor.

See also:

~QUndoCommand().

Method Detail

child

public final QUndoCommand child(int index)

Returns the child command at index.

See also:

childCount(), and QUndoStack::command().

childCount

public final int childCount()

Returns the number of child commands in this command.

See also:

child().

setText

public final void setText(java.lang.String 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.

See also:

text(), QUndoStack::createUndoAction(), and QUndoStack::createRedoAction().

text

public final java.lang.String text()

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:

setText(), QUndoStack::createUndoAction(), and QUndoStack::createRedoAction().

id

public int id()

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 doesn't support compression.

If the command supports compression this function 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(), and QUndoStack::push().

mergeWith

public boolean mergeWith(QUndoCommand other)

Attempts to merge this command with command. Returns true on success; otherwise returns false.

If this function returns true, calling this command's redo() must have the same effect as redoing both this command and command. Similarly, calling this command's undo() must have the same effect as undoing command and this 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.

        public boolean mergeWith(QUndoCommand other)
        {
            if (other.id() != id()) // make sure other is also an AppendText command
                return false;
            m_text += ((AppendText)other).m_text;
            return true;
        }

See also:

id(), and QUndoStack::push().

redo

public void redo()

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

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

See also:

undo().

undo

public void undo()

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 beahavior.

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

See also:

redo().