QUndoStack¶

The QUndoStack class is a stack of QUndoCommand objects. More…

Synopsis¶

Functions¶

def beginMacro (text)
def canRedo ()
def canUndo ()
def cleanIndex ()
def clear ()
def command (index)
def count ()
def createRedoAction (parent[, prefix=””])
def createUndoAction (parent[, prefix=””])
def endMacro ()
def index ()
def isActive ()
def isClean ()
def push (cmd)
def redoText ()
def setUndoLimit (limit)
def text (idx)
def undoLimit ()
def undoText ()

Slots¶

def redo ()
def resetClean ()
def setActive ([active=true])
def setClean ()
def setIndex (idx)
def undo ()

Signals¶

def canRedoChanged (canRedo)
def canUndoChanged (canUndo)
def cleanChanged (clean)
def indexChanged (idx)
def redoTextChanged (redoText)
def undoTextChanged (undoText)

Detailed Description¶

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

An undo stack maintains a stack of commands that have been applied to a document.

New commands are pushed on the stack using push() . Commands can be undone and redone using undo() and redo() , or by triggering the actions returned by createUndoAction() and createRedoAction() .

QUndoStack keeps track of the current command. This is the command which will be executed by the next call to redo() . The index of this command is returned by index() . The state of the edited object can be rolled forward or back using setIndex() . If the top-most command on the stack has already been redone, index() is equal to count() .

QUndoStack provides support for undo and redo actions, command compression, command macros, and supports the concept of a clean state.

Undo and Redo Actions¶

QUndoStack provides convenient undo and redo QAction objects, which can be inserted into a menu or a toolbar. When commands are undone or redone, QUndoStack updates the text properties of these actions to reflect what change they will trigger. The actions are also disabled when no command is available for undo or redo. These actions are returned by createUndoAction() and createRedoAction() .

Command Compression and Macros¶

Command compression is useful when several commands can be compressed into a single command that can be undone and redone in a single operation. For example, when a user types a character in a text editor, a new command is created. This command inserts the character into the document at the cursor position. However, it is more convenient for the user to be able to undo or redo typing of whole words, sentences, or paragraphs. Command compression allows these single-character commands to be merged into a single command which inserts or deletes sections of text. For more information, see mergeWith() and push() .

A command macro is a sequence of commands, all of which are undone and redone in one go. Command macros are created by giving a command a list of child commands. Undoing or redoing the parent command will cause the child commands to be undone or redone. Command macros may be created explicitly by specifying a parent in the QUndoCommand constructor, or by using the convenience functions beginMacro() and endMacro() .

Although command compression and macros appear to have the same effect to the user, they often have different uses in an application. Commands that perform small changes to a document may be usefully compressed if there is no need to individually record them, and if only larger changes are relevant to the user. However, for commands that need to be recorded individually, or those that cannot be compressed, it is useful to use macros to provide a more convenient user experience while maintaining a record of each command.

Clean State¶

QUndoStack supports the concept of a clean state. When the document is saved to disk, the stack can be marked as clean using setClean() . Whenever the stack returns to this state through undoing and redoing commands, it emits the signal cleanChanged() . This signal is also emitted when the stack leaves the clean state. This signal is usually used to enable and disable the save actions in the application, and to update the document’s title to reflect that it contains unsaved changes.

Obsolete Commands¶

QUndoStack is able to delete commands from the stack if the command is no longer needed. One example may be to delete a command when two commands are merged together in such a way that the merged command has no function. This can be seen with move commands where the user moves their mouse to one part of the screen and then moves it to the original position. The merged command results in a mouse movement of 0. This command can be deleted since it serves no purpose. Another example is with networking commands that fail due to connection issues. In this case, the command is to be removed from the stack because the redo() and undo() functions have no function since there was connection issues.

A command can be marked obsolete with the setObsolete() function. The isObsolete() flag is checked in push() , undo() , redo() , and setIndex() after calling undo() , redo() and QUndoCommand :mergeWith() where applicable.

If a command is set obsolete and the clean index is greater than or equal to the current command index, then the clean index will be reset when the command is deleted from the stack.

See also

QUndoCommand QUndoView

class PySide6.QtGui.QUndoStack([parent=None])¶

Parameters: parent – PySide6.QtCore.QObject

Constructs an empty undo stack with the parent parent. The stack will initially be in the clean state. If parent is a QUndoGroup object, the stack is automatically added to the group.

See also

push()

PySide6.QtGui.QUndoStack.beginMacro(text)¶

Parameters: text – str

Begins composition of a macro command with the given text description.

An empty command described by the specified text is pushed on the stack. Any subsequent commands pushed on the stack will be appended to the empty command’s children until endMacro() is called.

Calls to and endMacro() may be nested, but every call to must have a matching call to endMacro() .

While a macro is being composed, the stack is disabled. This means that:

indexChanged() and cleanChanged() are not emitted,

canUndo() and canRedo() return false,

calling undo() or redo() has no effect,

the undo/redo actions are disabled.

The stack becomes enabled and appropriate signals are emitted when endMacro() is called for the outermost macro.

stack.beginMacro("insert red text")
stack.push(InsertText(document, idx, text))
stack.push(SetColor(document, idx, text.length(), Qt.red))
stack.endMacro() # indexChanged() is emitted

This code is equivalent to:

insertRed = QUndoCommand() # an empty command()
insertRed.setText("insert red text")
InsertText(document, idx, text, insertRed) # becomes child of insertRed
SetColor(document, idx, text.length(), Qt.red, insertRed)
stack.push(insertRed)

Table of Contents

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QUndoStack¶

Synopsis¶

Functions¶

Slots¶

Signals¶

Detailed Description¶

Undo and Redo Actions¶

Command Compression and Macros¶

Clean State¶

Obsolete Commands¶