What is Memento design pattern?
What is Memento design pattern?
feature in an object. In a Memento pattern there are three
objects:
Originator: This is the object that has an internal state.
Caretaker: This is the object that can change the state of
Originator. But it wants to have control over rolling back
the change.
Memento: This is the object that Caretaker gets from
Originator, before making and change. If Caretaker wants
to Rollback the change it gives Memento back to
Originator. Originator can use Memento to restore its own
state to the original state.
E.g. One good use of memento is in online Forms. If we
want to show to user a form pre-populated with some
data, we keep this copy in memento. Now user can
update the form. But at any time when user wants to reset
the form, we use memento to make the form in its original
pre-populated state. If user wants to just save the form
we save the form and update the memento. Now onwards
any new changes to the form can be rolled back to the
last saved Memento object.
xxxxxxxxxxclass Editor is private field text, curX, curY, selectionWidth method setText(text) is this.text = text method setCursor(x, y) is this.curX = x this.curY = y method setSelectionWidth(width) is this.selectionWidth = width // Saves the current state inside a memento. method createSnapshot():Snapshot is // Memento is an immutable object; that's why the // originator passes its state to the memento's // constructor parameters. return new Snapshot(this, text, curX, curY, selectionWidth)// The memento class stores the past state of the editor.class Snapshot is private field editor: Editor private field text, curX, curY, selectionWidth constructor Snapshot(editor, text, curX, curY, selectionWidth) is this.editor = editor this.text = text this.curX = x this.curY = y this.selectionWidth = selectionWidth // At some point, a previous state of the editor can be // restored using a memento object. method restore() is editor.setText(text) editor.setCursor(curX, curY) editor.setSelectionWidth(selectionWidth)// A command object can act as a caretaker. In that case, the// command gets a memento just before it changes the// originator's state. When undo is requested, it restores the// originator's state from a memento.class Command is private field backup: Snapshot method makeBackup() is backup = editor.createSnapshot() method undo() is if (backup != null) backup.restore() // ...
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