Memento em Java

O Memento é um padrão de projeto comportamental que permite tirar um “retrato” do estado de um objeto e restaurá-lo no futuro.

O Memento não compromete a estrutura interna do objeto com o qual trabalha, nem os dados mantidos dentro dos retratos.

Saiba mais sobre o Memento

Complexidade:

Popularidade:

Exemplos de uso: O princípio do Memento pode ser alcançado usando a serialização, o que é bastante comum em Java. Embora não seja a única e a maneira mais eficiente de tirar retratos do estado de um objeto, ainda permite armazenar backups de estado, protegendo a estrutura do originador de outros objetos.

Aqui estão alguns exemplos do padrão nas principais bibliotecas Java:

All java.io.Serializable implementações podem simular o Memento.
Todas as implementações de javax.faces.component.StateHolder.

Editor de formas e desfazer/refazer complexo

Este editor gráfico permite alterar a cor e a posição das formas na tela. No entanto, qualquer modificação pode ser desfeita e repetida.

O “desfazer” é baseado na colaboração entre os padrões Memento e Command. O editor rastreia um histórico dos comandos executados. Antes de executar qualquer comando, ele faz um backup e o conecta ao objeto de comando. Após a execução, ele adiciona o comando executado para o histórico.

Quando um usuário solicita o desfazer, o editor busca um comando recente do histórico e restaura o estado do backup mantido dentro desse comando. Se o usuário solicitar outro desfazer, o editor utilizará o próximo comando do histórico e assim por diante.

Os comandos revertidos são mantidos no histórico até que o usuário faça algumas modificações nas formas na tela. Isso é crucial para refazer comandos desfeitos.

editor

editor/Editor.java: Código do editor

package refactoring_guru.memento.example.editor;

import refactoring_guru.memento.example.commands.Command;
import refactoring_guru.memento.example.history.History;
import refactoring_guru.memento.example.history.Memento;
import refactoring_guru.memento.example.shapes.CompoundShape;
import refactoring_guru.memento.example.shapes.Shape;

import javax.swing.*;
import java.io.*;
import java.util.Base64;

public class Editor extends JComponent {
    private Canvas canvas;
    private CompoundShape allShapes = new CompoundShape();
    private History history;

    public Editor() {
        canvas = new Canvas(this);
        history = new History();
    }

    public void loadShapes(Shape... shapes) {
        allShapes.clear();
        allShapes.add(shapes);
        canvas.refresh();
    }

    public CompoundShape getShapes() {
        return allShapes;
    }

    public void execute(Command c) {
        history.push(c, new Memento(this));
        c.execute();
    }

    public void undo() {
        if (history.undo())
            canvas.repaint();
    }

    public void redo() {
        if (history.redo())
            canvas.repaint();
    }

    public String backup() {
        try {
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            ObjectOutputStream oos = new ObjectOutputStream(baos);
            oos.writeObject(this.allShapes);
            oos.close();
            return Base64.getEncoder().encodeToString(baos.toByteArray());
        } catch (IOException e) {
            return "";
        }
    }

    public void restore(String state) {
        try {
            byte[] data = Base64.getDecoder().decode(state);
            ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(data));
            this.allShapes = (CompoundShape) ois.readObject();
            ois.close();
        } catch (ClassNotFoundException e) {
            System.out.print("ClassNotFoundException occurred.");
        } catch (IOException e) {
            System.out.print("IOException occurred.");
        }
    }
}

editor/Canvas.java: Código da tela

package refactoring_guru.memento.example.editor;

import refactoring_guru.memento.example.commands.ColorCommand;
import refactoring_guru.memento.example.commands.MoveCommand;
import refactoring_guru.memento.example.shapes.Shape;

import javax.swing.*;
import javax.swing.border.Border;
import java.awt.*;
import java.awt.event.*;
import java.awt.image.BufferedImage;

class Canvas extends java.awt.Canvas {
    private Editor editor;
    private JFrame frame;
    private static final int PADDING = 10;

    Canvas(Editor editor) {
        this.editor = editor;
        createFrame();
        attachKeyboardListeners();
        attachMouseListeners();
        refresh();
    }

    private void createFrame() {
        frame = new JFrame();
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setLocationRelativeTo(null);

        JPanel contentPanel = new JPanel();
        Border padding = BorderFactory.createEmptyBorder(PADDING, PADDING, PADDING, PADDING);
        contentPanel.setBorder(padding);
        contentPanel.setLayout(new BoxLayout(contentPanel, BoxLayout.Y_AXIS));
        frame.setContentPane(contentPanel);

        contentPanel.add(new JLabel("Select and drag to move."), BorderLayout.PAGE_END);
        contentPanel.add(new JLabel("Right click to change color."), BorderLayout.PAGE_END);
        contentPanel.add(new JLabel("Undo: Ctrl+Z, Redo: Ctrl+R"), BorderLayout.PAGE_END);
        contentPanel.add(this);
        frame.setVisible(true);
        contentPanel.setBackground(Color.LIGHT_GRAY);
    }

    private void attachKeyboardListeners() {
        addKeyListener(new KeyAdapter() {
            @Override
            public void keyPressed(KeyEvent e) {
                if ((e.getModifiers() & KeyEvent.CTRL_MASK) != 0) {
                    switch (e.getKeyCode()) {
                        case KeyEvent.VK_Z:
                            editor.undo();
                            break;
                        case KeyEvent.VK_R:
                            editor.redo();
                            break;
                    }
                }
            }
        });
    }

    private void attachMouseListeners() {
        MouseAdapter colorizer = new MouseAdapter() {
            @Override
            public void mousePressed(MouseEvent e) {
                if (e.getButton() != MouseEvent.BUTTON3) {
                    return;
                }
                Shape target = editor.getShapes().getChildAt(e.getX(), e.getY());
                if (target != null) {
                    editor.execute(new ColorCommand(editor, new Color((int) (Math.random() * 0x1000000))));
                    repaint();
                }
            }
        };
        addMouseListener(colorizer);

        MouseAdapter selector = new MouseAdapter() {
            @Override
            public void mousePressed(MouseEvent e) {
                if (e.getButton() != MouseEvent.BUTTON1) {
                    return;
                }

                Shape target = editor.getShapes().getChildAt(e.getX(), e.getY());
                boolean ctrl = (e.getModifiers() & ActionEvent.CTRL_MASK) == ActionEvent.CTRL_MASK;

                if (target == null) {
                    if (!ctrl) {
                        editor.getShapes().unSelect();
                    }
                } else {
                    if (ctrl) {
                        if (target.isSelected()) {
                            target.unSelect();
                        } else {
                            target.select();
                        }
                    } else {
                        if (!target.isSelected()) {
                            editor.getShapes().unSelect();
                        }
                        target.select();
                    }
                }
                repaint();
            }
        };
        addMouseListener(selector);


        MouseAdapter dragger = new MouseAdapter() {
            MoveCommand moveCommand;

            @Override
            public void mouseDragged(MouseEvent e) {
                if ((e.getModifiersEx() & MouseEvent.BUTTON1_DOWN_MASK) != MouseEvent.BUTTON1_DOWN_MASK) {
                    return;
                }
                if (moveCommand == null) {
                    moveCommand = new MoveCommand(editor);
                    moveCommand.start(e.getX(), e.getY());
                }
                moveCommand.move(e.getX(), e.getY());
                repaint();
            }

            @Override
            public void mouseReleased(MouseEvent e) {
                if (e.getButton() != MouseEvent.BUTTON1 || moveCommand == null) {
                    return;
                }
                moveCommand.stop(e.getX(), e.getY());
                editor.execute(moveCommand);
                this.moveCommand = null;
                repaint();
            }
        };
        addMouseListener(dragger);
        addMouseMotionListener(dragger);
    }

    public int getWidth() {
        return editor.getShapes().getX() + editor.getShapes().getWidth() + PADDING;
    }

    public int getHeight() {
        return editor.getShapes().getY() + editor.getShapes().getHeight() + PADDING;
    }

    void refresh() {
        this.setSize(getWidth(), getHeight());
        frame.pack();
    }

    public void update(Graphics g) {
        paint(g);
    }

    public void paint(Graphics graphics) {
        BufferedImage buffer = new BufferedImage(this.getWidth(), this.getHeight(), BufferedImage.TYPE_INT_RGB);
        Graphics2D ig2 = buffer.createGraphics();
        ig2.setBackground(Color.WHITE);
        ig2.clearRect(0, 0, this.getWidth(), this.getHeight());

        editor.getShapes().paint(buffer.getGraphics());

        graphics.drawImage(buffer, 0, 0, null);
    }
}

history

history/History.java: Histórico armazena comandos e mementos

package refactoring_guru.memento.example.history;

import refactoring_guru.memento.example.commands.Command;

import java.util.ArrayList;
import java.util.List;

public class History {
    private List<Pair> history = new ArrayList<Pair>();
    private int virtualSize = 0;

    private class Pair {
        Command command;
        Memento memento;
        Pair(Command c, Memento m) {
            command = c;
            memento = m;
        }

        private Command getCommand() {
            return command;
        }

        private Memento getMemento() {
            return memento;
        }
    }

    public void push(Command c, Memento m) {
        if (virtualSize != history.size() && virtualSize > 0) {
            history = history.subList(0, virtualSize - 1);
        }
        history.add(new Pair(c, m));
        virtualSize = history.size();
    }

    public boolean undo() {
        Pair pair = getUndo();
        if (pair == null) {
            return false;
        }
        System.out.println("Undoing: " + pair.getCommand().getName());
        pair.getMemento().restore();
        return true;
    }

    public boolean redo() {
        Pair pair = getRedo();
        if (pair == null) {
            return false;
        }
        System.out.println("Redoing: " + pair.getCommand().getName());
        pair.getMemento().restore();
        pair.getCommand().execute();
        return true;
    }

    private Pair getUndo() {
        if (virtualSize == 0) {
            return null;
        }
        virtualSize = Math.max(0, virtualSize - 1);
        return history.get(virtualSize);
    }

    private Pair getRedo() {
        if (virtualSize == history.size()) {
            return null;
        }
        virtualSize = Math.min(history.size(), virtualSize + 1);
        return history.get(virtualSize - 1);
    }
}

history/Memento.java: Classe Memento

package refactoring_guru.memento.example.history;

import refactoring_guru.memento.example.editor.Editor;

public class Memento {
    private String backup;
    private Editor editor;

    public Memento(Editor editor) {
        this.editor = editor;
        this.backup = editor.backup();
    }

    public void restore() {
        editor.restore(backup);
    }
}

commands

commands/Command.java: Classe Comando base

package refactoring_guru.memento.example.commands;

public interface Command {
    String getName();
    void execute();
}

commands/ColorCommand.java: Muda a forma selecionada

package refactoring_guru.memento.example.commands;

import refactoring_guru.memento.example.editor.Editor;
import refactoring_guru.memento.example.shapes.Shape;

import java.awt.*;

public class ColorCommand implements Command {
    private Editor editor;
    private Color color;

    public ColorCommand(Editor editor, Color color) {
        this.editor = editor;
        this.color = color;
    }

    @Override
    public String getName() {
        return "Colorize: " + color.toString();
    }

    @Override
    public void execute() {
        for (Shape child : editor.getShapes().getSelected()) {
            child.setColor(color);
        }
    }
}

commands/MoveCommand.java: Move a forma selecionada

package refactoring_guru.memento.example.commands;

import refactoring_guru.memento.example.editor.Editor;
import refactoring_guru.memento.example.shapes.Shape;

public class MoveCommand implements Command {
    private Editor editor;
    private int startX, startY;
    private int endX, endY;

    public MoveCommand(Editor editor) {
        this.editor = editor;
    }

    @Override
    public String getName() {
        return "Move by X:" + (endX - startX) + " Y:" + (endY - startY);
    }

    public void start(int x, int y) {
        startX = x;
        startY = y;
        for (Shape child : editor.getShapes().getSelected()) {
            child.drag();
        }
    }

    public void move(int x, int y) {
        for (Shape child : editor.getShapes().getSelected()) {
            child.moveTo(x - startX, y - startY);
        }
    }

    public void stop(int x, int y) {
        endX = x;
        endY = y;
        for (Shape child : editor.getShapes().getSelected()) {
            child.drop();
        }
    }

    @Override
    public void execute() {
        for (Shape child : editor.getShapes().getSelected()) {
            child.moveBy(endX - startX, endY - startY);
        }
    }
}

shapes: Várias formas

shapes/Shape.java

package refactoring_guru.memento.example.shapes;

import java.awt.*;
import java.io.Serializable;

public interface Shape extends Serializable {
    int getX();
    int getY();
    int getWidth();
    int getHeight();
    void drag();
    void drop();
    void moveTo(int x, int y);
    void moveBy(int x, int y);
    boolean isInsideBounds(int x, int y);
    Color getColor();
    void setColor(Color color);
    void select();
    void unSelect();
    boolean isSelected();
    void paint(Graphics graphics);
}

shapes/BaseShape.java

package refactoring_guru.memento.example.shapes;

import java.awt.*;

public abstract class BaseShape implements Shape {
    int x, y;
    private int dx = 0, dy = 0;
    private Color color;
    private boolean selected = false;

    BaseShape(int x, int y, Color color) {
        this.x = x;
        this.y = y;
        this.color = color;
    }

    @Override
    public int getX() {
        return x;
    }

    @Override
    public int getY() {
        return y;
    }

    @Override
    public int getWidth() {
        return 0;
    }

    @Override
    public int getHeight() {
        return 0;
    }

    @Override
    public void drag() {
        dx = x;
        dy = y;
    }

    @Override
    public void moveTo(int x, int y) {
        this.x = dx + x;
        this.y = dy + y;
    }

    @Override
    public void moveBy(int x, int y) {
        this.x += x;
        this.y += y;
    }

    @Override
    public void drop() {
        this.x = dx;
        this.y = dy;
    }

    @Override
    public boolean isInsideBounds(int x, int y) {
        return x > getX() && x < (getX() + getWidth()) &&
                y > getY() && y < (getY() + getHeight());
    }

    @Override
    public Color getColor() {
        return color;
    }

    @Override
    public void setColor(Color color) {
        this.color = color;
    }

    @Override
    public void select() {
        selected = true;
    }

    @Override
    public void unSelect() {
        selected = false;
    }

    @Override
    public boolean isSelected() {
        return selected;
    }

    void enableSelectionStyle(Graphics graphics) {
        graphics.setColor(Color.LIGHT_GRAY);

        Graphics2D g2 = (Graphics2D) graphics;
        float[] dash1 = {2.0f};
        g2.setStroke(new BasicStroke(1.0f,
                BasicStroke.CAP_BUTT,
                BasicStroke.JOIN_MITER,
                2.0f, dash1, 0.0f));
    }

    void disableSelectionStyle(Graphics graphics) {
        graphics.setColor(color);
        Graphics2D g2 = (Graphics2D) graphics;
        g2.setStroke(new BasicStroke());
    }

    @Override
    public void paint(Graphics graphics) {
        if (isSelected()) {
            enableSelectionStyle(graphics);
        }
        else {
            disableSelectionStyle(graphics);
        }

        // ...
    }
}

shapes/Circle.java

package refactoring_guru.memento.example.shapes;

import java.awt.*;

public class Circle extends BaseShape {
    private int radius;

    public Circle(int x, int y, int radius, Color color) {
        super(x, y, color);
        this.radius = radius;
    }

    @Override
    public int getWidth() {
        return radius * 2;
    }

    @Override
    public int getHeight() {
        return radius * 2;
    }

    @Override
    public void paint(Graphics graphics) {
        super.paint(graphics);
        graphics.drawOval(x, y, getWidth() - 1, getHeight() - 1);
    }
}

shapes/Dot.java

package refactoring_guru.memento.example.shapes;

import java.awt.*;

public class Dot extends BaseShape {
    private final int DOT_SIZE = 3;

    public Dot(int x, int y, Color color) {
        super(x, y, color);
    }

    @Override
    public int getWidth() {
        return DOT_SIZE;
    }

    @Override
    public int getHeight() {
        return DOT_SIZE;
    }

    @Override
    public void paint(Graphics graphics) {
        super.paint(graphics);
        graphics.fillRect(x - 1, y - 1, getWidth(), getHeight());
    }
}

shapes/Rectangle.java

package refactoring_guru.memento.example.shapes;

import java.awt.*;

public class Rectangle extends BaseShape {
    private int width;
    private int height;

    public Rectangle(int x, int y, int width, int height, Color color) {
        super(x, y, color);
        this.width = width;
        this.height = height;
    }

    @Override
    public int getWidth() {
        return width;
    }

    @Override
    public int getHeight() {
        return height;
    }

    @Override
    public void paint(Graphics graphics) {
        super.paint(graphics);
        graphics.drawRect(x, y, getWidth() - 1, getHeight() - 1);
    }
}

shapes/CompoundShape.java

package refactoring_guru.memento.example.shapes;

import java.awt.*;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class CompoundShape extends BaseShape {
    private List<Shape> children = new ArrayList<>();

    public CompoundShape(Shape... components) {
        super(0, 0, Color.BLACK);
        add(components);
    }

    public void add(Shape component) {
        children.add(component);
    }

    public void add(Shape... components) {
        children.addAll(Arrays.asList(components));
    }

    public void remove(Shape child) {
        children.remove(child);
    }

    public void remove(Shape... components) {
        children.removeAll(Arrays.asList(components));
    }

    public void clear() {
        children.clear();
    }

    @Override
    public int getX() {
        if (children.size() == 0) {
            return 0;
        }
        int x = children.get(0).getX();
        for (Shape child : children) {
            if (child.getX() < x) {
                x = child.getX();
            }
        }
        return x;
    }

    @Override
    public int getY() {
        if (children.size() == 0) {
            return 0;
        }
        int y = children.get(0).getY();
        for (Shape child : children) {
            if (child.getY() < y) {
                y = child.getY();
            }
        }
        return y;
    }

    @Override
    public int getWidth() {
        int maxWidth = 0;
        int x = getX();
        for (Shape child : children) {
            int childsRelativeX = child.getX() - x;
            int childWidth = childsRelativeX + child.getWidth();
            if (childWidth > maxWidth) {
                maxWidth = childWidth;
            }
        }
        return maxWidth;
    }

    @Override
    public int getHeight() {
        int maxHeight = 0;
        int y = getY();
        for (Shape child : children) {
            int childsRelativeY = child.getY() - y;
            int childHeight = childsRelativeY + child.getHeight();
            if (childHeight > maxHeight) {
                maxHeight = childHeight;
            }
        }
        return maxHeight;
    }

    @Override
    public void drag() {
        for (Shape child : children) {
            child.drag();
        }
    }

    @Override
    public void drop() {
        for (Shape child : children) {
            child.drop();
        }
    }

    @Override
    public void moveTo(int x, int y) {
        for (Shape child : children) {
            child.moveTo(x, y);
        }
    }

    @Override
    public void moveBy(int x, int y) {
        for (Shape child : children) {
            child.moveBy(x, y);
        }
    }

    @Override
    public boolean isInsideBounds(int x, int y) {
        for (Shape child : children) {
            if (child.isInsideBounds(x, y)) {
                return true;
            }
        }
        return false;
    }

    @Override
    public void setColor(Color color) {
        super.setColor(color);
        for (Shape child : children) {
            child.setColor(color);
        }
    }

    @Override
    public void unSelect() {
        super.unSelect();
        for (Shape child : children) {
            child.unSelect();
        }
    }

    public Shape getChildAt(int x, int y) {
        for (Shape child : children) {
            if (child.isInsideBounds(x, y)) {
                return child;
            }
        }
        return null;
    }

    public boolean selectChildAt(int x, int y) {
        Shape child = getChildAt(x,y);
        if (child != null) {
            child.select();
            return true;
        }
        return false;
    }

    public List<Shape> getSelected() {
        List<Shape> selected = new ArrayList<>();
        for (Shape child : children) {
            if (child.isSelected()) {
                selected.add(child);
            }
        }
        return selected;
    }

    @Override
    public void paint(Graphics graphics) {
        if (isSelected()) {
            enableSelectionStyle(graphics);
            graphics.drawRect(getX() - 1, getY() - 1, getWidth() + 1, getHeight() + 1);
            disableSelectionStyle(graphics);
        }

        for (Shape child : children) {
            child.paint(graphics);
        }
    }
}

Demo.java: Código de inicialização

package refactoring_guru.memento.example;

import refactoring_guru.memento.example.editor.Editor;
import refactoring_guru.memento.example.shapes.Circle;
import refactoring_guru.memento.example.shapes.CompoundShape;
import refactoring_guru.memento.example.shapes.Dot;
import refactoring_guru.memento.example.shapes.Rectangle;

import java.awt.*;

public class Demo {
    public static void main(String[] args) {
        Editor editor = new Editor();
        editor.loadShapes(
                new Circle(10, 10, 10, Color.BLUE),

                new CompoundShape(
                        new Circle(110, 110, 50, Color.RED),
                        new Dot(160, 160, Color.RED)
                ),

                new CompoundShape(
                        new Rectangle(250, 250, 100, 100, Color.GREEN),
                        new Dot(240, 240, Color.GREEN),
                        new Dot(240, 360, Color.GREEN),
                        new Dot(360, 360, Color.GREEN),
                        new Dot(360, 240, Color.GREEN)
                )
        );
    }
}