Multiple calls of paintComponent()

问题

I have these two classes:

public class Pencil extends JComponent implements MouseListener, MouseMotionListener{
Plansa plansa;
Graphics g;
public Pencil(Plansa newCanvas){
    this.plansa = newCanvas;
    this.plansa.setFlagShape(false);
}
@Override
public void mouseDragged(MouseEvent arg0) {
    plansa.setMouseDragged(arg0);
    this.plansa.setFlagShape(false);
    plansa.paintComponent(plansa.getGraphics());
}

@Override
public void mouseClicked(MouseEvent e) {
}

@Override
public void mousePressed(MouseEvent arg0) {
    plansa.setMousePressed(arg0);
}

@Override
public void mouseReleased(MouseEvent arg0) {
    // TODO Auto-generated method stub
    plansa.setMouseReleased(arg0);
    this.plansa.setFlagShape(true);
    plansa.paintComponent(plansa.getGraphics());

}

@Override
public void mouseEntered(MouseEvent e) {
}

@Override
public void mouseExited(MouseEvent e) {
}

@Override
public void mouseMoved(MouseEvent e) {
}

}

And this one:

public class Plansa extends JPanel{
Image image;
Pencil pencil;
 //...
 void init(){
    this.setShape("freeLine");
    this.setColor(Color.BLACK);
    this.size=1;
    this.type="round";
    this.fill=false;
    this.setBackground(Color.WHITE);
    pencil = new Pencil(this);
    addMouseListener(pencil);
    addMouseMotionListener(pencil);
    flagPaint = true;
    flagShape = false;
}
public Plansa(){
    this.setSize(800, 600);
    init();
}

//...

       @Override
    public void paintComponent(Graphics g) {
    g.setColor(currentColor);
    Graphics2D g2d = (Graphics2D) g;
    g2d.setStroke(setBrush(size,type));
    super.paintComponent(g);
    switch(shape){
        default: break;
        case "freeLine":{ 
            g.drawLine(xDragged, yDragged, xCurrent, yCurrent); 
            break;
        }
            case "rectangle":{
                if(flagShape == true){
            g.drawRect(xPressed, yPressed, Math.max(xCurrent-xPressed,xPressed-xCurrent), Math.max(yCurrent-yPressed,yPressed-yCurrent));
            if(fill == true) g.fillRect(xPressed, yPressed, Math.max(xCurrent-xPressed,xPressed-xCurrent), Math.max(yCurrent-yPressed,yPressed-yCurrent));
                }
            break;
        }
        case "circle":{
            if(flagShape == true){
            int radius = (int)Math.sqrt(Math.max(xCurrent-xPressed,xPressed-xCurrent)*Math.max(xCurrent-xPressed,xPressed-xCurrent)+Math.max(yCurrent-yPressed,yPressed-yCurrent)*Math.max(yCurrent-yPressed,yPressed-yCurrent));
            g.drawOval(xPressed, yPressed, radius, radius);
            if(fill == true) g.fillOval(xPressed, yPressed, radius, radius);
            }
            break;
        }
        case "oval":{
            if(flagShape == true){
            g.drawOval(xPressed, yPressed, Math.max(xCurrent-xPressed,xPressed-xCurrent), Math.max(yCurrent-yPressed,yPressed-yCurrent));
            if(fill == true) g.fillOval(xPressed, yPressed, Math.max(xCurrent-xPressed,xPressed-xCurrent), Math.max(yCurrent-yPressed,yPressed-yCurrent));
            }
            break;
        }
        case "line":{
            if(flagShape == true){
            g.drawLine(xPressed, yPressed, xCurrent, yCurrent);
            }
            break;
        }
    }

}

 //...
}

My problem is that every time I call paintComponent() method, the JPanel clears and the only item that remains is the one I just drawn. Is there any way to avoid this?

回答1:

Store all the objects in an ArrayList and iterate through it when you are drawing.

You could have them all implement a custom interface, Drawable for example, then store in an ArrayList<Drawable>.

回答2:

After close inspection of the code, it appears that you're trying to use Components as "painters", it's a bit like using a sports car as a go-kart, a lot of extras for little gain.

Instead, you should define yourself a interface of some kine that provides the basic requirements of what you want to draw/paint and then define concrete class implementations that implement the functionality.

You would then maintain a List of some kind of all the drawings that can be painted.

While simple, the example below provides a jumping off point that could be enhanced that allow selection of drawings, re-ordering and removal (should you want it).

This, is, essentially, the same concept proposed by Doorknob (+1)

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import java.awt.geom.Line2D;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class TestDraw {

    public static void main(String[] args) {
        new TestDraw();
    }

    public TestDraw() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new Pencil());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class Pencil extends JPanel implements MouseListener, MouseMotionListener {

        private List<Drawable> drawables;
        private Drawable activeDrawable;

        private Point clickPoint;

        public Pencil() {
            drawables = new ArrayList<>(5);
            addMouseListener(this);
            addMouseMotionListener(this);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(400, 400);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            for (Drawable drawable : drawables) {
                drawable.paint(g2d);
            }
            g2d.dispose();
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            if (activeDrawable != null) {
                Point p = e.getPoint();
                Rectangle bounds = activeDrawable.getBounds();

                int x = bounds.x;
                int y = bounds.y;
                int width = p.x - clickPoint.x;
                int height = p.y - clickPoint.y;
                if (width < 0) {
                    width *= -1;
                    x = p.x;
                }
                if (height < 0) {
                    height *= -1;
                    y = p.y;
                }
                bounds = new Rectangle(x, y, width, height);
                System.out.println(bounds);
                activeDrawable.setBounds(bounds);
                repaint();
            }
        }

        @Override
        public void mouseClicked(MouseEvent e) {
        }

        protected Drawable createActiveShape(MouseEvent e) {

            System.out.println("Anchor = " + e.getPoint());
            Drawable drawable = new FreeLine(e.getPoint());
            drawable.setLocation(e.getPoint());
            return drawable;

        }

        @Override
        public void mousePressed(MouseEvent e) {
            // You could also check to see if the clicked on a drawable...
            clickPoint = e.getPoint();
            activeDrawable = createActiveShape(e);
            drawables.add(activeDrawable);
            repaint();
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            if (activeDrawable != null) {
                Rectangle bounds = activeDrawable.getBounds();
                if (bounds.width == 0 || bounds.height == 0) {
                    drawables.remove(activeDrawable);
                }
            }
            clickPoint = null;
            activeDrawable = null;
        }

        @Override
        public void mouseEntered(MouseEvent e) {
        }

        @Override
        public void mouseExited(MouseEvent e) {
        }

        @Override
        public void mouseMoved(MouseEvent e) {
        }
    }

    public interface Drawable {

        public void setLocation(Point p);

        public void setSize(Dimension dim);

        public void setBounds(Rectangle bounds);

        public Rectangle getBounds();

        public void paint(Graphics2D g2d);
    }

    public abstract class AbstractDrawable implements Drawable {

        private Rectangle bounds;

        public AbstractDrawable() {
            bounds = new Rectangle();
        }

        @Override
        public void setLocation(Point p) {
            bounds.setLocation(p);
        }

        @Override
        public void setBounds(Rectangle bounds) {
            this.bounds = bounds;
        }

        @Override
        public void setSize(Dimension dim) {
            bounds.setSize(dim);
        }

        @Override
        public Rectangle getBounds() {
            return bounds;
        }
    }

    public class FreeLine extends AbstractDrawable {

        private Point anchor;

        public FreeLine(Point anchor) {
            this.anchor = anchor;
        }

        @Override
        public void paint(Graphics2D g2d) {
            Rectangle bounds = getBounds();
            Point p1 = new Point(anchor);
            Point p2 = new Point(bounds.getLocation());
            if (p1.x > p2.x) {
                p2.x = p1.x - bounds.width;
            } else {
                p2.x = p1.x + bounds.width;
            }
            if (p1.y > p2.y) {
                p2.y = p1.y - bounds.height;
            } else {
                p2.y = p1.y + bounds.height;
            }
            g2d.draw(new Line2D.Float(p1, p2));
        }
    }

回答3:

A better idea generally for progressive drawings is to use a BufferedImage as the canvas, as seen here.

回答4:

Don't use/comment out:

super.paintComponent(g);

That line is the one that is doing the clearing.

来源：https://stackoverflow.com/questions/15890704/multiple-calls-of-paintcomponent