如何使用Swing运行后台任务？

Question

Im currently making some sort of "Paint" with Swing components. It can only draw ellipses, rectangles and lines. The problem is, everytime I switch figures, the other figures won't remain on the JPanel. Every figure of the same type remains, for example: If I start drawing lines, they remain in the JPanel until I change of figure. Every figure I'm drawing gets saved to a class where I have defined the attributes of:

• StartPoint
• LastPoint
• TypeOfFigure
• OutLineColor
• FillingColor

And every object of that class is saved to an ArrayList. How can I redraw the elements of that arraylist everytime I switch figures? Heres my draw() method

private void draw() {
        mMouseIsDragging = false;
        mMouseReleased = false;
        MyPanel.this.addMouseListener(new MouseAdapter() {
            private Coordenadas mDraw;
            @Override
            public void mousePressed(MouseEvent e) {
                mCoordenadas.add(new Coordenadas());
                mDraw = (Coordenadas) MyPanel.this.mCoordenadas.get(MyPanel.this.mCoordenadas.size() - 1);
                mDraw.setColor(mColorLinea);
                mDraw.setColorRelleno(mColorRelleno);
                mDraw.setTipo(mOpcion);
                mStartPoint = e.getPoint();
                mDraw.setFirstPoint(e.getPoint());
                mMouseIsDragging = true;
           }
            @Override
            public void mouseReleased(MouseEvent e) {
                mDraw = (Coordenadas) MyPanel.this.mCoordenadas.get(MyPanel.this.mCoordenadas.size() - 1);
                mPuntoFinal = e.getPoint();
                mDraw.setLastPoint(e.getPoint());
                mMouseReleased = true;
                mMouseIsDragging = false;
                repaint();
            }
        });
        MyPanel.this.addMouseMotionListener(new MouseMotionAdapter() {
            @Override
            public void mouseDragged(MouseEvent e) {
                mPuntoFinal = e.getPoint();
                mMouseReleased = false;
                mMouseIsDragging = true;
                repaint();
            }
        });
    }

And this is the paintComponent method parts making the draw of these figures:

if (mMouseReleased == true && mMouseIsDragging == false) {
            for (int i = 0; i < mCoordenadas.size(); i++) {
                Coordenadas mCoord = (Coordenadas) mCoordenadas.get(i);
                switch (mCoord.getTipo()) {
                    case 1:
                        mG2D.setColor(mCoord.getColor());
                        mG2D.drawLine(mCoord.getFirstPoint().x, mCoord.getFirstPoint().y, mCoord.getLastPoint().x, mCoord.getLastPoint().y);
                        break;
                    case 2:
                        if (mCoord.getColorRelleno() != null) {
                            mG2D.setPaint(mCoord.getColorRelleno());
                            mG2D.fill(new Rectangle2D.Double(mCoord.getFirstPoint().x, mCoord.getFirstPoint().y, Math.abs(mCoord.getFirstPoint().x - mCoord.getLastPoint().x), Math.abs(mCoord.getFirstPoint().y - mCoord.getLastPoint().y)));
                        } else {
                            mG2D.setColor(mCoord.getColor());
                            mG2D.drawRect(mCoord.getFirstPoint().x, mCoord.getFirstPoint().y, Math.abs(mCoord.getFirstPoint().x - mCoord.getLastPoint().x), Math.abs(mCoord.getFirstPoint().y - mCoord.getLastPoint().y));
                        }
                        break;
                    case 3:
                        if (mCoord.getColorRelleno() != null) {
                            mG2D.setPaint(mCoord.getColorRelleno());
                            mG2D.fill(new Ellipse2D.Double(mCoord.getFirstPoint().y, mCoord.getFirstPoint().x, mCoord.getLastPoint().x, mCoord.getLastPoint().y));
                        } else {
                            mG2D.setColor(mCoord.getColor());
                            mG2D.drawOval(mCoord.getFirstPoint().x, mCoord.getFirstPoint().y, mCoord.getLastPoint().x, mCoord.getLastPoint().y);
                        }
                        break;
                }
            }
        }
if (mMouseReleased == false && mMouseIsDragging == true) {
            if (mColorRelleno != null) {
                mG2D.setBackground(mColorRelleno);
            } else {
                mG2D.setColor(mColorLinea);
            }
            switch (mOpcion) {
                case 1:
                    mG2D.drawLine(mPuntoInicio.x, mPuntoInicio.y, mPuntoFinal.x, mPuntoFinal.y);
                    break;
                case 2:
                    mG2D.drawRect(mPuntoInicio.x, mPuntoInicio.y, Math.abs(mPuntoInicio.x - mPuntoFinal.x), Math.abs(mPuntoInicio.y - mPuntoFinal.y));
                    break;
                case 3:
                    mG2D.drawOval(mPuntoInicio.x, mPuntoInicio.y, mPuntoFinal.x, mPuntoFinal.y);
                    break;
            }
        }

Sorry for mixing english and spanish here, I just made this D: The first if enters when the mouse is released and not dragging obviously, but somehow when I switch figures, it doesn't paint the figures that are inside the arraylist, which in this case is mCoordenadas. How can I run this in another thread? Or better expressed, do I need a thread to handle this?

Answer 1

Imagine a painter working on a canvas. From time to time he grabs a bucket with paint, and pours that over his canvas. Everything that was there before ... "vanishes".

That is what you do when calling

mG2D.fill(new Rectangle2D.Double(mCoord.getFirstPoint().x, ...                         

within your paintComponent() method.

Thing is: you have a certain (complicated) condition there; and what happens is basically: whenever the correct conditions come together, you flood-fill your canvas; thereby "overwriting" any prior content.

Beyond that: this is probably a good example of the DRY principle: you have the same code in various places; and lost track of that.

Instead: make sure that anything your code should do is written up in exactly one place.

In other words: you shouldn't have multiple calls to fill. Instead: step back, and identify "blocks of common behavior" - and extract them into a method.

Answer 2

Or better expressed, do I need a thread to handle this?

Threads has nothing to do with this.

mCoordenadas.add(new Coordenadas());

Don't add a new panel. All painting must be done on the same panel.

The ArrayList must contain a generic "shape" object. So the ArrayList can contain ellipse, rectangle and line shapes. Each "shape" must know how to paint itself.

What might change when you switch figures is the MouseListener logic. You might remove the old listener and add the new listener for the newly selected shape.

it doesn't paint the figures that are inside the arraylist,

Another option is to paint directly to a BufferedImage , then you don't have to worry about repainting objects.

Check out Custom Painting Approaches for working examples of both approaches: 1) painting from an ArrayList and 2) painting to a BufferedImage.