java - 鼠标拖拽

Question

我在显示所有三角形时遇到问题。我使用鼠标拖动绘制了一个三角形。每次我画一个新的三角形，以前的三角形就消失了。怎样才能让三角形留下来，这样绘图面板上就会出现很多三角形？

.....
private class PaintSurface extends JComponent {
   Point startDrag, endDrag, midPoint;
   Polygon triangle;
   public PaintSurface() {
      this.addMouseListener(new MouseAdapter() {
      public void mousePressed(MouseEvent e) {
         startDrag = new Point(e.getX(), e.getY());
         endDrag = startDrag;
         repaint();
      }

      public void mouseReleased(MouseEvent e) {
         Shape r = makeRectangle(startDrag.x, startDrag.y, e.getX(), e.getY());          
         shapes.add(r); 
         if (startDrag.x > endDrag.x)
            midPoint = new Point((endDrag.x +(Math.abs(startDrag.x - endDrag.x)/2)),e.getY());
         else 
            midPoint = new Point((endDrag.x -(Math.abs(startDrag.x - endDrag.x)/2)),e.getY()); 
         int[] xs = { startDrag.x, endDrag.x, midPoint.x };
         int[] ys = { startDrag.y, startDrag.y, midPoint.y };          
         triangle = new Polygon(xs, ys, 3);               
         startDrag = null;
         endDrag = null;
         repaint();
        }
     });

     this.addMouseMotionListener(new MouseMotionAdapter() {
        public void mouseDragged(MouseEvent e) {
           endDrag = new Point(e.getX(), e.getY());
           repaint();
        }
     });
  }

  public void paint(Graphics g) {
     Graphics2D g2 = (Graphics2D) g;
     g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
     paintBackground(g2);
     Color[] colors = { Color.YELLOW, Color.MAGENTA, Color.CYAN , Color.RED, Color.BLUE, Color.PINK};
     int colorIndex = 0;
     g2.setStroke(new BasicStroke(1));
     g2.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 0.50f));
     g2.fillPolygon(triangle);        
     }
}

Answer 1

您在屏幕上看到的就是您在绘图函数中绘制的内容。目前，您只能在 triangle 变量中存储一个三角形，不断替换和绘制它。

你需要的是存储一个三角形列表，并在 mouseReleased 中每次添加一个新的到列表中。以下是要更改的内容:

private class PaintSurface extends JComponent {
   ...
   //Polygon triangle;
   List<Polygon> triangles = new LinkedList<Polygon>();
   ...

   public PaintSurface() {

      public void mouseReleased(MouseEvent e) {
         ...
         //triangle = new Polygon(xs, ys, 3);
         triangles.add( new Polygon(xs, ys, 3); );
         ...
        }
     });
     ...
  }

  public void paint(Graphics g) {
     ...
     //g2.fillPolygon(triangle);
     for (Polygon triangle : triangles) g2.fillPolygon(triangle);
     ...
   }
}