當鼠標釋放時，線程在繪圖程序中消失

Question

下面寫在BasicLinePix類中我嘗試創建一個程序，允許用戶在按住Shift鍵的同時按住鼠標按鍵並將其拖動到端點來繪製線。問題是，當鼠標被釋放時，該行消失。我試圖讓面板顯示該行，並且以相同的方式顯示多行。

我的代碼如下所示：

// this method overrides the paint method defined in JFrame 
    public void paint(Graphics g) { 
     super.paint(g); 


    } 


    // Inner class - instances of this class handle action events 
    private class EventHandler implements ActionListener, MouseListener, 
      MouseMotionListener { 

     private Point startPoint = null; // line's start point 
     private Point endPoint = null; // line's most recent end point 

     public void actionPerformed(ActionEvent arg0) { 
      if (arg0.getActionCommand().equals("Exit")) { 
       statusLabel.setText("Exiting program..."); 
       System.exit(0); 
      } 

     } 

     @Override 
     public void mouseClicked(MouseEvent arg0) { 
      // TODO Auto-generated method stub 

     } 

     @Override 
     public void mouseEntered(MouseEvent arg0) { 
      // TODO Auto-generated method stub 

     } 

     @Override 
     public void mouseExited(MouseEvent arg0) { 
      // TODO Auto-generated method stub 

     } 

     @Override 
     public void mousePressed(MouseEvent e) { 

      if (e.isShiftDown()) { 

       // record starting point for line 
       startPoint = new Point(e.getX(), e.getY()); 

       // initialize endPoint 
       endPoint = startPoint; 
      } 

      if (e.isControlDown()) { 
       Graphics g = drawingPanel.getGraphics(); 
       g.drawString("Hello", e.getX(), e.getY()); 
      } 

     } 

     @Override 
     public void mouseReleased(MouseEvent arg0) { 
      //repaint the frame and its contents 
      //this executes the paint method defined above 
      repaint(); 
     } 

     @Override 
     public void mouseDragged(MouseEvent e) { 

      if (e.isShiftDown()) { 
       // Implement rubber-band cursor 
       Graphics g = drawingPanel.getGraphics(); 
       g.setColor(Color.black); 

       g.setXORMode(drawingPanel.getBackground()); 

       // REDRAW the line that was drawn 
       // most recently during this drag 
       // XOR mode means that yellow pixels turn black 
       // essentially erasing the existing line 
       drawLine(g, startPoint, endPoint); 

       // Update the end point of the line to current mouse position 
       endPoint = new Point(e.getX(), e.getY()); 

       // Draw line to current mouse position 
       // XOR mode: yellow pixels become black 
       // black pixels, like those from existing lines, temporarily 
       // become 
       // yellow 
       drawLine(g, startPoint, endPoint); 

      } 

     } 

     @Override 
     public void mouseMoved(MouseEvent arg0) { 
      // TODO Auto-generated method stub 

     } 

     private void drawLine(Graphics g, Point start, Point end) { 
      if (startPoint != null && endPoint != null) { 
       int startX = ((Double) start.getX()).intValue(); 
       int startY = ((Double) start.getY()).intValue(); 

       int endX = ((Double) end.getX()).intValue(); 
       int endY = ((Double) end.getY()).intValue(); 

       g.drawLine(startX, startY, endX, endY); 
      } 
     } 

    } 

} 

任何幫助將不勝感激！

Answer 1

不要使用組件的getGraphics()調用來獲取組件的圖形。在paintComponent(...)方法中繪製。您將在本網站和圖形教程中多次找到此建議。

問題是從組件上調用getGraphics()獲得的Graphics對象不是持久的Graphics對象，並且隨着下一次重繪而消失。

請注意，您發佈的代碼中有80％與您的問題無關，並且完全沒有必要且令人分心。

編輯：請注意，它看起來像你的程序可能受益於使用BufferedImage，繪製它，然後在JPanel的paintComponent方法中顯示BufferedImage。如果你確實走這條路線，那麼可以，只要你在完成處置後就可以通過getGraphics()得到BufferedImage的圖形對象。編輯2：儘管刪除不相關的代碼是一個好主意，但不要刪除太多讓代碼無法編譯和無法運行的代碼。看看，而不是創建一個sscce。