如何在JLayeredPane的任何一层上使用图形(Piant组件)

Question

我试图在运行时在JlayerdPane的任何一层上画一条线。我现在面临的是，一旦我释放鼠标，画的线就会自动删除。我希望绘制的线一直在那里，直到我再次单击鼠标为止。

我打电话给下面的书面课，这边是

    iDimension = new getDimension();
    iDimension.setBounds(1, 12,  441, 380);
    //iDimension.setOpaque(true);
    iDimension.setBackground(new Color(0,0,0,100));

我已经添加了iDimension与分层窗格在这种方式

    layeredPane.add(iDimension, new Integer(1),0);

这是getDimension类

public class getDimension extends JPanel {
    public getDimension() {
        setDoubleBuffered(true);
        this.setBorder(UIManager.getBorder("ComboBox.border"));
        this.repaint();
    }
    Point pointStart = null;
    Point pointEnd = null;
    {
        addMouseListener(new MouseAdapter() {
            public void mousePressed(MouseEvent e) {
                pointStart = e.getPoint();
            }
            public void mouseReleased(MouseEvent e) {
                pointStart = null;
            }
        });
        addMouseMotionListener(new MouseMotionAdapter() {
            public void mouseMoved(MouseEvent e) {
                pointEnd = e.getPoint();
            }
            public void mouseDragged(MouseEvent e) {
                pointEnd = e.getPoint();
                repaint();
            }
        });
    }
    public void paint(Graphics g) {
        super.paint(g);
        if (pointStart != null) {
            g.setColor(Color.GREEN);
            g.drawLine(pointStart.x, pointStart.y, pointEnd.x, pointEnd.y);
            //  System.out.println("" + pointStart.x +"," + pointStart.y +"," + pointEnd.x +"," +pointEnd.y);
        }
    }
}

我是java的新手。如果我的问题有歧义，请纠正。

Answer 1

我现在面临的是，一旦我释放鼠标，画的线就会自动删除。我希望绘制的线一直在那里，直到我再次单击鼠标为止。

代码只做你让它做的事情：

addMouseListener(new MouseAdapter() {
    public void mousePressed(MouseEvent e) {
        pointStart = e.getPoint();
    }
    public void mouseReleased(MouseEvent e) {
        pointStart = null;
    }
});

和：

    public void paint(Graphics g) {
        super.paint(g); 
        if (pointStart != null) {  // *********
            g.setColor(Color.GREEN);
            g.drawLine(pointStart.x, pointStart.y, pointEnd.x, pointEnd.y);
        }     
    }      

请注意，如果pointStart是null，则不绘制该线，而是在mouseReleased上将其设置为null！解决方案——不要那样做。

例如

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.UIManager;
@SuppressWarnings("serial")
public class GetDimension extends JPanel {
    private static final int PREF_W = 600;
    private static final int PREF_H = PREF_W;
    private Point pointStart = null;
    private Point pointEnd = null;
    public GetDimension() {
        this.setBorder(UIManager.getBorder("ComboBox.border"));
        this.repaint();
        MouseAdapter myMouse = new MouseAdapter() {
            public void mousePressed(MouseEvent e) {
                pointStart = e.getPoint();
                repaint();
            }
            public void mouseDragged(MouseEvent e) {
                pointEnd = e.getPoint();
                repaint();
            }
        };
        addMouseListener(myMouse);
        addMouseMotionListener(myMouse);
    }
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (pointStart != null && pointEnd != null) {
            g.setColor(Color.GREEN);
            g.drawLine(pointStart.x, pointStart.y, pointEnd.x, pointEnd.y);
        }
    }
    @Override
    public Dimension getPreferredSize() {
        if (isPreferredSizeSet()) {
            return super.getPreferredSize();
        }
        return new Dimension(PREF_W, PREF_H);
    }
    private static void createAndShowGui() {
        GetDimension mainPanel = new GetDimension();
        JFrame frame = new JFrame("GetDimension");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }
    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            createAndShowGui();
        });
    }
}