在分层窗格上绘制到JPanel

Question

JFrame frame = new JFrame();
frame.add(new CustomPaintComponent());

pane.add(new CustomPaintComponent(),JLayeredPane.POPUP_LAYER);

package example;

import javax.swing.*;

public class Example {

    public static void main(String[] args) {
        
        JLayeredPane forLayers = new JLayeredPane();
        forLayers.setSize(300, 300);
        
        JFrame showsDrawnImage = new JFrame ("JFrame");
        showsDrawnImage.setSize(300, 300);
        
        JPanel panelDrawnOn = new DrawImage();
        
        //CODE BELOW WORKS - and returns string line in output//
        
        showsDrawnImage.add(panelDrawnOn); //blank these out to show the layeredpane example
        showsDrawnImage.setVisible(true); //blank these out to show the layeredpane example
        
        //CODE BELOW DOESN'T WORK - only shows the JFrame and DOES NOT return string line in output//

        forLayers.add(panelDrawnOn, JLayeredPane.PALETTE_LAYER);
        showsDrawnImage.add(forLayers);
        forLayers.setVisible(true);
        showsDrawnImage.setVisible(true);

    }
    
}

DrawImage类进行实际绘制

package example;

import java.awt.*;
import java.awt.geom.*;
import javax.swing.JPanel;

public class DrawImage extends JPanel{
    
    public void paintComponent( Graphics g ) {
        
                    System.out.println("This is what I try to access/draw");
                    
                    super.paintComponent(g);
                    Graphics2D g2 = (Graphics2D)g;

                    Line2D line = new Line2D.Double(10, 10, 40, 40);
                    g2.setColor(Color.blue);
                    g2.setStroke(new BasicStroke(10));
                    g2.draw(line);
 
                 }
    
}

Answer 1

我想主要问题是您的自定义组件没有“大小”，所以没有什么可绘制的。

当您将组件添加到JFrame时，布局管理器会给组件一个大小。JLayeredPane不使用布局管理器，因此现在您有责任给组件指定大小和位置。

阅读Swing教程中关于如何为一个工作示例使用分层窗格的部分。