java - 如果添加到Jpanel中，则无法使用paintComponent进行绘制，但在JFrame中可以正常工作

我想画简单的椭圆形。在以下代码中，如果我在Jpanel上添加一个椭圆形，则它不起作用，但是如果我在框架上绘制，则可以正常工作。以下是有效的代码。

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;

import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.LineBorder;

public class Controller {

    public Controller() {
        View view = new View();
        Model model = new Model(100, 100, 100, 100);
        view.paintModel(model);
    }

    public static void main(String[] args) {
        Controller c = new Controller();
    }

    public class View extends JFrame {
        private JPanel jpanel;

        public View() {
            this.setBounds(0, 0, 500, 500);
            this.setLayout(new BorderLayout());

            jpanel = new JPanel();
            jpanel.setBackground(Color.WHITE);
            jpanel.setLayout(null);
            this.add(jpanel, BorderLayout.CENTER);

            this.setVisible(true);
            this.validate();
        }

        public void paintModel(Model model) {
            jpanel.add(new ModelView(model));
            jpanel.validate();
            jpanel.repaint();
        }

    }

    public class Model {
        public int xPos;
        public int yPos;
        public int height;
        public int width;

        public Model(int xPos, int yPos, int height, int width) {
            super();
            this.xPos = xPos;
            this.yPos = yPos;
            this.height = height;
            this.width = width;
        }
    }

    public class ModelView extends JComponent {

        private Model model;

        public ModelView(Model model) {
            super();
            setBorder(new LineBorder(Color.RED));
            this.model = model;
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(model.xPos + model.width, model.yPos
                    + model.height);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.setColor(Color.black);
            g.fillOval(model.xPos, model.yPos, model.width, model.height);
        }
    }

}

最佳答案

首先，您没有在super.paintComponent类中调用ModelView，这非常重要，尤其是在处理诸如JComponent之类的透明组件时

其次，您没有提供ModelView的布局信息，也就是说，该组件没有首选，最小或最大大小提示，因此许多布局管理器会认为它的大小为0x0，即这里发生了什么。

JPanel默认情况下使用FlowLayout，它将使用组件的首选大小进行布局。

更新ModelView以提供一些大小调整提示（在大多数情况下，覆盖getPreferredSize应该就足够了），或者在要添加JPanel的ModelView上使用其他布局管理器，而不必在意这些事情（例如BorderLayout）

更新

此外，您“可能”正在绘画超出组件的可视范围，例如，使用Model model = new Model(100, 100, 100, 100)创建模型，然后使用g.fillOval(model.xPos, model.yPos, model.width, model.height)渲染该模型，但是组件的preferredSize仅为，实际上是在组件的右下边缘（外部）绘制椭圆形

除非打算编写自己的布局管理器来进行管理，否则在计算组件的首选大小时应考虑x / y偏移，例如new Dimension(model.width, model.height)

例如...抱歉，我为您的代码添加了一些代码以填充模型...



红线只是new Dimension(model.xPos + model.width, model.yPos + model.height)，它显示LineBorder组件的边界

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.border.LineBorder;

public class View extends JFrame {

    private JPanel jpanel;

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                new View();

            }
        });
    }

    public View() {
        this.setBounds(0, 0, 500, 500);
        this.setLayout(new BorderLayout());

        jpanel = new JPanel();
        jpanel.setBackground(Color.WHITE);
        this.add(jpanel, BorderLayout.CENTER);

        Model model = new Model(100, 100, 100, 100);
        paintModel(model);

        this.setVisible(true);
    }

    public void paintModel(Model model) {
        jpanel.add(new ModelView(model));
        jpanel.validate();
        jpanel.repaint();
    }

    public class ModelView extends JComponent {

        private Model model;

        public ModelView(Model model) {
            super();
            setBorder(new LineBorder(Color.RED));
            this.model = model;
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(model.xPos + model.width, model.yPos + model.height);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.setColor(Color.black);
            g.fillOval(model.xPos, model.yPos, model.width, model.height);
        }
    }

    public class Model {

        public int xPos;
        public int yPos;
        public int height;
        public int width;

        public Model(int xPos, int yPos, int height, int width) {
            super();
            this.xPos = xPos;
            this.yPos = yPos;
            this.height = height;
            this.width = width;
        }
    }
}