JComponent 一旦离开屏幕就停止渲染

我正在尝试制作一个简单的动画，其中一个矩形从屏幕开始（在屏幕右边缘的右侧）并向左移动。因此，在本例中，我的框架的宽度为 1000，墙壁的 x 值为 1100。显然，首先，矩形不应该是 我们可见。但当矩形向左移动时，它最终应该变得可见。然而，这个动画并没有这样做。即使墙的 x 值在屏幕范围内，它也不会被渲染。

我尝试放一个println()中的声明paintComponent()墙的方法，我发现paintComponent()没有被框架调用repaint()方法。我认为当墙第一次添加到框架中时，Swing 认为由于它不在屏幕上，所以不需要渲染，因此即使墙最终出现在屏幕上，Swing 也认为它不需要渲染得到渲染。

我尝试过重新验证和无效框架和组件，但没有任何效果。请帮我解决这个问题。下面是代码：

package graphics.simpleAnimation;

public class Simple_Animation implements Runnable {

    private UI ui; // The UI (frame)

    private Wall wall; // Wall object that moves across the screen

    private Simple_Animation() {

        // Initialize the wall object (intentionally given an x value that is greater than the frame's width)
        wall = new Wall(1100, 400, 200, 400);

        // Initialize the UI (width is only 1000)
        ui = new UI(1000, 600, "Geometry Dash");

        // Add the wall to the ui (the frame)
        ui.add(wall);

    }

    public void run() {
        // Set the frame visible
        ui.setVisible(true);

        // Repaint the frame and move the wall
        while (true) {
            ui.repaint();
            wall.moveWall(-2, 0);

            try {
                Thread.sleep(16);
            } catch (InterruptedException IE) {
                System.out.println(IE);
            }

        }

    }

    // Starts the program in a new thread
    public static void main(String[] args) {
        Simple_Animation simpleAnimation = new Simple_Animation();
        new Thread(simpleAnimation).start();
    }

}


package graphics.simpleAnimation;

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

public class UI extends JFrame {

    // Variables storing the width and height of the content pane (where the components are being rendered)
    public int content_pane_width;
    public int content_pane_height;

    public UI(int frameW, int frameH, String frameTitle) {

        setTitle(frameTitle);
        setSize(frameW, frameH);
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        setLayout(null);

        content_pane_width = getContentPane().getWidth();
        content_pane_height = getContentPane().getHeight();

    }

    @Override
    public void paint(Graphics g) {
        super.paint(g);
    }

}

package graphics.simpleAnimation;

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

public class Wall extends JComponent {

    private int wallX;
    private int wallY;
    private int wallW;
    private int wallH;


    Wall(int x, int y, int sizeX, int sizeY) {
        wallX = x;
        wallY = y;
        wallW = sizeX;
        wallH = sizeY;

        setSize(getPreferredSize());
    }

    public void moveWall(int moveX, int moveY) {
        wallX += moveX;
        wallY += moveY;
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(wallW, wallH);
    }

    @Override
    public void paintComponent(Graphics g) {
        Graphics2D g2d = (Graphics2D) g;

        setLocation(wallX, wallY);
        g2d.fillRect(0, 0, wallW, wallH);
    }
}

我在你的程序中发现了一些错误

1. 您正在使用null layout， 请参见空布局是邪恶的以及答案这个问题看看为什么你应该避免使用它。（可能不是在这种情况下，根据下面的 @MadProgrammer 的评论），这只是另一种方法

2. while (true) {这条线可能会阻塞事件调度线程 (EDT)沿着这一行：Thread.sleep(16);, See 课程：Swing 中的并发了解更多关于和如何使用 Swing 定时器。您还应该将您的程序放在 EDT 上，可以这样做：

   public static void main(String[] args) {
       SwingUtilities.invokeLater(new Runnable() {
           @Override
           public void run() {
               //Your constructor here
           }
       });
   }
   

3. 你没有打电话super.paintComponent()在你的paintComponent()的方法Wall可能会破坏绘画链的类，总是先调用它。

4. 你正在延长JComponent，最好延长JPanel并使用在其上进行自定义绘画Shapes API

考虑到上述所有内容，您就可以编写如下代码：

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.geom.Rectangle2D;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;

public class SingleAnimation {

    private JFrame frame;
    private Timer timer;

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new SingleAnimation().createAndShowGui();
            }
        });
    }

    public void createAndShowGui() {
        frame = new JFrame(getClass().getSimpleName());

        Wall wall = new Wall(300, 0);

        timer = new Timer(16, new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                wall.moveWall(-2, 0);
            }
        });

        timer.setInitialDelay(0);
        timer.start();

        frame.add(wall);
        frame.pack();
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}

class Wall extends JPanel {
    private int xCoord;
    private int yCoord;

    public int getxCoord() {
        return xCoord;
    }

    public void setxCoord(int xCoord) {
        this.xCoord = xCoord;
    }

    public int getyCoord() {
        return yCoord;
    }

    public void setyCoord(int yCoord) {
        this.yCoord = yCoord;
    }

    public Wall(int x, int y) {
        this.xCoord = x;
        this.yCoord = y;
    }

    public void moveWall(int xUnits, int yUnits) {
        xCoord += xUnits;
        yCoord += yUnits;
        repaint();
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(400, 400);
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;
        g2d.setColor(Color.BLUE);

        g2d.fill(new Rectangle2D.Double(xCoord, yCoord, 100, 20));
    }
}

这将产生类似的输出，如下所示：