如何防止绘制的形状在 Java 中消失？

Question

我正在做一个项目来创建一个可以绘制随机形状的屏幕保护程序。我有几个问题，但我现在主要关心的是如何让形状留在屏幕上，而不是在创建后消失。这是我的代码。我不能使用任何循环，也不想改变我所拥有的任何功能（除了可能的 shapeDrawn）。

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

public class ScreenSaver2 extends JPanel implements ActionListener {
    private JFrame frame = new JFrame("FullSize");
    private Rectangle rectangle;
    boolean full;


    protected void paintComponent(Graphics g) {
        int r = (int)(Math.random() * 255);
        int gr = (int)(Math.random() * 255);
        int b = (int)(Math.random() * 255);
        Color color = new Color(r, gr, b);
        int width = 10 + (int)(Math.random() * 40);
        int height = 10 + (int)(Math.random() * 40);
        int x = (int)(Math.random() * (getWidth() - width));
        int y = (int)(Math.random() * (getHeight() - height));
        int whichShape = (int)(Math.random() * 3);
        int shapesDrawn = 0;

        super.paintComponent(g);
        if (shapesDrawn >= 30) {
            shapesDrawn = 0;
        }

        switch (whichShape) {
        case 0:
            g.setColor(color);
            g.drawLine(x, y, x + width, y + height);
            shapesDrawn++;
            break;
        case 1:
            g.setColor(color);
            g.drawRect(x, y, width, height);
            shapesDrawn++;
            break;
        case 2:
            g.setColor(color);
            g.drawRoundRect(x, y, width, height, 25, 25);
            shapesDrawn++;
            break;
        case 3:
            g.setColor(color);
            g.drawOval(x, y, width, height);
            shapesDrawn++;
            break;
        }

    }


    ScreenSaver2() {
        // Remove the title bar, min, max, close stuff
        frame.setUndecorated(true);
        // Add a Key Listener to the frame
        frame.addKeyListener(new KeyHandler());
        // Add this panel object to the frame
        frame.add(this);
        // Get the dimensions of the screen
        rectangle = GraphicsEnvironment.getLocalGraphicsEnvironment()
        .getDefaultScreenDevice().getDefaultConfiguration().getBounds();
        // Set the size of the frame to the size of the screen
        frame.setSize(rectangle.width, rectangle.height);
        frame.setVisible(true);
        // Remember that we are currently at full size
        full = true;
        // set and initialize timer
        Timer t = new Timer(500, this);
        t.setDelay(500);
        t.start();

    }

    // This method will run when any key is pressed in the window
    class KeyHandler extends KeyAdapter {
        public void keyPressed(KeyEvent e) {
            // Terminate the program.
            if (e.getKeyChar() == 'x') {
                System.out.println("Exiting");
                System.exit(0);
            }
            // Change background color
            else if (e.getKeyChar() == 'r') {
                System.out.println("Change background color");
                setBackground(new Color((int)(Math.random() * 256), (int)(Math.random() * 256), (int)(Math.random() * 256)));
                repaint();
            }
            // Resize to half-screen
            else if (e.getKeyChar() == 'z') {
                System.out.println("Resizing");
                frame.setSize((int)rectangle.getWidth() / 2, (int)rectangle.getHeight());
            }
        }
    }

    public void actionPerformed(ActionEvent e) {
        repaint();
    }

    public static void main(String[] args) {
        ScreenSaver2 obj = new ScreenSaver2();
    }
}

Answer 1

不要将paintComponent 中的每个新形状直接绘制到Graphics 上下文中，而是使用后备缓冲区先将形状渲染到Graphics 上下文中。

这样，您只需在每次呈现新形状时维护一个简单的计数器。

这也很麻烦，因为paintComponent 可能由于多种原因被调用，而您无法控制很多原因，这意味着您的程序可以在任何计时器滴答实际发生之前绘制更多形状...

更新示例

您剩下的唯一选择是创建一个后备缓冲区，您可以根据需要在其上绘制每个更改。这将“存储”油漆周期之间的每个变化。然后，您可以简单地将其绘制到屏幕上...

private BufferedImage img;

//...

@Override
protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    Graphics2D g2d = (Graphics2D) g.create();
    // Create the backing buffer
    // This is a little cheat, creating a new image when the number of shapes
    // exceeds the requirements, but it saves messing about with clearing
    // a alpha image ;)
    if (img == null || shapesDrawn >= 30) {
        img = new BufferedImage(getWidth(), getHeight(), BufferedImage.TYPE_INT_ARGB);
        shapesDrawn = 0;
    } else if (img.getWidth() != getWidth() || img.getHeight() != img.getHeight()) {
        // Update the backing buffer to meet the requirements of the changed screen size...
        BufferedImage buffer = new BufferedImage(getWidth(), getHeight(), BufferedImage.TYPE_INT_ARGB);
        Graphics2D gbuffer = buffer.createGraphics();
        gbuffer.drawImage(img, 0, 0, this);
        gbuffer.dispose();
        img = buffer;
    }

    // Get a reference to the backing buffers graphics context...
    Graphics2D gbuffer = img.createGraphics();

    // Paint the shapes to the backing buffer...
    int r = (int) (Math.random() * 255);
    int gr = (int) (Math.random() * 255);
    int b = (int) (Math.random() * 255);
    Color color = new Color(r, gr, b);
    int width = 10 + (int) (Math.random() * 40);
    int height = 10 + (int) (Math.random() * 40);
    int x = (int) (Math.random() * (getWidth() - width));
    int y = (int) (Math.random() * (getHeight() - height));
    int whichShape = (int) (Math.random() * 3);
    int shapesDrawn = 0;

    switch (whichShape) {
        case 0:
            gbuffer.setColor(color);
            gbuffer.drawLine(x, y, x + width, y + height);
            shapesDrawn++;
            break;
        case 1:
            gbuffer.setColor(color);
            gbuffer.drawRect(x, y, width, height);
            shapesDrawn++;
            break;
        case 2:
            gbuffer.setColor(color);
            gbuffer.drawRoundRect(x, y, width, height, 25, 25);
            shapesDrawn++;
            break;
        case 3:
            gbuffer.setColor(color);
            gbuffer.drawOval(x, y, width, height);
            shapesDrawn++;
            break;
    }
    // Dispose of the buffers graphics context, this frees up memory for us
    gbuffer.dispose();
    // Paint the image to the screen...
    g2d.drawImage(img, 0, 0, this);
    g2d.dispose();
}

可能是我曾经给过的最糟糕的建议

改变...

super.paintComponent(g);
if (shapesDrawn >= 30) {
    shapesDrawn = 0;
}

到...

if (shapesDrawn >= 30) {
    super.paintComponent(g);
    shapesDrawn = 0;
}