用 mouseDragged 绘制圆圈轨迹

Question

我正在尝试编写一个程序，用 mouseDragged 绘制一条圆圈，就像 MS 画图一样。我已经成功地让我的程序在我点击时画了一个圆圈。当我拖动鼠标时，我也成功地让我的程序画了一个圆圈；但是，这不会在我拖动的任何地方留下一行圆圈。它只是简单地拖动同一个圆圈。我试图让我的程序在我拖动的位置后面留下一圈圆圈，但我很困惑为什么我的程序不会这样做。

package assignment_11;

import java.awt.*;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import javax.swing.*;


public class Canvas extends JComponent implements MouseListener, MouseMotionListener{

private int x, x1;
private int y, y1;

public Canvas() {
    addMouseMotionListener(this);
    addMouseListener(this);
}

public static void main(String[] args) {
    //creates new JFrame, sets Exit On Close, sets visible
    JFrame window = new JFrame();
    window.add(new Canvas());
    window.pack();
    window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    window.setVisible(true);
}

public Dimension getPreferredSize() {
    return new Dimension(640,480);
}

@Override
public void mouseClicked(MouseEvent arg0) {
    System.out.println(arg0);
    x = arg0.getX();
    y = arg0.getY();
    repaint();

}

@Override
public void mouseEntered(MouseEvent arg0) {
    System.out.println(arg0);

}

@Override
public void mouseExited(MouseEvent arg0) {
    System.out.println(arg0);

}

@Override
public void mousePressed(MouseEvent arg0) {
    System.out.println(arg0);

}

@Override
public void mouseReleased(MouseEvent arg0) {
    System.out.println(arg0);

}

public void paintComponent(Graphics g) {
    g.fillOval(x, y, 10, 10);
    g.fillOval(x1, y1, 10, 10);
}

@Override
public void mouseDragged(MouseEvent arg0) {
    System.out.println(arg0);
    x1 = arg0.getX();
    y1 = arg0.getY();
    repaint();
}

@Override
public void mouseMoved(MouseEvent arg0) {
    System.out.println(arg0);
}


}

感谢任何帮助！

Answer 1

绘画具有破坏性。也就是说，每次调用 paintComponent 时，您都需要重新绘制组件的整个状态。

这引发了问题 - 您需要某种方式来存储每次调用 paintComponent 时要绘制的状态。

为此，一个简单的 ArrayList 就可以很好地完成这项工作。它将允许您存储所有您感兴趣的点，并允许您在每次调用 paintComponent 时重新绘制它们，例如...

import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JComponent;
import javax.swing.JFrame;

public class Canvas extends JComponent implements MouseListener, MouseMotionListener {

    private List<Point> points;

    public Canvas() {
        points = new ArrayList<>(25);
        addMouseMotionListener(this);
        addMouseListener(this);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater((new Runnable() {
            @Override
            public void run() {
                JFrame window = new JFrame();
                window.add(new Canvas());
                window.pack();
                window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                window.setVisible(true);
            }
        }));
    }

    public Dimension getPreferredSize() {
        return new Dimension(640, 480);
    }

    @Override
    public void mouseClicked(MouseEvent arg0) {
        System.out.println(arg0);
        points.add(arg0.getPoint());
        repaint();

    }

    @Override
    public void mouseEntered(MouseEvent arg0) {
        System.out.println(arg0);

    }

    @Override
    public void mouseExited(MouseEvent arg0) {
        System.out.println(arg0);

    }

    @Override
    public void mousePressed(MouseEvent arg0) {
        System.out.println(arg0);

    }

    @Override
    public void mouseReleased(MouseEvent arg0) {
        System.out.println(arg0);

    }

    public void paintComponent(Graphics g) {
        for (Point p : points) {
            g.fillOval(p.x, p.y, 10, 10);
        }
    }

    @Override
    public void mouseDragged(MouseEvent arg0) {
        System.out.println(arg0);
        points.add(arg0.getPoint());
        repaint();
    }

    @Override
    public void mouseMoved(MouseEvent arg0) {
        System.out.println(arg0);
    }

}

现在，随着问题的复杂性增加，您可以将 "shapes" 存储在 List 中，它具有某种关于如何绘制自身的概念，允许添加更复杂的形状

您还应该看看 Painting in AWT and Swing 以更好地了解 Swing 中绘画的实际工作原理

用 mouseDragged 绘制圆圈轨迹

Draw trail of circles with mouseDragged

java

swing

jcomponent