在 Panel 中绘制一个球并使用 mouseDragged 移动球

Question

我正在练习在Panel中画一个球，并在拖动球时显示球的坐标。

这是我第一次练习绘画练习（？）

这是我的代码。

import java.awt.*;
 import java.awt.event.*;

public class MovingBall extends Frame {
Panel ballPanel = new Panel();
Label ballLabel = new Label();
Panel coordinatePanel = new Panel();
Label coordinateLabel = new Label();
int x0=0,y0 =0, x=20,y=30;
int nowX, nowY;
Label nowXcoordinateLabel = new Label("Now X :"+nowX);
Label nowYcoordinateLabel = new Label("Now Y :"+nowY);
MovingBall(){
    setLayout(new GridLayout(1,1));
    ballPanel.add(ballLabel); coordinatePanel.add(coordinateLabel);

    add(ballPanel);
    add(coordinatePanel);
    ballPanel.setBackground(Color.white);

    coordinatePanel.setBackground(Color.LIGHT_GRAY);
    nowXcoordinateLabel.setBackground(Color.WHITE);
    nowYcoordinateLabel.setBackground(Color.WHITE);
    coordinatePanel.add(nowXcoordinateLabel);
    coordinatePanel.add(nowYcoordinateLabel);
    setVisible(true);
    setSize(400,400);

    MouseMotionListener ml = new MouseMotionAdapter(){
   public void mouseDragged(MouseEvent e){
      Point p = new Point();
      nowX = (int) p.getX();
      nowY = (int) p.getY();
  }  
};
    addWindowListener(new WindowAdapter() {
    public void windowClosing(WindowEvent we) {
        dispose();
     }
    }
   );
  }
   public void paintComponent(Graphics2D gg){
   // super.paintComponents(gg);
    ballPanel.paintComponents(gg);
    gg.setColor(Color.BLUE);
    gg.fillOval(x0, y0, 10, 10);

}



public static void main(String[]arg){
    MovingBall mb = new MovingBall();
}
}

我有两个问题

1. 我使用fillOval 和paintComponent 绘制并显示了一个球，但我在屏幕上看不到它。为什么？
2. 如果我想使用mouseDragged 移动球，知道如何移动球吗？我需要一些线程吗？

Answer 1

开始吧

• Frame 没有 paintComponent 方法，所以永远不会调用它。
• 即使是这样，位置 0x0 也会在框架边框下绘制圆圈，因此您不会看到它
• 您应该从MouseEvent 中获取Point，而不是从您创建的新Point 对象中获取
• 框架不负责管理鼠标拖动或绘画，框架负责提供初始容器，其他所有内容都添加到该容器中

从此，您应该将绘画和鼠标拖动管理的功能移至它自己的类。这为您提供了两件事，首先，您可以在其上绘制的表面，它将包含在框架边框内，并且鼠标事件将自动转换为面板上下文（0x0 将是面板的左上角)

这就提出了如何更新标签的问题。好吧，您可以从 AWT API 中获取一个叶子，并使用一个简单的观察者模式来在坐标更改时生成事件，例如

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.Label;
import java.awt.Panel;
import java.awt.Point;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;
import java.awt.event.MouseMotionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.util.ArrayList;
import java.util.EventObject;
import java.util.List;

public class MovingBall extends Frame {

    BallPane ballPanel = new BallPane();
    Label ballLabel = new Label();
    int x0 = 0, y0 = 0, x = 20, y = 30;
    int nowX, nowY;
    Label nowXcoordinateLabel = new Label("Now X :" + nowX);
    Label nowYcoordinateLabel = new Label("Now Y :" + nowY);

    MovingBall() {
        setLayout(new BorderLayout());
        ballPanel.add(ballLabel);

        add(ballPanel);
        ballPanel.setBackground(Color.white);

        nowXcoordinateLabel.setBackground(Color.WHITE);
        nowYcoordinateLabel.setBackground(Color.WHITE);
        setVisible(true);
        setSize(400, 400);
        addWindowListener(new WindowAdapter() {
            @Override
            public void windowClosed(WindowEvent e) {
                dispose();
            }
        });

        Panel coordinates = new Panel(new FlowLayout());
        coordinates.add(nowXcoordinateLabel);
        coordinates.add(nowYcoordinateLabel);
        coordinates.setBackground(Color.LIGHT_GRAY);
        add(coordinates, BorderLayout.SOUTH);

        ballPanel.addCoordinateListene(new CoordinateListener() {
            @Override
            public void coordinatesChanged(CoordinateEvent evt) {
                nowXcoordinateLabel.setText("Now X: " + evt.getCoordinate().getX());
                nowYcoordinateLabel.setText("Now X: " + evt.getCoordinate().getY());
                revalidate();
                repaint();
            }
        });

    }

    public static void main(String[] arg) {
        MovingBall mb = new MovingBall();
    }

    public class CoordinateEvent extends EventObject {
        private final Point p;

        public CoordinateEvent(Object source, Point p) {
            super(source);
            this.p = p;
        }

        public Point getCoordinate() {
            return p;
        }
    }

    public interface CoordinateListener {
        public void coordinatesChanged(CoordinateEvent evt);
    }

    public class BallPane extends Panel {

        int x0 = 0, y0 = 0, x = 20, y = 30;
        private List<CoordinateListener> coordinateListeners;

        public BallPane() {
            MouseMotionListener ml = new MouseMotionAdapter() {
                public void mouseDragged(MouseEvent e) {
                    x0 = (int) e.getX();
                    y0 = (int) e.getY();
                    fireCoordinateChange(new Point(e.getPoint()));
                    repaint();
                }
            };
            addMouseMotionListener(ml);
            coordinateListeners = new ArrayList<>(5);
        }

        @Override
        public void paint(Graphics g) {
            super.paint(g);
            g.setColor(Color.BLUE);
            g.fillOval(x0, y0, 10, 10);
        }

        public void addCoordinateListene(CoordinateListener listener) {
            coordinateListeners.add(listener);
        }

        public void removeCoordinateListene(CoordinateListener listener) {
            coordinateListeners.remove(listener);
        }

        protected void fireCoordinateChange(Point p) {
            CoordinateEvent evt = new CoordinateEvent(this, p);
            for (CoordinateListener listener : coordinateListeners) {
                listener.coordinatesChanged(evt);
            }
        }

    }
}

Answer 2

使您的类扩展Panel 并准备好使用覆盖paint 方法进行绘图并将MouseMotionListener 添加到列出您的面板。获取X 和Y 坐标以在paint 方法中使用，最后添加您的绘图面板到Frame。

简单代码：已更新

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridLayout;
import java.awt.Label;
import java.awt.Panel;
import java.awt.RenderingHints;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;

public class TestClass extends Panel {


        /**
         * 
         */
        private static final long serialVersionUID = 1L;

        Panel ballPanel = new Panel();
        Label ballLabel = new Label();
        Panel coordinatePanel = new Panel();
        Label coordinateLabel = new Label();
        int nowX, nowY;
        Label nowXcoordinateLabel = new Label("Now X :");
        Label nowYcoordinateLabel = new Label("Now Y :");

        TestClass() {

            coordinatePanel.add(coordinateLabel);
            nowXcoordinateLabel.setBackground(Color.WHITE);
            nowYcoordinateLabel.setBackground(Color.WHITE);
            nowXcoordinateLabel.setPreferredSize(new Dimension(100, 25));
            nowYcoordinateLabel.setPreferredSize(new Dimension(100, 25));
            coordinatePanel.setPreferredSize(new Dimension(400, 30));
            coordinatePanel.setBackground(Color.LIGHT_GRAY);
            coordinatePanel.add(nowXcoordinateLabel);
            coordinatePanel.add(nowYcoordinateLabel);

            MouseAdapter ml = new MouseAdapter() {

                @Override
                public void mouseMoved(MouseEvent e) {
                    nowXcoordinateLabel.setText("Now X :" + e.getX());
                    nowYcoordinateLabel.setText("Now Y :" + e.getY());
                    nowX = e.getX();
                    nowY = e.getY();
                    repaint();
                    super.mouseMoved(e);
                }

            };


            setLayout(new GridLayout(1, 1));
            setBackground(Color.WHITE);
            addMouseMotionListener(ml);
            setVisible(true);
            setSize(400, 400);
        }


        @Override
        public void paint(Graphics g) {
            Graphics2D gg = (Graphics2D) g;
            gg.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
            gg.setColor(Color.BLUE);
            gg.fillOval(nowX, nowY, 20, 20);

        }

        public static void main(String[] arg) {
            TestClass mb = new TestClass();
            Frame frame = new Frame("Test drawing");
            frame.addWindowListener(new WindowAdapter() {


                @Override
                public void windowClosing(WindowEvent e) {
                     frame.dispose();
                    super.windowClosing(e);
                }


            });
            frame.setLayout(new GridLayout(1, 1));
            frame.add(mb);
            frame.add(mb.coordinatePanel);
            frame.setSize(800, 600);
            frame.setVisible(true);
        }


}

Answer 3

获取鼠标的位置，而不是：

nowX = (int) p.getX();

这样写：

nowX = (int) e.getX();

你还需要在每次用户触发鼠标拖动事件时重新绘制椭圆