窗口的内容在最小化时消失

Question

我有一个简单的类,当鼠标拖动时绘制一条线或鼠标按下(释放)时画一个点.

当我最小化应用程序然后恢复它时,窗口的内容消失,除了最后一个点(像素).我知道super.paint(g)每次窗口改变时该方法都会重新绘制背景,但无论我是否使用它,结果似乎都是一样的.它们之间的区别在于,当我不使用它时,窗口上绘制的不仅仅是一个像素,而不是我的所有绘画.我怎样才能解决这个问题？

这是班级.

package painting;

import java.awt.*;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;
import javax.swing.JFrame;
import javax.swing.JPanel;

class CustomCanvas extends Canvas{   
    Point oldLocation= new Point(10, 10);
    Point location= new Point(10, 10);
    Dimension dimension = new Dimension(2, 2);     
    CustomCanvas(Dimension dimension){  
        this.dimension = dimension;   
        this.init();
        addListeners();
    }    
    private void init(){                     
        oldLocation= new Point(0, 0);
        location= new Point(0, 0);
    }
    public void paintLine(){
        if ((location.x!=oldLocation.x) || (location.y!=oldLocation.y)) {         
            repaint(location.x,location.y,1,1);                                   
        } 
    }
    private void addListeners(){
        addMouseListener(new MouseAdapter(){
            @Override
            public void mousePressed(MouseEvent me){                   
                oldLocation = location;
                location = new Point(me.getX(), me.getY());
                paintLine();
            }
            @Override
            public void mouseReleased(MouseEvent me){                
                oldLocation = location;
                location = new Point(me.getX(), me.getY());
                paintLine();
            }
        });
        addMouseMotionListener(new MouseMotionAdapter() {
            @Override
            public void mouseDragged(MouseEvent me){                
                oldLocation = location;
                location = new Point(me.getX(), me.getY());
                paintLine();
            }
        });
    }
    @Override
    public void paint(Graphics g){  
        super.paint(g);
        g.setColor(Color.red);       
        g.drawLine(location.x, location.y, oldLocation.x, oldLocation.y);                
    }
    @Override
    public Dimension getMinimumSize() {
        return dimension; 
    }
    @Override
    public Dimension getPreferredSize() {
        return dimension;
    }

}
class CustomFrame extends JPanel {
    JPanel displayPanel = new JPanel(new BorderLayout());
    CustomCanvas canvas = new CustomCanvas(new Dimension(200, 200));        
    public CustomFrame(String titlu) {            
        canvas.setBackground(Color.white);
        displayPanel.add(canvas, BorderLayout.CENTER);            
        this.add(displayPanel);
    }   
}
public class CustomCanvasFrame {
    public static void main(String args[]) {
        CustomFrame panel = new CustomFrame("Test Paint");
        JFrame f = new JFrame();
        f.add(panel);
        f.pack();
        SwingConsole.run(f, 700, 700);
    }
}

Answer 1

您没有存储您正在绘制的点的状态.重新绘制面板时,它仅显示其绘制的最后一点的信息.

回复评论:

例如,您需要拥有一组Points ArrayList<Point> location = new ArrayList<Point>();

然后,在你的听众中: location.add(new Point(me.getX(), me.getY()));

最后,在paintLine()中:

for (Point location : locations) {
  repaint(location.x,location.y,1,1); 
}

该集合locations通常称为显示列表.大多数图形程序都使用它们

回复评论:

是的,我希望如此.我只是根据你的代码抛出一个想法给你一个起点.完全按照我的描述做几乎肯定是个坏主意.

Answer 2

这是不是意味着每次按或拖动鼠标时我都会绘制所有点(而不是一个点)？

是的,但@ Dave的方法对于数千个节点来说是完全令人满意的,如可以看到的那样GraphPanel.除此之外,请考虑渲染器使用的flyweight模式,并在此处进行说明.JTable

附录:关注您的AWTPainting问题,下面的变化可能说明系统和应用程序触发的绘画之间的差异.拖动鼠标时,repaint()调用update(),调用paint(); 这是应用程序触发的.在调整窗口大小时,仅paint()调用(不绘制红色数字); 这是系统触发的.请注意,调整大小后释放鼠标时会出现闪烁.

当整个组件的背景被清除并重新绘制时,通常会发生闪烁:

 4.如果组件并没有重写update(),默认实现update()清除组件的背景(如果它不是一个轻量级组件),并简单地调用paint().

import java.awt.Canvas;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.Panel;
import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.util.ArrayList;
import java.util.List;

public class AWTPainting {

    public static void main(String args[]) {
        CustomPanel panel = new CustomPanel();
        Frame f = new Frame();
        f.addWindowListener(new WindowAdapter() {

            @Override
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });
        f.add(panel);
        f.pack();
        f.setVisible(true);
    }
}

class CustomPanel extends Panel {

    public CustomPanel() {
        this.add(new CustomCanvas(new Dimension(320, 240)));
    }
}

class CustomCanvas extends Canvas {

    private MouseAdapter handler = new MouseHandler();
    private List<Point> locations = new ArrayList<Point>();
    private Point sentinel = new Point();
    private Dimension dimension;

    CustomCanvas(Dimension dimension) {
        this.dimension = dimension;
        this.setBackground(Color.white);
        this.addMouseListener(handler);
        this.addMouseMotionListener(handler);
        this.locations.add(sentinel);
    }

    @Override
    public void paint(Graphics g) {
        g.setColor(Color.blue);
        Point p1 = locations.get(0);
        for (Point p2 : locations.subList(1, locations.size())) {
            g.drawLine(p1.x, p1.y, p2.x, p2.y);
            p1 = p2;
        }
    }

    @Override
    public void update(Graphics g) {
        paint(g);
        g.clearRect(0, getHeight() - 24, 50, 20); // to background
        g.setColor(Color.red);
        g.drawString(String.valueOf(locations.size()), 8, getHeight() - 8);
    }

    private class MouseHandler extends MouseAdapter {

        @Override
        public void mousePressed(MouseEvent e) {
            if (locations.get(0) == sentinel) { // reference identity
                locations.set(0, new Point(e.getX(), e.getY()));
            }
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            locations.add(new Point(e.getX(), e.getY()));
            repaint();
        }
    }

    @Override
    public Dimension getPreferredSize() {
        return dimension;
    }
}