为什么当我想重绘我的画时 repaint() 不起作用？

Question

这是我的第一个 java 动画，我希望我的标志开始使用 repaint()，但是该方法什么也不做，我不知道为什么。它与 moveFlag 方法有关，它应该使旗帜运动起来，让人联想到旗帜在风中飘扬。应用程序只是显示标志，但不会移动它。public class Testing extends JPanel implements ActionListener {    private int x,y, xinc, yinc;    public void paintComponent(Graphics g){        xinc = 10;        yinc = 10;        x = 101;        y = 151;        Graphics2D gimg = (Graphics2D) g;        Rectangle2D rect = new Rectangle2D.Double(50,50,10,300);        CubicCurve2D cub1 = new         CubicCurve2D.Double(60,50,x,10,y,100,200,50);        gimg.draw(cub1);        ubicCurve2D cub2 = new         CubicCurve2D.Double(60,100,x,60,y,150,200,100);        gimg.draw(cub2);        CubicCurve2D cub3 = new         CubicCurve2D.Double(60,150,x,110,y,200,200,150);        gimg.draw(cub3);        Line2D l1 = new Line2D.Double(200,50,200,150);        gimg.draw(l1);        GeneralPath gp1 = new GeneralPath();            GeneralPath gp2 = new GeneralPath();        gp1.moveTo(60,50);        gp1.curveTo(x,10,y,100,200,50);        gp1.lineTo(200,100);        gp1.curveTo(y,150,x,60,60,100);        gp1.lineTo(60,50);        gimg.setColor(Color.WHITE);        gimg.fill(gp1);        gimg.setColor(Color.GRAY);        gimg.fill(rect);            gp2.moveTo(60,100);           gp2.curveTo(x,60,y,150,200,100);        gp2.lineTo(200,150);        gp2.curveTo(y,200,x,110,60,150);        gp2.lineTo(60,100);        gimg.setColor(Color.RED);        gimg.fill(gp2);    }        public void moveFlag(){  //waving animation            Timer timer = new Timer(20, this);        timer.start();        x = x + xinc;        y = y + yinc;        if(x<=60||(x+xinc)>=130)            xinc*=-1;        if(y<=50||(y+yinc)>=230)            yinc*=-1;        //revalidate();        repaint();    }           @Override    public void actionPerformed(ActionEvent e) {    }  

Answer 1

你的想法是对的，但有些代码放错了地方。我会在评论中解释。

    import java.awt.*;

    import java.awt.event.*;

    import java.awt.geom.*;

    import javax.swing.*;

    public class Testing extends JPanel implements ActionListener {

       private int x, y, xinc, yinc;

       public void paintComponent(Graphics g) {

          // added this statement.

          super.paintComponent(g);

          Graphics2D gimg = (Graphics2D) g;

          Rectangle2D rect = new Rectangle2D.Double(50, 50, 10, 300);

          CubicCurve2D cub1 =

                new CubicCurve2D.Double(60, 50, x, 10, y, 100, 200, 50);

          gimg.draw(cub1);

          CubicCurve2D cub2 =

                new CubicCurve2D.Double(60, 100, x, 60, y, 150, 200, 100);

          gimg.draw(cub2);

          CubicCurve2D cub3 =

                new CubicCurve2D.Double(60, 150, x, 110, y, 200, 200, 150);

          gimg.draw(cub3);

          Line2D l1 = new Line2D.Double(200, 50, 200, 150);

          gimg.draw(l1);

          GeneralPath gp1 = new GeneralPath();

          GeneralPath gp2 = new GeneralPath();

          gp1.moveTo(60, 50);

          gp1.curveTo(x, 10, y, 100, 200, 50);

          gp1.lineTo(200, 100);

          gp1.curveTo(y, 150, x, 60, 60, 100);

          gp1.lineTo(60, 50);

          gimg.setColor(Color.WHITE);

          gimg.fill(gp1);

          gimg.setColor(Color.GRAY);

          gimg.fill(rect);

          gp2.moveTo(60, 100);

          gp2.curveTo(x, 60, y, 150, 200, 100);

          gp2.lineTo(200, 150);

          gp2.curveTo(y, 200, x, 110, 60, 150);

          gp2.lineTo(60, 100);

          gimg.setColor(Color.RED);

          gimg.fill(gp2);

       }

       public void moveFlag() { // waving animation

          Timer timer = new Timer(20, this);

          timer.start();

          // moved the next four statements from paintComponents.  They

          // are initializations.  In paintComponent you kept resetting them.

          xinc = 10;

          yinc = 10;

          x = 101;

          y = 151;

          repaint();

       }

       @Override

       public void actionPerformed(ActionEvent e) {

         //moved all of these from moveFlag to here. This is called by

         //the timer event to update the x and y coordinates.

          x = x + xinc;

          y = y + yinc;

          if (x <= 60 || (x + xinc) >= 130)

             xinc *= -1;

          if (y <= 50 || (y + yinc) >= 230)

             yinc *= -1;

          // revalidate();

          repaint();

       }

      public static void main(String[] args) {

          JFrame frame = new JFrame();

          Testing test = new Testing();

          frame.setContentPane(test);

          frame.setSize(500, 500);

          frame.setVisible(true);

          test.moveFlag();

       }

    }

我必须添加的唯一一段代码是 super.paintComponent(g) 调用。这是必要的，因为您要覆盖 paintComponent()，您需要确保在绘制图形之前调用父方法。