Java（Graphics2D）：创建的Graphics2D绘制的三角形直到第二次重绘才可见

Question

我有以下最少的代码来绘制带有箭头的线：

package gui;

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Polygon;
import java.awt.RenderingHints;
import java.awt.geom.AffineTransform;
import java.awt.geom.Line2D;

import javax.swing.JPanel;

public class StateBtn extends JPanel {

    private static final long serialVersionUID = -431114028667352251L;

    @Override
    protected void paintComponent(Graphics g) {

        super.paintComponent(g);

        // enable antialiasing
        Graphics2D g2 = (Graphics2D) g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR);


        // draw the arrow
        Line2D.Double line = new Line2D.Double(0, getHeight()/2, 20, getHeight()/2);            
        drawArrowHead(g2, line);
        g2.draw(line);

        // If I call repaint() here (like in my answer below), it works

    }

    private void drawArrowHead(Graphics2D g2d, Line2D.Double line) {  
        AffineTransform tx = new AffineTransform();

        tx.setToIdentity();
        double angle = Math.atan2(line.y2-line.y1, line.x2-line.x1);
        tx.translate(line.x2, line.y2);
        tx.rotate((angle-Math.PI/2d));  

        Polygon arrowHead = new Polygon();  
        arrowHead.addPoint(0,5);
        arrowHead.addPoint(-5,-5);
        arrowHead.addPoint(5,-5);

        Graphics2D g = (Graphics2D) g2d.create();
        g.setTransform(tx);   
        g.fill(arrowHead);
        g.dispose();
    }

}

它是这样创建的：

package gui;

import java.awt.EventQueue;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;

public class Main extends JFrame {

    private static final long serialVersionUID = 4085389089535850911L;
    private JPanel contentPane;

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    Main frame = new Main();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    public Main() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        setSize(500, 500);
        setLocation(0, 0);

        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        setContentPane(contentPane);
        contentPane.setLayout(null);

        StateBtn stateBtn = new StateBtn();
        stateBtn.setBounds(200,200,35,35);
        contentPane.add(stateBtn);
    }
}

线条已正确绘制，但箭头不可见，直到我调用repaint（）为止。 问题在于该元素是可拖动元素，因此每次位置更改时，我都必须两次调用repaint（）。 这将使代码更加复杂，并且GUI会变得很落后。

为什么不能只将箭头与直线绘制在一起？ 真的没有人可以帮助我吗？

Answer 1

您尚未发布真正的MCVE ，因此不可能知道您可能做错了什么，但是您不需要在答案中使用过的麻烦，您可以在paintComponent中调用repaint() 。 如果您仍然需要有关自己代码的帮助，请发布有效的MCVE代码，我们可以对其进行编译和运行，而无需进行修改。 有关我所说的MCVE的示例，请阅读MCVE链接，然后看一下我在下面的答案中发布的MCVE示例。

说了这么多，请理解，Swing图形通常是被动的，这意味着您将让程序根据事件更改其状态，然后调用repaint() ，这建议 Swing重绘管理器调用paint。 不能保证会进行绘制，因为可能会忽略由于“堆叠”而又由于许多调用而备份的重新绘制请求。

因此，根据您的情况，我们可以使用您的代码并对其进行修改以查看其工作原理。 假设我给我的JPanel一个MouseAdapter-一个既是MouseListener又是MouseMotionListener的类，并且在此适配器中，我只设置了两个Point实例字段，即p0（用于最初按下鼠标的位置）和p1（用于鼠标的位置）。拖动或释放。 我可以设置这些字段，然后调用repaint，然后让我的绘画方法使用p0和p1绘制箭头。 因此鼠标调整器可能如下所示：

private class MyMouse extends MouseAdapter {
    private boolean settingMouse = false;

    @Override
    public void mousePressed(MouseEvent e) {
        if (e.getButton() != MouseEvent.BUTTON1) {
            return;
        }
        p0 = e.getPoint();
        p1 = null;
        settingMouse = true; // drawing a new arrow
        repaint();
    }

    @Override
    public void mouseReleased(MouseEvent e) {
        setP1(e);
        settingMouse = false; // no longer drawing the new arrow
    }

    @Override
    public void mouseDragged(MouseEvent e) {
        setP1(e);
    }

    private void setP1(MouseEvent e) {
        if (settingMouse) {
            p1 = e.getPoint();
            repaint();
        }
    }
}

然后在绘画代码中，我将使用您的代码，对其进行修改以使其使用我的p0和p1点：

@Override
protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    Graphics2D g2 = (Graphics2D) g;
    g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
    g2.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR);

    if (p0 != null && p1 != null) {
        Line2D.Double line = new Line2D.Double(p0.x, p0.y, p1.x, p1.y);

        drawArrowHead(g2, line);
        g2.draw(line);          
    }

}

整个shebang看起来像这样：

import java.awt.*;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.geom.*;
import javax.swing.*;

@SuppressWarnings("serial")
public class StateBtn extends JPanel {
    // constants to size the JPanel
    private static final int PREF_W = 800;
    private static final int PREF_H = 650;
    private static final int AH_SIZE = 5; // size of arrow head -- avoid "magic"
                                            // numbers!

    // our start and end Points for the arrow
    private Point p0 = null;
    private Point p1 = null;

    public StateBtn() {
        // create and add a label to tell the user what to do
        JLabel label = new JLabel("Click Mouse and Drag");
        label.setFont(new Font(Font.SANS_SERIF, Font.BOLD, 42));
        label.setForeground(new Color(0, 0, 0, 50));
        setLayout(new GridBagLayout());
        add(label); // add it to the center

        // create our MouseAdapater and use it as both MouseListener and
        // MouseMotionListener
        MyMouse myMouse = new MyMouse();
        addMouseListener(myMouse);
        addMouseMotionListener(myMouse);
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR);

        // only do this if there are points to draw!
        if (p0 != null && p1 != null) {
            Line2D.Double line = new Line2D.Double(p0.x, p0.y, p1.x, p1.y);

            drawArrowHead(g2, line);
            g2.draw(line);
        }

    }

    private void drawArrowHead(Graphics2D g2d, Line2D.Double line) {
        AffineTransform tx = new AffineTransform();

        tx.setToIdentity();
        double angle = Math.atan2(line.y2 - line.y1, line.x2 - line.x1);
        tx.translate(line.x2, line.y2);
        tx.rotate((angle - Math.PI / 2d));

        Polygon arrowHead = new Polygon();
        arrowHead.addPoint(0, AH_SIZE); // again avoid "magic" numbers
        arrowHead.addPoint(-AH_SIZE, -AH_SIZE);
        arrowHead.addPoint(AH_SIZE, -AH_SIZE);

        Graphics2D g = (Graphics2D) g2d.create();
        g.setTransform(tx);
        g.fill(arrowHead);
        g.dispose(); // we created this, so we can dispose of it
        // we should **NOT** dispose of g2d since the JVM gave us that
    }

    @Override
    public Dimension getPreferredSize() {
        // size our JPanel
        return new Dimension(PREF_W, PREF_H);
    }

    private class MyMouse extends MouseAdapter {
        private boolean settingMouse = false;

        @Override
        public void mousePressed(MouseEvent e) {
            // if we press the wrong mouse button, exit
            if (e.getButton() != MouseEvent.BUTTON1) {
                return;
            }
            p0 = e.getPoint();  // set the start point
            p1 = null;  // clear the end point
            settingMouse = true; // tell mouse listener we're creating a new arrow
            repaint();  // suggest a repaint
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            setP1(e);
            settingMouse = false; // no longer drawing the new arrow
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            setP1(e);
        }

        private void setP1(MouseEvent e) {
            if (settingMouse) {
                p1 = e.getPoint(); // set the end point
                repaint();  // and paint!
            }
        }
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }

    private static void createAndShowGui() {
        StateBtn mainPanel = new StateBtn();
        JFrame frame = new JFrame("StateBtn");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(mainPanel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
}

这段代码就是我的MCVE示例的含义。 实际上，对于像样的MCVE来说，它有点大，但它确实可以。 请编译并运行代码以查看它是否有效。 如果这对您没有帮助，如果您仍然必须在重涂调用中使用kudge，那么我敦促您创建自己的MCVE并将其与您的问题一起发布，然后对我发表评论，以便我可以看到它。

drawArrowHead(...) ，有人问是否可以在drawArrowHead(...)方法中创建一个新的Graphics对象，是否可以，是的，不仅可以，而且是处理AffineTransforms时的首选方法，因为这种方式您不必担心转换可能对共享原始Graphics对象的边框和子组件产生的下游影响。 同样，只要您遵循处理您自己创建的 Graphics对象并且不处理JVM给您的 Graphics对象的规则， 就可以了 。

Answer 2

好的，看来，除了再次调用重绘，别无其他方法。 我在paintComponent方法的结尾确实喜欢这样：

if (repaint == false) {
    repaint = true;
} else {
    repaint = false;
    repaint();
}

因此，它只重画一次。 但是，没有清洁的解决方案吗？