当我在其 ViewPort 中绘制 JPanel 时，JScrollPane 不滚动

Question

我正在尝试在作为 JScrollPane 的客户端（ViewPort 视图）的 JPanel 子类上绘制一些树结构。我可以很好地绘制树，但即使它比视口大，JScrollPane 也不会滚动。客户端是 JTextArea 的相同 JScrollPane 工作正常。

下面的代码创建了一个框架，其中有 2 个并排的滚动窗格——一个有效，另一个无效。（为了简单起见，我只是绘制文本。）我是 Swing GUI 的新手，所以我无疑在做一些愚蠢的事情，但是几天的搜索和阅读并没有告诉我我的错误。

import javax.swing.JPanel;
import javax.swing.JTextArea;
import javax.swing.JScrollPane;
import java.awt.Graphics;

public class ScrollTest extends javax.swing.JFrame {

    public ScrollTest() {
        initComponents();
        prepComponents();
    }

    void prepComponents() {
        leftScrollPaneContent = new JTextArea();
        leftScrollPane.setViewportView(leftScrollPaneContent);
        rightScrollPaneContent = new TestPanel();
        rightScrollPane.setViewportView(rightScrollPaneContent);
        String s = "Test of JTextArea setText()\n\n";
        for (int i = 1; i < 30; i++) {
            s += "Line " + i + ": This is a test.\n";
        }
        leftScrollPaneContent.setText(s);
        rightScrollPaneContent.repaint();
    }

    @SuppressWarnings("unchecked")
    private void initComponents() {
        leftPanel = new javax.swing.JPanel();
        jPanelLabel = new javax.swing.JLabel();
        leftScrollPane = new javax.swing.JScrollPane();
        rightPanel = new javax.swing.JPanel();
        subclassLabel = new javax.swing.JLabel();
        rightScrollPane = new javax.swing.JScrollPane();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        jPanelLabel.setText("JTextArea in a ScrollPane");

        leftScrollPane.setHorizontalScrollBarPolicy(javax.swing.ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);
        leftScrollPane.setVerticalScrollBarPolicy(javax.swing.ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);

        javax.swing.GroupLayout leftPanelLayout = new javax.swing.GroupLayout(leftPanel);
        leftPanel.setLayout(leftPanelLayout);
        leftPanelLayout.setHorizontalGroup(
                leftPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                .addGroup(leftPanelLayout.createSequentialGroup()
                        .addContainerGap()
                        .addComponent(leftScrollPane)
                        .addContainerGap())
                .addGroup(leftPanelLayout.createSequentialGroup()
                        .addGap(101, 101, 101)
                        .addComponent(jPanelLabel, javax.swing.GroupLayout.PREFERRED_SIZE, 180, javax.swing.GroupLayout.PREFERRED_SIZE)
                        .addContainerGap(114, Short.MAX_VALUE))
        );
        leftPanelLayout.setVerticalGroup(
                leftPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                .addGroup(leftPanelLayout.createSequentialGroup()
                        .addContainerGap()
                        .addComponent(jPanelLabel)
                        .addGap(18, 18, 18)
                        .addComponent(leftScrollPane, javax.swing.GroupLayout.DEFAULT_SIZE, 332, Short.MAX_VALUE)
                        .addContainerGap())
        );

        subclassLabel.setText("JPanel subclass in a ScrollPane");

        rightScrollPane.setHorizontalScrollBarPolicy(javax.swing.ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);
        rightScrollPane.setVerticalScrollBarPolicy(javax.swing.ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);

        javax.swing.GroupLayout rightPanelLayout = new javax.swing.GroupLayout(rightPanel);
        rightPanel.setLayout(rightPanelLayout);
        rightPanelLayout.setHorizontalGroup(
                rightPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                .addGroup(rightPanelLayout.createSequentialGroup()
                        .addContainerGap()
                        .addComponent(rightScrollPane)
                        .addContainerGap())
                .addGroup(rightPanelLayout.createSequentialGroup()
                        .addGap(66, 66, 66)
                        .addComponent(subclassLabel, javax.swing.GroupLayout.PREFERRED_SIZE, 200, javax.swing.GroupLayout.PREFERRED_SIZE)
                        .addContainerGap(77, Short.MAX_VALUE))
        );
        rightPanelLayout.setVerticalGroup(
                rightPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                .addGroup(rightPanelLayout.createSequentialGroup()
                        .addComponent(subclassLabel)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                        .addComponent(rightScrollPane)
                        .addContainerGap())
        );

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
                layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                .addGroup(layout.createSequentialGroup()
                        .addContainerGap()
                        .addComponent(leftPanel, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                        .addGap(18, 18, 18)
                        .addComponent(rightPanel, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                        .addContainerGap(14, Short.MAX_VALUE))
        );
        layout.setVerticalGroup(
                layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                .addGroup(layout.createSequentialGroup()
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                                .addGroup(layout.createSequentialGroup()
                                        .addContainerGap()
                                        .addComponent(leftPanel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
                                .addGroup(layout.createSequentialGroup()
                                        .addGap(17, 17, 17)
                                        .addComponent(rightPanel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)))
                        .addContainerGap())
        );

        pack();
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new ScrollTest().setVisible(true);
            }
        });
    }

    private javax.swing.JLabel jPanelLabel;
    private javax.swing.JPanel leftPanel;
    private javax.swing.JScrollPane leftScrollPane;
    private javax.swing.JPanel rightPanel;
    private javax.swing.JScrollPane rightScrollPane;
    private javax.swing.JLabel subclassLabel;
    // End of variables declaration                   
    private JTextArea leftScrollPaneContent;
    private JPanel rightScrollPaneContent;

    public class TestPanel extends JPanel {

        public void paint(Graphics g) {
            super.paint(g);
            g.drawString("Test of JPanel g.drawString()\n", 10, 20);
            for (int i = 3; i < 33; i++) {
                g.drawString("Line " + (i-2) + ": This is a test.", 10, 20 * i);
            }
        }
    }
}

Answer 1

您的 JTextArea leftScrollPaneContent 覆盖 getPreferredSize() 哪个 return size 巧妙源自实际的文本和字体。周围的JScrollPane用这个大小来决定如果需要滚动条。
此外，JTextArea（因为从 JTextComponent 延伸）还实现了 Scrollable 接口。这为周围的环境提供了一些额外的提示 JScollPane 以获得更好的滚动行为。

您的 TestPanel rightScrollPaneContent 没有做任何事情。因此它从 JPanel 继承了一个 getPreferredSize() 方法 return只有 10x10 的小尺寸（这对于空 JPanel，没有子组件）。因此周围的JScrollPane决定没有需要为这个所谓的微小内容显示任何滚动条。

因此，最重要的是，在您的 TestPanel 中，您需要覆盖 getPreferredSize() 和 return 那里有一个合理的大尺寸。作为奖励（为了更好的滚动行为）你也可以实现 Scrollable 界面。或者您选择从另一个 class 扩展您的 class 它已经实现了 Scrollable 接口。

当我在其 ViewPort 中绘制 JPanel 时，JScrollPane 不滚动

JScrollPane does not scroll when I paint the JPanel in its ViewPort

java

swing

jscrollpane