java - 智能 JScrollPane 跟随输出文本

Question

我有一个智能 JScrollPane，但与其他所有解决方案不同，我想知道如果滚动 Pane 不在底部并且文本是否可以跟随(向上滚动)查看的组件(JTextArea)缓冲区已满。包含功能齐全的代码片段。谢谢。

public class ScrollingJTextAreaExample extends JFrame {
    // Worker thread to help periodically append example messages to JTextArea
    Timer timer = new Timer();
    // Merely informative counter, will be displayed with the example messages
    int messageCounter = 0;
    // GUI components
    JScrollPane jScrollPane;
    MyJTextArea jTextArea;

    public ScrollingJTextAreaExample() {
        initComponents(); // Boiler plate GUI construction and layout

        // Configure JTextArea to not update the cursor position after
        // inserting or appending text to the JTextArea. This disables the
        // JTextArea's usual behavior of scrolling automatically whenever
        // inserting or appending text into the JTextArea: we want scrolling
        // to only occur at our discretion, not blindly. NOTE that this
        // breaks normal typing into the JTextArea. This approach assumes
        // that all updates to the ScrollingJTextArea are programmatic.
        DefaultCaret caret = (DefaultCaret) jTextArea.getCaret();
        caret.setUpdatePolicy(DefaultCaret.NEVER_UPDATE);

        // Schedule a task to periodically append example messages to jTextArea
        timer.schedule(new TextGeneratorTask(), 250, 250);

        // This DocumentListener takes care of re-scrolling when appropriate
        Document document = jTextArea.getDocument();
        document.addDocumentListener(new ScrollingDocumentListener());
    }

    // Boring, vanilla GUI construction and layout code
    private void initComponents() {
        jScrollPane = new javax.swing.JScrollPane();
        jTextArea = new MyJTextArea();
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        jScrollPane.setViewportView(jTextArea);
        getContentPane().add(jScrollPane, java.awt.BorderLayout.CENTER);
        setSize(320, 240);
        setLocationRelativeTo(null);
    }

    // ScrollingDocumentListener takes care of re-scrolling when appropriate
    class ScrollingDocumentListener implements DocumentListener {
        public void changedUpdate(DocumentEvent e) {
            maybeScrollToBottom();
        }

        public void insertUpdate(DocumentEvent e) {
            maybeScrollToBottom();
        }

        public void removeUpdate(DocumentEvent e) {
            maybeScrollToBottom();
        }

        private void maybeScrollToBottom() {
            JScrollBar scrollBar = jScrollPane.getVerticalScrollBar();
            boolean scrollBarAtBottom = isScrollBarFullyExtended(scrollBar);
            boolean scrollLock = Toolkit.getDefaultToolkit()
                    .getLockingKeyState(KeyEvent.VK_SCROLL_LOCK);
            if (scrollBarAtBottom && !scrollLock) {
                // Push the call to "scrollToBottom" back TWO PLACES on the
                // AWT-EDT queue so that it runs *after* Swing has had an
                // opportunity to "react" to the appending of new text:
                // this ensures that we "scrollToBottom" only after a new
                // bottom has been recalculated during the natural
                // revalidation of the GUI that occurs after having
                // appending new text to the JTextArea.
                EventQueue.invokeLater(new Runnable() {
                    public void run() {
                        EventQueue.invokeLater(new Runnable() {
                            public void run() {
                                scrollToBottom(jTextArea);
                            }
                        });
                    }
                });
            }
        }
    }

    class TextGeneratorTask extends TimerTask {
        public void run() {
            EventQueue.invokeLater(new Runnable() {
                public void run() {
                    String message = (++messageCounter)
                            + " Lorem ipsum dolor sit amet, consectetur adipisicing elit. \n";
                    jTextArea.appendText(message);
                }
            });
        }
    }

    public static boolean isScrollBarFullyExtended(JScrollBar vScrollBar) {
        BoundedRangeModel model = vScrollBar.getModel();
        return (model.getExtent() + model.getValue()) == model.getMaximum();
    }

    public static void scrollToBottom(JComponent component) {
        Rectangle visibleRect = component.getVisibleRect();
        visibleRect.y = component.getHeight() - visibleRect.height;
        component.scrollRectToVisible(visibleRect);
    }

    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new ScrollingJTextAreaExample().setVisible(true);
            }
        });
    }


    public class MyJTextArea extends JTextArea {
        DefaultCaret caret;
        static final int BUFFERSIZE = 100;

        public MyJTextArea() {
            setBorder(BorderFactory.createEtchedBorder());      
            setBackground(Color.black);
            setForeground(Color.white);
            setCaretColor(getForeground());
            setFont(new Font("Lucida Console", Font.PLAIN, 12));
            setLineWrap(false);            
            setEditable(false);
        }

        public void appendText(String text) {
            // When the line count in the text window > size of the buffer, 
            // replace the first line in buffer to give way to the last line, 
            // all lines in buffer shift one line up
            if (getLineCount() > BUFFERSIZE) {
                try {
                    replaceRange(null, getLineStartOffset(0), getLineEndOffset(0));
                } catch (Exception e) {
                    append("An exception occured in replacing the topmost entry in the buffer");
                    e.printStackTrace();
                    return;
                }
            }
            append(text);            
        }
    }
}

Answer 1

在您的示例中，您有 100 行的缓冲区。假设打开滚动锁定时，视口(viewport)中的第一行位于第 40 行。

如果我理解您的要求，那么一旦您开始从文档中删除行，您希望第 40 行保留在视口(viewport)的顶部。因此，当行被删除时，滚动条将移动到滚动 Pane 的顶部。当第 40 行最终被删除时，滚动条将位于滚动 Pane 的顶部。

如果是这样，下面的(粗略的)代码可以给你一个想法:

public void removeUpdate(DocumentEvent e) {
    //maybeScrollToBottom();
    int lineHeight = jTextArea.getPreferredSize().height / jTextArea.getLineCount();
    JViewport viewport = jScrollPane.getViewport();
    Point position = viewport.getViewPosition();
    position.y -= lineHeight;

    if (position.y > 0 )
        viewport.setViewPosition(position);
}

我怀疑这段代码是否完全按照您想要的方式工作，因为一旦缓冲区已满，您就不能只是将滚动条向下拖动到底部以使视口(viewport)在附加行时不断滚动。但它可能会给你一些想法。