Question

我正在尝试模仿Adium和我见过的大多数其他聊天客户端的功能，其中当新邮件进入时滚动条会向前移动，但前提是你已经在那里。换句话说，如果您向上滚动了几行并正在阅读，当有新消息进入时它不会将您的位置跳到屏幕底部;这会很烦人。但如果您滚动到底部，程序会正确地假设您希望始终查看最新的消息，因此会相应地自动滚动。

我有一段时间试图模仿这个;该平台似乎不惜一切代价来对抗这种行为。我能做的最好的事情如下：

在构造函数中：

JTextArea chatArea = new JTextArea();
JScrollPane chatAreaScrollPane = new JScrollPane(chatArea);

// We will manually handle advancing chat window
DefaultCaret caret = (DefaultCaret) chatArea.getCaret();
caret.setUpdatePolicy(DefaultCaret.NEVER_UPDATE);

处理新文本的方法：

boolean atBottom = isViewAtBottom();

// Append the text using styles etc to the chatArea

if (atBottom) {
    scrollViewportToBottom();
} 


public boolean isAtBottom() {
    // Is the last line of text the last line of text visible?
    Adjustable sb = chatAreaScrollPane.getVerticalScrollBar();

    int val = sb.getValue();
    int lowest = val + sb.getVisibleAmount();
    int maxVal = sb.getMaximum();

    boolean atBottom = maxVal == lowest;
    return atBottom;
}


private void scrollToBottom() {
    chatArea.setCaretPosition(chatArea.getDocument().getLength());
}

现在，这种方法有效，但由于两个原因，它很笨拙而不理想。

通过设置插入位置，可以删除用户在聊天区域中可能具有的任何选择。我可以想象，如果他试图复制/粘贴，这将是非常恼人的。
由于滚动窗格的前进发生在插入文本之后，滚动条处于错误位置会有一瞬间，然后它会在视觉上向末端跳跃。这不太理想。

在你问之前，是的，我已经在Text Area Scrolling上阅读了这篇博客文章，但默认滚动到最低行为不是我想要的。

其他相关（但在我看来，在这方面并不完全有用）问题： Setting scroll bar on a jscrollpane Making a JScrollPane automatically scroll all the way down.

非常感谢这方面的任何帮助。

编辑：

根据Devon_C_Miller的建议，我有一种改进的滚动方式，解决问题＃1。

private void scrollToBottom() {
    javax.swing.SwingUtilities.invokeLater(new Runnable() {
       public void run() {
           try {
               int endPosition = chatArea.getDocument().getLength();
               Rectangle bottom = chatArea.modelToView(endPosition);
               chatArea.scrollRectToVisible(bottom);
           }
           catch (BadLocationException e) {
               System.err.println("Could not scroll to " + e);
           }
       }
    });
}

我仍有问题＃2。

Answer 1

查看scrollRectToVisible(Rectangle r)和modelToView(int pos)

这应该可以在不影响用户选择的情况下为您提供所需的信息。

对于滚动条，尝试强制滚动并在不同事件上发生追加。例如：

if (atBottom) {
    // append new line & do scroll
    SwingUtilities.invokerLater(new Runnable(){
        public void run() {
            // append text
        }});
} else {
    // append newline
    // append text
}

Answer 2

基于之前的答案和进一步的Google，我做了一个测试，其中一个Thread继续将字符串附加到JScrollPane中的JTextArea。我认为在这里使用invokeLater很重要，因为在滚动到该值之前，需要使用新的最大值更新JScrollBar。使用invokeLater，AWT Thread将处理这个问题。

private class AppendThread extends Thread {
    public void run() {
      while (true) {
        String s = "random number = " + Math.random() + "\n";
        boolean scrollDown = textAreaBottomIsVisible();
        textArea.append(s);
        if (scrollDown) {
          javax.swing.SwingUtilities.invokeLater(new Runnable() {
                  public void run() {
                      JScrollBar bar = scrollPane.getVerticalScrollBar();
                      bar.setValue(bar.getMaximum());
                  }
              }
          );
        }
        try {
          Thread.sleep(1000);
        } catch (Exception e) {
          System.err.println("exception = " + e);
        }
      }
    }

    private boolean textAreaBottomIsVisible() {
      Adjustable sb = scrollPane.getVerticalScrollBar();
      int val = sb.getValue();
      int lowest = val + sb.getVisibleAmount();
      int maxVal = sb.getMaximum();
      boolean atBottom = maxVal == lowest;
      return atBottom;
    }
}

Answer 3

查看此example。

虽然，我会更改代码以使用更有效的以下内容：

caret.setDot(textArea.getDocument().getLength());

Answer 4

我花了最近几天尝试不同的方法解决这个问题。我发现的最佳解决方案是替换JScrollPane视口的布局管理器，并在那里实现所有需要的滚动行为。跳跃的滚动条旋钮消失了，而且速度非常快 - 它创造了另一种竞争条件我不得不解决。详细信息如下：http://www.java-forums.org/awt-swing/56808-scroll-bar-knob-jumpy-when-component-growing.html

Answer 5

有点晚了，但我找到的是http://www.camick.com/java/source/SmartScroller.java

实质上，您可以使用以下代码：

JScrollPane scrollPane = new JScrollPane(myOversizedComponent);
// Injects smartscroll behaviour to your scrollpane
new SmartScroller(scrollPane);

这里是SmartScroller类：

import java.awt.Component;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.text.*;

/**
 *  The SmartScroller will attempt to keep the viewport positioned based on
 *  the users interaction with the scrollbar. The normal behaviour is to keep
 *  the viewport positioned to see new data as it is dynamically added.
 *
 *  Assuming vertical scrolling and data is added to the bottom:
 *
 *  - when the viewport is at the bottom and new data is added,
 *    then automatically scroll the viewport to the bottom
 *  - when the viewport is not at the bottom and new data is added,
 *    then do nothing with the viewport
 *
 *  Assuming vertical scrolling and data is added to the top:
 *
 *  - when the viewport is at the top and new data is added,
 *    then do nothing with the viewport
 *  - when the viewport is not at the top and new data is added, then adjust
 *    the viewport to the relative position it was at before the data was added
 *
 *  Similiar logic would apply for horizontal scrolling.
 */
public class SmartScroller implements AdjustmentListener
{
    public final static int HORIZONTAL = 0;
    public final static int VERTICAL = 1;

    public final static int START = 0;
    public final static int END = 1;

    private int viewportPosition;

    private JScrollBar scrollBar;
    private boolean adjustScrollBar = true;

    private int previousValue = -1;
    private int previousMaximum = -1;

    /**
     *  Convenience constructor.
     *  Scroll direction is VERTICAL and viewport position is at the END.
     *
     *  @param scrollPane the scroll pane to monitor
     */
    public SmartScroller(JScrollPane scrollPane)
    {
        this(scrollPane, VERTICAL, END);
    }

    /**
     *  Convenience constructor.
     *  Scroll direction is VERTICAL.
     *
     *  @param scrollPane the scroll pane to monitor
     *  @param viewportPosition valid values are START and END
     */
    public SmartScroller(JScrollPane scrollPane, int viewportPosition)
    {
        this(scrollPane, VERTICAL, viewportPosition);
    }

    /**
     *  Specify how the SmartScroller will function.
     *
     *  @param scrollPane the scroll pane to monitor
     *  @param scrollDirection indicates which JScrollBar to monitor.
     *                         Valid values are HORIZONTAL and VERTICAL.
     *  @param viewportPosition indicates where the viewport will normally be
     *                          positioned as data is added.
     *                          Valid values are START and END
     */
    public SmartScroller(JScrollPane scrollPane, int scrollDirection, int viewportPosition)
    {
        if (scrollDirection != HORIZONTAL
        &&  scrollDirection != VERTICAL)
            throw new IllegalArgumentException("invalid scroll direction specified");

        if (viewportPosition != START
        &&  viewportPosition != END)
            throw new IllegalArgumentException("invalid viewport position specified");

        this.viewportPosition = viewportPosition;

        if (scrollDirection == HORIZONTAL)
            scrollBar = scrollPane.getHorizontalScrollBar();
        else
            scrollBar = scrollPane.getVerticalScrollBar();

        scrollBar.addAdjustmentListener( this );

        //  Turn off automatic scrolling for text components

        Component view = scrollPane.getViewport().getView();

        if (view instanceof JTextComponent)
        {
            JTextComponent textComponent = (JTextComponent)view;
            DefaultCaret caret = (DefaultCaret)textComponent.getCaret();
            caret.setUpdatePolicy(DefaultCaret.NEVER_UPDATE);
        }
    }

    @Override
    public void adjustmentValueChanged(final AdjustmentEvent e)
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            public void run()
            {
                checkScrollBar(e);
            }
        });
    }

    /*
     *  Analyze every adjustment event to determine when the viewport
     *  needs to be repositioned.
     */
    private void checkScrollBar(AdjustmentEvent e)
    {
        //  The scroll bar listModel contains information needed to determine
        //  whether the viewport should be repositioned or not.

        JScrollBar scrollBar = (JScrollBar)e.getSource();
        BoundedRangeModel listModel = scrollBar.getModel();
        int value = listModel.getValue();
        int extent = listModel.getExtent();
        int maximum = listModel.getMaximum();

        boolean valueChanged = previousValue != value;
        boolean maximumChanged = previousMaximum != maximum;

        //  Check if the user has manually repositioned the scrollbar

        if (valueChanged && !maximumChanged)
        {
            if (viewportPosition == START)
                adjustScrollBar = value != 0;
            else
                adjustScrollBar = value + extent >= maximum;
        }

        //  Reset the "value" so we can reposition the viewport and
        //  distinguish between a user scroll and a program scroll.
        //  (ie. valueChanged will be false on a program scroll)

        if (adjustScrollBar && viewportPosition == END)
        {
            //  Scroll the viewport to the end.
            scrollBar.removeAdjustmentListener( this );
            value = maximum - extent;
            scrollBar.setValue( value );
            scrollBar.addAdjustmentListener( this );
        }

        if (adjustScrollBar && viewportPosition == START)
        {
            //  Keep the viewport at the same relative viewportPosition
            scrollBar.removeAdjustmentListener( this );
            value = value + maximum - previousMaximum;
            scrollBar.setValue( value );
            scrollBar.addAdjustmentListener( this );
        }

        previousValue = value;
        previousMaximum = maximum;
    }
}

Answer 6

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

您可以在此处找到完整的详细信息：How to make JTextPane autoscroll only when scroll bar is at bottom and scroll lock is off?

Answer 7

DefaultCaret caret = (DefaultCaret)textArea.getCaret();  
caret.setUpdatePolicy(DefaultCaret.ALWAYS_UPDATE);

Swing：滚动到JScrollPane的底部，以当前视口位置为条件

7 个答案: