Java Swing：滚动到JScrollPane的底部，以当前视口位置为条件

我试图模仿Adium和我见过的大多数其他聊天客户端的功能，当新消息进来时，滚动条会移到底部，但只有当你已经在那里的时候。换句话说，如果你已经滚动了几行并且正在阅读，当一条新消息进来时，它不会将你的位置跳到屏幕底部；那会很烦人。但是如果滚动到底部，程序会正确地假设您希望随时查看最新的消息，因此自动滚动

我花了一段时间试图模仿这一点；该平台似乎不惜一切代价打击这种行为。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);

在处理新输入文本的方法中：

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，这是可行的，但它很简陋，不理想，原因有二

通过设置插入符号位置，用户在聊天区中的任何选择都会被删除。我可以想象，如果他试图复制/粘贴，这会非常恼人

由于滚动窗格的前进发生在插入文本之后，因此在一瞬间滚动条处于错误的位置，然后它在视觉上向末尾跳跃。这并不理想 在你问之前，是的，我已经在上读了这篇博文，但是默认的滚动到底的行为不是我想要的

其他相关（但在我看来，在这方面并非完全有用）问题：

在这方面的任何帮助都将不胜感激

编辑：

根据Devon_C#u Miller的建议，我有一种改进的滚动到底部的方法，解决问题1

我仍然有问题#2。

请查看和

这应该可以在不干扰用户选择的情况下满足您的需求

对于滚动条，尝试在不同的事件上强制滚动和追加。例如：

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

看看这个

尽管如此，我还是会更改代码以使用以下更高效的代码：

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

---

基于前面的回答和进一步的谷歌搜索，我做了一个测试，其中一个线程在JScrollPane中连续地向JTextArea追加字符串。我认为在这里使用invokeLater是很重要的，因为在我们滚动到该值之前，JScrollBar需要用其新的最大值进行更新。使用invokeLater，AWT线程将处理这个问题

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;
    }
}

---

过去几天我一直在尝试不同的方法来解决这个问题。我找到的最佳解决方案是替换JScrollPane视口的布局管理器，并在那里实现所有所需的滚动行为。跳跃的滚动条旋钮不见了，它燃烧得很快——如此之快，以至于它创造了另一个我不得不解决的比赛条件。详情如下:

---

您可以在这里找到完整的详细信息：

有点晚了，但我发现了一些东西：

本质上，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;
    }
}

---

该示例再次放弃选择。此外，滚动条仍在跳跃。我没有看到“滚动条跳跃”。如果找到所选内容，则始终可以更改代码以不重置插入符号位置。然后视口将不会自动滚动，但我认为这是您想要的行为。如果用户当前正在选择文本，滚动不应该是自动的。我不明白你对不同事件的意思。。。我将滚动作为一个单独的事件。拆分滚动和追加的目的是让GUI有机会在滚动和添加文本之间重新绘制。这允许滚动条在文本出现之前移动到底部。我正在拆分滚动条和追加，但显然追加必须在滚动之前发生，否则追加文本时我们将不在视图底部。请查看我上面发布的代码中的注释。添加一个换行符并在一个事件中滚动，然后在invokeLater中添加没有前面换行符的文本。您可以在此处查看我的答案：

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

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

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;
    }
}