Swing：滚动到 JScrollPane 的底部，以当前视口位置为条件

Question

我试图模仿 Adium 和我见过的大多数其他聊天客户端的功能，其中当新消息进入时滚动条会前进到底部，但前提是您已经在那里。换句话说，如果您向上滚动几行并正在阅读，当收到新消息时，它不会将您的位置跳到屏幕底部；而是将您的位置跳到屏幕底部。那会很烦人。但是，如果您滚动到底部，程序会正确地假设您希望始终看到最新的消息，因此会相应地自动滚动。

我曾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);

在处理传入的新文本的方法中：

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，这可以工作，但它很卡顿并且不理想，原因有两个。

1. 通过设置插入符位置，用户在聊天区域中可能进行的任何选择都将被删除。我可以想象，如果他尝试复制/粘贴，这会非常令人恼火。
2. 由于滚动窗格的前进是在插入文本之后发生的，因此滚动条有一瞬间位于错误的位置，然后它在视觉上跳向末尾。这并不理想。

在你问之前，是的，我已经阅读了这篇关于 文本区域 的博客文章滚动，但默认滚动到底部的行为不是我想要的。

其他相关的（但在我看来，在这方面并不完全有帮助）问题： 在 jscrollpane 上设置滚动条 使 JScrollPane 自动一直向下滚动。

非常感谢在这方面的任何帮助。

编辑：

根据 Devon_C_Miller 的建议，我有一种改进的滚动到底部的方法，解决了问题 #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);
           }
       }
    });
}

我仍然有问题#2。

Answer 1

看一下 scrollRectToVisible(矩形 r) 和 modelToView(int pos)

这应该可以让您找到所需的内容，而不会干扰用户的选择。

至于滚动条，尝试强制滚动和追加发生在不同的事件上。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

根据之前的答案和进一步的 Google 搜索，我做了一个测试，其中线程连续将字符串附加到 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 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;
    }
}

Answer 3

有点晚了，但我发现了一些东西： http://www.camick.com/java /source/SmartScroller.java

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;
    }
}

Answer 4

查看此示例。

尽管如此，我会更改代码以使用以下更有效的代码：

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

Answer 5

在过去的几天里，我一直在尝试不同的方法来解决这个问题。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所需的滚动行为。跳动的滚动条旋钮消失了，而且速度非常快——快到它创造了另一个我必须解决的竞争条件。详细信息在这里： http ://www.java-forums.org/awt-swing/56808-scroll-bar-knob-jumpy-when-component-grinding.html

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：如何让JTextPane仅在滚动条位于底部且滚动锁定关闭时自动滚动？

Answer 7

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