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| 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;
}
} |
Auto-scroll JTextArea Swing
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