smart scrolling the chat text area

gui/griffon-app/views/com/muwire/gui/ChatRoomView.groovy

@@ -32,6 +32,7 @@ class ChatRoomView {
     def parent
     def sayField
     def roomTextArea
+    def textScrollPane
     def membersTable
     def lastMembersTableSortEvent
     
@@ -42,7 +43,7 @@ class ChatRoomView {
                 borderLayout()
                 panel(constraints : BorderLayout.CENTER) {
                     gridLayout(rows : 1, cols : 1)
-                    scrollPane {
+                    textScrollPane = scrollPane {
                         roomTextArea = textArea(editable : false, lineWrap : true, wrapStyleWord : true)
                     }
                 }
@@ -72,7 +73,7 @@ class ChatRoomView {
                         }
                         panel {
                             gridLayout(rows : 1, cols : 1)
-                            scrollPane {
+                            textScrollPane = scrollPane {
                                 roomTextArea = textArea(editable : false, lineWrap : true, wrapStyleWord : true)
                             }
                         }
@@ -87,6 +88,8 @@ class ChatRoomView {
 
             }
         }
+        
+        SmartScroller smartScroller = new SmartScroller(textScrollPane)
     }
     
     void mvcGroupInit(Map<String,String> args) {

gui/src/main/java/com/muwire/gui/SmartScroller.java

@@ -0,0 +1,174 @@
+package com.muwire.gui;
+
+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;
+    }
+}