e822. 监听JScrollPane的滚动

A scrollbar in a scroll pane fires adjustment events whenever its value changes.

    // Create a scrollable text area
    JTextArea textArea = new JTextArea();
    JScrollPane pane = new JScrollPane(textArea);
    
    // Listen for value changes in the scroll pane's scrollbars
    AdjustmentListener listener = new MyAdjustmentListener();
    pane.getHorizontalScrollBar().addAdjustmentListener(listener);
    pane.getVerticalScrollBar().addAdjustmentListener(listener);
    
    class MyAdjustmentListener implements AdjustmentListener {
        // This method is called whenever the value of a scrollbar is changed,
        // either by the user or programmatically.
        public void adjustmentValueChanged(AdjustmentEvent evt) {
            Adjustable source = evt.getAdjustable();
    
            // getValueIsAdjusting() returns true if the user is currently
            // dragging the scrollbar's knob and has not picked a final value
            if (evt.getValueIsAdjusting()) {
                // The user is dragging the knob
                return;
            }
    
            // Determine which scrollbar fired the event
            int orient = source.getOrientation();
            if (orient == Adjustable.HORIZONTAL) {
                // Event from horizontal scrollbar
            } else {
                // Event from vertical scrollbar
            }
    
            // Determine the type of event
            int type = evt.getAdjustmentType();
            switch (type) {
              case AdjustmentEvent.UNIT_INCREMENT:
                  // Scrollbar was increased by one unit
                  break;
              case AdjustmentEvent.UNIT_DECREMENT:
                  // Scrollbar was decreased by one unit
                  break;
              case AdjustmentEvent.BLOCK_INCREMENT:
                  // Scrollbar was increased by one block
                  break;
              case AdjustmentEvent.BLOCK_DECREMENT:
                  // Scrollbar was decreased by one block
                  break;
              case AdjustmentEvent.TRACK:
                  // The knob on the scrollbar was dragged
                  break;
            }
    
            // Get current value
            int value = evt.getValue();
        }
    }

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