Java 布局JScrollPane时何时调用getPreferredScrollableViewportSize（）？

实现可滚动接口需要实现getPreferredScrollableViewportSize（）方法。这通常是通过转发对getPreferredSize（）的调用来完成的-除非可滚动的其他参数可能会影响首选的JViewport大小，例如JTree中的setVisibleRowCount（）方法

我认为这个方法可以帮助我实现我的目标，但是我的getPreferredScrollableViewportSize（）实现中的一个简单的print语句证实了它从未被调用过。搜索JScrollPane、ScrollPaneLayout和JViewport确认没有（直接）调用该方法。然而，JScrollPane中的注释明确指出ScrollPanelLayout使用了它，我可以确认它在JTree中的实现与预期一致

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它是什么时候调用的，由哪个类（大概是LayoutManager）调用，以及何时调用？我使用的是JDK1.7_07，我还没有时间搜索所有的源代码，但是在测试中，似乎在打包GUI时调用了该方法

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

@SuppressWarnings("serial")
public class TestScrollable extends JPanel {
   private static final int REPACK_COUNT = 10;
   protected static final int RESIZE_COUNT = 5;

   public TestScrollable() {
      MyScrollable mainScrollable = new MyScrollable("Main Scrollable");
      mainScrollable.setLayout(new GridLayout(0, 1));

      int rowCount = 100;
      for (int i = 0; i < rowCount; i++) {
         JPanel rowPanel = new JPanel();
         String name = "Row Panel " + i;
         rowPanel.setName(name);
         rowPanel.setBorder(BorderFactory.createLineBorder(Color.blue));
         rowPanel.setLayout(new BorderLayout());
         rowPanel.add(new JLabel(rowPanel.getName()));
         mainScrollable.add(rowPanel);
      }

      JViewport viewport = new JViewport() {
         @Override
         public void doLayout() {
            System.out.println("viewport doLayout called");
            super.doLayout();
         }

      };
      viewport.setView(mainScrollable);

      JScrollPane scrollPane = new JScrollPane() {
         @Override
         public void doLayout() {
            System.out.println("scrollpane doLayout called");
            super.doLayout();
         }
      };
      scrollPane.setViewport(viewport);
      setLayout(new BorderLayout());
      add(scrollPane);
   }

   private static void createAndShowGui() {
      TestScrollable mainPanel = new TestScrollable();

      final JFrame frame = new JFrame("TestScrollable") {
         @Override
         public void pack() {
            System.out.println("JFrame pack() called");
            super.pack();
         }
      };
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);

      int delay = 1000;
      // re-test pack()
      new Timer(delay, new ActionListener() {
         private int timerCount = 0;

         @Override
         public void actionPerformed(ActionEvent e) {
            System.out.println("timer count: " + timerCount);

            if (timerCount == RESIZE_COUNT) {
               int newWidth = frame.getSize().width * 2;
               int newHeight = frame.getSize().height * 2;
               Dimension newSize = new Dimension(newWidth, newHeight);
               frame.setSize(newSize);
               frame.repaint();
            }

            if (timerCount == REPACK_COUNT) {
               System.out.println("calling pack again");
               frame.pack();
               ((Timer) e.getSource()).stop();
            }
            timerCount++;
         }
      }).start();
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

@SuppressWarnings("serial")
class MyScrollable extends JComponent implements Scrollable {
   public static final int VP_WIDTH = 600;
   private static final int ROW_COUNT = 10;

   public MyScrollable(String name) {
      super.setName(name);
   }

   @Override
   public Dimension getPreferredScrollableViewportSize() {
      System.out.println(getName()
            + " getPreferredScrollableViewportSize called");
      Component[] comps = getComponents();
      if (comps.length > 0) {
         int height = ROW_COUNT * comps[0].getPreferredSize().height;
         return new Dimension(VP_WIDTH, height);
      }

      return super.getPreferredSize();
   }

   @Override
   public Dimension getPreferredSize() {
      System.out.println(getName() + " getPreferredSize called");
      return super.getPreferredSize();
   }

   @Override
   public int getScrollableBlockIncrement(Rectangle visibleRect,
         int orientation, int direction) {
      if (orientation == SwingConstants.HORIZONTAL) {
         return VP_WIDTH;
      }
      Component[] comps = getComponents();
      if (comps.length > 0) {
         return comps[0].getHeight() * (3 * ROW_COUNT / 4);
      }

      return getSize().height / 3;
   }

   @Override
   public boolean getScrollableTracksViewportHeight() {
      return false;
   }

   @Override
   public boolean getScrollableTracksViewportWidth() {
      return true;
   }

   @Override
   public int getScrollableUnitIncrement(Rectangle visibleRect,
         int orientation, int direction) {
      if (orientation == SwingConstants.HORIZONTAL) {
         return VP_WIDTH;
      }
      Component[] comps = getComponents();
      if (comps.length > 0) {
         return comps[0].getHeight();
      }
      return getSize().height / 3;
   }

}

编辑2
当我将getPreferredScrollableViewportSize覆盖更改为：

@Override
public Dimension getPreferredScrollableViewportSize() {
  System.out.println(getName()
        + " getPreferredScrollableViewportSize called");
  StackTraceElement[] foo = Thread.currentThread().getStackTrace();
  int maxTraces = 10;
  for (int i = 0; i < foo.length && i < maxTraces ; i++) {
     System.out.printf("%02d: %s%n", i, foo[i]);
  }
  if (getComponentCount() > 0) {
     Component[] comps = getComponents();
     int height = ROW_COUNT * comps[0].getPreferredSize().height;
     return new Dimension(VP_WIDTH, height);
  }

  return super.getPreferredSize();
}

建议ViewportLayout类的preferredLayoutSize调用Scrollable的getPreferredScrollableViewportSize方法

编辑3
事实上，政府支持这一点：

   86     public Dimension preferredLayoutSize(Container parent) {
   87         Component view = ((JViewport)parent).getView();
   88         if (view == null) {
   89             return new Dimension(0, 0);
   90         }
   91         else if (view instanceof Scrollable) {
   92             return ((Scrollable)view).getPreferredScrollableViewportSize();
   93         }
   94         else {
   95             return view.getPreferredSize();
   96         }
   97     }

---

请参见我的答案的编辑2。看来是ViewportLayout调用了您的方法。@HovercraftFullOfEels干得不错，谢谢。是的，看起来确实如此。与JScrollPane直接将ScrollPanelLayout设置为其LayoutManager不同，JViewport不硬编码ViewportLayout，而是依赖JComponent查询其UI，该UI决定了JViewport的LayoutManager。我还没有完全了解整个UI，所以这可能不是完全准确的。

Main Scrollable getPreferredScrollableViewportSize called
00: java.lang.Thread.getStackTrace(Unknown Source)
01: pkg.MyScrollable.getPreferredScrollableViewportSize(TestScrollable.java:115)
02: javax.swing.ViewportLayout.preferredLayoutSize(Unknown Source)
03: java.awt.Container.preferredSize(Unknown Source)
04: java.awt.Container.getPreferredSize(Unknown Source)
05: javax.swing.JComponent.getPreferredSize(Unknown Source)
06: javax.swing.ScrollPaneLayout.preferredLayoutSize(Unknown Source)
07: java.awt.Container.preferredSize(Unknown Source)
08: java.awt.Container.getPreferredSize(Unknown Source)
09: javax.swing.JComponent.getPreferredSize(Unknown Source)

   86     public Dimension preferredLayoutSize(Container parent) {
   87         Component view = ((JViewport)parent).getView();
   88         if (view == null) {
   89             return new Dimension(0, 0);
   90         }
   91         else if (view instanceof Scrollable) {
   92             return ((Scrollable)view).getPreferredScrollableViewportSize();
   93         }
   94         else {
   95             return view.getPreferredSize();
   96         }
   97     }