Java – when to call getpreferredscrollableviewportsize() when laying out JScrollPane?
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Java
To implement the scrollable interface, you need to implement the getpreferredscrollableviewportsize () method This usually only needs to be done by forwarding the call to getpreferredsize() - unless other parameters of scrollable may affect the preferred jviewport size, such as the setvisiblerowcount() method in JTree
I have a situation where I think this method can help me achieve my goal, but a simple print statement in my getpreferredscrollableviewportsize () implementation confirms that it will never be called Search JScrollPane, scrollpanelayout and jviewport to confirm that the method is not called (directly) However, the comments in JScrollPane make it clear that scrollpanelayout uses it, and I can confirm that it is implemented as expected in JTree
When to call, what class (probably layoutmanager) and when? I am using JDK 1.7_ 07
Solution
I don't have time to search all the source code, but during the test, it seems that this method is called when the GUI is packaged
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 direction) {
if (orientation == SwingConstants.HORIZONTAL) {
return VP_WIDTH;
}
Component[] comps = getComponents();
if (comps.length > 0) {
return comps[0].getHeight();
}
return getSize().height / 3;
}
}
return:
JFrame pack() called Main Scrollable getPreferredScrollableViewportSize called Main Scrollable getPreferredSize called scrollpane doLayout called Main Scrollable getPreferredSize called Main Scrollable getPreferredSize called viewport doLayout called Main Scrollable getPreferredSize called scrollpane doLayout called Main Scrollable getPreferredSize called viewport doLayout called Main Scrollable getPreferredSize called timer count: 0 timer count: 1 timer count: 2 timer count: 3 timer count: 4 timer count: 5 scrollpane doLayout called Main Scrollable getPreferredSize called viewport doLayout called Main Scrollable getPreferredSize called scrollpane doLayout called Main Scrollable getPreferredSize called viewport doLayout called Main Scrollable getPreferredSize called timer count: 6 timer count: 7 timer count: 8 timer count: 9 timer count: 10 calling pack again JFrame pack() called Main Scrollable getPreferredScrollableViewportSize called Main Scrollable getPreferredSize called scrollpane doLayout called Main Scrollable getPreferredSize called viewport doLayout called Main Scrollable getPreferredSize called scrollpane doLayout called Main Scrollable getPreferredSize called viewport doLayout called Main Scrollable getPreferredSize called
Edit 2 when I change the getpreferredscrollableviewportsize override to:
@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();
}
This is what I see:
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)
It is recommended that the preferredlayoutsize of the viewportlayout class call the getpreferredscrollableviewportsize method of scrollable
Edit 3 in fact, the viewportlayout source code supports this:
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 }
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