[news.eclipse.newcomer] Re: Getting scroll pane to display bottom of tex

[eclipse@xxxxxxxxx (Peter Toye)]

In article <memo.20051024144938.2600A@xxxxxxxxxxxxxxxxxxxxx>, 
eclipse@xxxxxxxxx (Peter Toye) wrote:

> *From:* eclipse@xxxxxxxxx (Peter Toye)
> *Date:* Mon, 24 Oct 2005 14:49 +0100 (BST)
> 
> In article <djimkv$9c8$1@xxxxxxxxxxxxxxxx>, yaofahua@xxxxxxxxxxx (JACK 
> YAO) wrote:
> 
> > *From:* JACK YAO <yaofahua@xxxxxxxxxxx>
> > *Date:* Mon, 24 Oct 2005 21:12:59 +0800
> > 
> > Peter Toye wrote:
> > > In article <dj86ss$pj6$1@xxxxxxxxxxxxxxxx>, yaofahua@xxxxxxxxxxx 
> > > (JACK YAO) wrote:
> > > 
> > >> public class test extends JFrame implements ActionListener {
> > >>      JTextArea ta = new JTextArea(10, 10);
> > >>      int i = 0;
> > >>      JPanel p = new JPanel();
> > >>      JScrollPane sp = new JScrollPane(ta);
> > >>      public test() {
> > >>          p.add(sp);
> > >>          JButton btn = new JButton("test");
> > >>          btn.addActionListener(this);
> > >>          p.add(btn, BorderLayout.NORTH);
> > >>          this.add(p);
> > >>          pack();
> > >>          setVisible(true);
> > >>      }
> > >>      public void actionPerformed(ActionEvent e) {
> > >>          new AddThread().start();
> > >>      }
> > >>      class AddThread extends Thread {
> > >>          public void run() {
> > >>              for (int i = 0; i < 100; i++) {
> > >>                  ta.append("line :" + i + "\n");
> > >>                  //JScrollBar sb = sp.getVerticalScrollBar();
> > >>                  //sb.setValue(sb.getMaximum());
> > >>                  try {
> > >>                      Thread.sleep(100);
> > >>                  } catch (InterruptedException e) {
> > >>                      e.printStackTrace();
> > >>                  }
> > >>              }
> > >>          }
> > >>      }
> > >>      public static void main(String[] args) {
> > >>          new test();
> > >>      }
> > >> }
> > > Jack,
> > > 
> > > I've tried your program now, but modified it to append more than 
> > > one line. When you comment out the lines the slider bar stays at 
> > > the top. When you don't, last appended line isn't visible.
> > > 
> > > I'm obviously going to have to experiment more with my program, and 
> > > learn about threading.
> > > 
> > > On a wider point, this seems to be an issue which makes Swing 
> > > difficult to use - why isn't the obvious behaviour of the slider 
> > > bar on append made a default?
> > > 
> > > Peter
> > Peter, did you solve your problem ?  I change the for loop to :
> > //**************************************************************
> > for(int i=0;i<100;i++) {
> >      System.out.println(sb.getMaximum());//1
> > 
> >      ta.append("line :" + i + "\n");
> >      sleep(500);
> >      System.out.println(sb.getMaximum());//2
> > 
> >      ta.append("line :" + i + "\n");
> >      sleep(500);
> >      System.out.println(sb.getMaximum());//3
> > 
> >      ta.append("line :" + i + "\n");
> >      sleep(500);
> >      System.out.println(sb.getMaximum());//4
> > }
> > //***************************************************************
> > To my surprise, 3 4 which are the last two append are the same value. 
> > I think this test has no thread factor.
> > -- 
> > 
> > JACK YAO, Beijing, China
> > 
> Jack,
> 
> This is what I used for my test with threading. I found that the slider 
> doesn't go all the way down to the bottom after the multi-line append. 
> I think that I may bring this up as a Java bug report, but as a newbie 
> I wanted to be sure that it wasn't just me.
> 
> Peter
> 
>     class AddThread extends Thread {
>         public void run() {
>             for (int i = 0; i < 10; i++) {
>             	ta.append("Start of append\n");
>                 ta.append("line :" + i + "\n");
>                 ta.append("End of append\n");
>                 JScrollBar sb = sp.getVerticalScrollBar();
>                 sb.setValue(sb.getMaximum());
>                 try {
>                     Thread.sleep(1000);
>                 } catch (InterruptedException e) {
>                     e.printStackTrace();
>                 }
>             }
>         }
>     }
> 
> 
A further comment - if I put a delay before the sb.setValue statement it 
updates the scroll bar OK.

I suspect the problem is that the screen updating (and calculation of the 
size of the scrollbar) is done on a separate thread from the program, and 
the program gets the old value. I shall experiment using the invokeLater 
swing call to see if it will single-thread the app and give the expected 
result. I still think it's a Java bug that it doesn't happen 
automatically.

Peter