When I tried to scroll automatically with JScrollBar, the event handler was drawn only once.

It got stuck and was blown away for a few hours, so instead of a memo. Anyone who uses Swing seems to have failed once.

What was made

I was trying to make something like a self-made shell in Java, and I was trying to make JScrollPane scroll automatically. This is the source code I wrote while checking.

MyShell.java

class MyShell extends JFrame implements ActionListener{
    MyShell{
//(Omitted)
    String now = new File(".").getAbsoluteFile().getParent().toLowerCase();//Current directory

    JTextField text = new JTextField();//Input area
    text.addActionListener( this );
    JTextArea　area = new JTextArea(now+"->");//Display area
    JScrollPane scrollpane = new JScrollPane(area);

    JPanel p = new JPanel()
    p.add(text,BorderLayout.SOUTH);
    p.add(scrollpane);
//(Omitted)
    }
    public void actionPerformed(ActionEvent e){
  	    String input = text.getText().toLowerCase();
  	    if(input.equals("exit")){System.exit(0);}//For forced termination
        area.append(text.getText()+"\nPC->"+mani(input.split(" "))+"\n\n"+now+"->");//Added to the drawing area
        text.setText("");//Erase the contents of the input area

        JScrollBar scrollBar = scrollpane.getVerticalScrollBar();
        scrollBar.setValue(scrollBar.getMaximum());
    }
    public String mani(String[] input){
        //Command interpretation function
    }
//(Omitted)
}

• For the sake of clarity, it is written as declared in the constructor, but in reality, various variables are declared as instance fields.

For the time being, actionPerformed is called when the enter key is pressed in the input text field (reference link) ). Then, immediately after pressing the enter key, the command is interpreted and added to the drawing area. After that, it was scrolled by the height of the scroll bar, and the bottom of the JTextArea was displayed ...

What went wrong

In conclusion, the process registered as an event handler is drawn after all the processes are completed. In other words, the size that can be obtained with `scrollBar.getMaximum ()` is the size before redrawing, so you can only go to the bottom of the JTextArea before redrawing. If you try inserting Thread.sleep (1000) in the last line of ʻactionPerformed`, you can see the behavior well. Well, if you think about it carefully, it's better to do the drawing process as few times as possible, and of course it's natural.

Solution

(If you search the net properly, you can get as much information as you want ...)

Think of a single event handler as allowing only state changes and a single repaint call. Even if you call repaint multiple times, the paintComponent method will only be called once. Source: https://teratail.com/questions/75183

Upon further investigation, this article is ... Some people were stuck with the same thing ... I put the code in this article into ʻaction Performed` and it worked fine.

Swing also seemed to be a single thread design.

The invokeLater () method ... fires after all pending AWT events have been processed

Source: http://wisdom.sakura.ne.jp/system/java/swing/swing4.html

Summary

1. Don't try to do anything with an event handler
2. In the event handler, the drawing process is basically performed only once at the end.
3. Use invokeLater () if you want to do some drawing processing from other than the event handler thread in Swing.

Digression

As an aside, I also tried using paintImmediately. Apparently, only the drawing process is used, and other numerical values are updated after the event handler ends.

MyShell.java

actionPerformed(ActionEvent e){
    System.out.println(scrollbar.getValue()+"#"+scrollbar.getMaximum()+"#"+area.getHeight());
    p.paintImmediately(0,0,5000,5000);
    System.out.println(scrollbar.getValue()+"#"+scrollbar.getMaximum()+"#"+area.getHeight());
}