CODEWITHSUNDEEP
javaswingcardlayout
Haeri
Haeri

Reputation: 701

Java CardLayout .show() not showing

I use .show() before a "blocking" code like a while loop. But even though the .show gets called, the UI doesn't actually show the called panel.

Here is the code that shows the issue: (WARNING: This code contains a while true loop.)

import javax.swing.JFrame;
import java.awt.CardLayout;
import java.awt.event.ActionEvent;

import javax.swing.JPanel;
import javax.swing.JLabel;
import javax.swing.JButton;

public class CardTest extends JFrame{
public CardTest() {
    CardLayout cl = new CardLayout(0,0);
    getContentPane().setLayout(cl);

    JPanel panelA = new JPanel();
    getContentPane().add(panelA, "PanelA");

    JLabel lblPanelA = new JLabel("Panel A");
    panelA.add(lblPanelA);

    JButton btnSwitchToPanel = new JButton("Switch to Panel B");
    panelA.add(btnSwitchToPanel);

    JPanel panelB = new JPanel();
    getContentPane().add(panelB, "PanelB");

    JLabel lblPanelB = new JLabel("Panel B");
    panelB.add(lblPanelB);

    btnSwitchToPanel.addActionListener(new java.awt.event.ActionListener() {
        public void actionPerformed(ActionEvent event) {
            cl.show(getContentPane(), "PanelB");
            getContentPane().revalidate();

            // Here is the problem. Even though cl.show is called first, 
            // it still doesn't show, before the while loop has terminated.
            int i = 0;
            while(i < 1000000){
                i++;
                System.out.println(i);
            }
        }
    });

    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    pack();
    setVisible(true);
}

public static void main(String[] args){
    new CardTest();
}
}

If you are wondering, I need this for a downloader, where the while true loop (to download a file) is called after pressing a button in the first panel. The second panel contains the progress bar. But the progress panel never gets displayed even though the .show function is called before the download code.

UPDATE

I do know that putting the loop into a new thread, solves the draw problem, but it also introduces other problems, because I rely on sequential execution of functions after the loop(download file(loop), Unzipp file, move those files...).

The best solution would be to find a way to allow the .show() call to actually take the time to switch panes before continuing with the loop.

Upvotes: 1

Views: 870

Answers (3)

camickr
camickr

Reputation: 324197

I use .show() before a "blocking" code like a while loop. But even though the .show gets called, the UI doesn't actually show the called panel.

Yes, because you are "blocking" the Event Dispatch Thread (EDT) which is responsible for repainting the GUI. So the GUI can't repaint itself until the code finishes executing.

You need to create a separate Thread to executing the long running task so you don't block the EDT. One way to do this is to use a SwingWorker. The SwingWorker will create the Thread for you and will notify you when the task is complete so you can update the GUI.

Read the section from the Swing tutorial on Concurrency for more information and a working example.

Upvotes: 3

daiscog
daiscog

Reputation: 12077

This is because redrawing the UI is done in the same thread as event processing and doesn't happen until after the event processing has completed (i.e., all event handling methods have returned).

Best thing to do is move that "blocking" code into a runnable and execute it in a worker thread.

Upvotes: 2

Denis Lukenich
Denis Lukenich

Reputation: 3174

This happens because you are doing work on the EventDispatchingThread. This Thread is also responsible for actually drawing the GUI.

You have no other choice than doing your work in another thread.

E.g.: (Quick + Dirty)

new Thread(){
    @Override
    public void run() {
        while (...) {...}
    }
}.start();

Upvotes: 2

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