Re: setVisible() problem after button click
samuel.cheadle@gmail.com schrieb:
Hello,
I am having a big problem with the setVisible() method. It seems not
to work when an event such as a button click is involved in calling
it.
///// Pseudocode /////
display_messeges() {
loop {
add components to JFrame....
setVisible(true);
Sleep for a few seconds
}
}
The confusing thing is this will work when called from main, but does
not update the screen when called from actionPerformed method.
Am I doing somthing stupid here?...... it is driving me crazy
Yes, you do :)
In Swing there's a thread, called Event Dispatch(ing) Thread, EDT for
short. Most work is done on this thread - painting, platform independend
event handling (notifying listeners) and so on.
Therefore, if a button notifies it's listeners, this code also runs on
the EDT.
Q: So, what is the consequence if your program waits for a
few seconds within a listener like ActionListener?
A: Right, it forces the current thread to wait for a few seconds.
Q: And what's the current thread?
A: Right, the EDT.
Q: And since the EDT is responsible for painting (in Swing),
what happens?
A: Right, nothing gets painted until the listener's method returns.
The GUI is "blocked".
Q: Hmm. But how can you wait for a few seconds if one clicks
on a button without blocking the GUI?
A: Move the waiting procedure into a new thread. But be careful:
Only one thread may manipulate the GUI: the EDT. This leads
us to the last question:
Q: How do I manipulate the GUI from a non-EDT thread?
A: Short: Never :)
Long: You have to ensure that the UI manipulating code runs
on the EDT. Either your code runs on the EDT already
(like the actionPerformed method) or you put your
code into the event dispatching queue which is processed
by the EDT. This can be done via
java.awt.EventQueue.invokeLater or
javax.swing.Utilities.invokeLater
(the latter just calls java.awt.EventQueue.invokeLater).
I've attached some (untested - even not compiled) code.
You may also want to use SwingWorker instead of the horrible
Runnable-cascades.
Bye
Michael
public class Test {
private JButton button;
private void initComponents() {
button = new JButton("Hide");
button.addActionListener( new ActionListener() {
public void actionPerformed( ActionEvent e ) {
// first: make the button invisible
button.setVisible(false);
// now we have to return quickly so we
// don't block the EDT.
// Therefore create another Thread.
Thread t = new Thread( new Runnable() {
public void run() {
try {
Thread.sleep(2000);
} catch ( InterruptedException ie ) {
}
// after the two seconds we want
// to show the button again.
// since we're not on the EDT,
// let's put the setVisible-code
// into the Event Dispatching Queue
SwingUtilities.invokeLater( new Runnable() {
public void run() {
button.setVisible(true);
}
});
}
});
// and start it:
t.setPriority( Thread.NORM_PRIORITY );
t.start();
}
});
}
public void createAndShowGUI() {
initComponents();
JFrame frame = new JFrame("Test");
frame.setDefaultCloseOperation( JFrame.DISPOSE_ON_CLOSE );
frame.add( button );
frame.pack();
frame.setVisible(true);
}
public static final void main( String args[] ) {
// we're on the main thread, so we ensure that
// GUI creation is done on the EDT
SwingUtilities.invokeLater( new Runnable() {
public void run() {
new Test().createAndShowGUI();
}
});
}
}