How to make a button to do something in Blackberry - blackberry

Question

I am trying to make a button that shows a message when it is pressed, but cannot get it to work.
Here is my button:
HorizontalFieldManager buttonManager =
new HorizontalFieldManager(ButtonField.FIELD_HCENTER);
messageButton = new ButtonField("Press me", ButtonField.CONSUME_CLICK);
messageButton.setChangeListener(this);
buttonManager.add(messageButton);
add(buttonManager);
And here is the method that prints the message:
public void fieldChanged(Field field, int context) {
if (field == messageButton) {
showMessage();
}
}
private void showMessage() {
Dialog.inform("The button was pressed");
}
Am I doing something wrong in the showMessage() method, or id the error elsewhere?

Answer 1

// just paste this class and run
package mypackage;
import java.io.IOException;
import javax.microedition.media.MediaException;
import javax.microedition.media.Player;
import net.rim.device.api.ui.Field;
import net.rim.device.api.ui.FieldChangeListener;
import net.rim.device.api.ui.UiApplication;
import net.rim.device.api.ui.component.ButtonField;
import net.rim.device.api.ui.component.Dialog;
import net.rim.device.api.ui.container.HorizontalFieldManager;
import net.rim.device.api.ui.container.MainScreen;
public final class MyScreen extends MainScreen
{
public MyScreen()
{
// Set the displayed title of the screen
HorizontalFieldManager horizontalFieldManager= new HorizontalFieldManager();
ButtonField buttonField= new ButtonField("click to show dialog",Field.FIELD_VCENTER);
horizontalFieldManager.add(buttonField);
buttonField.setChangeListener(buttonchangelisners);
add(horizontalFieldManager);
}
private FieldChangeListener buttonchangelisners= new FieldChangeListener() {
public void fieldChanged(Field field, int context) {
// TODO Auto-generated method stub
showDialog("show your message");
}
};
public void showDialog(final String message) {
UiApplication.getUiApplication().invokeLater(new Runnable() {
public void run() {
Dialog.alert(message);
}
});
}
}

Answer 2

Check the event log (Alt + LGLG), I suspect you'll see an error about pushing a modal screen on a non-event thread. If that is the case just change showMessage to
private void showMessage()
{
UiApplication.getUiApplication.invokeLater(new Runnable()
{
public void run()
{
Dialog.inform("The button was pressed");
}
});
}

Answer 3

I think the problem might be the flag CONSUME_CLICK you are passing to the constructor. If you want to do something with the button this is certainly not a flag you'd want to use.
Other than this, I'd advise against using FieldChangeListener for fields. Maybe for buttons is ok, but for other components the fieldChanged method might be called without user interaction during layout. This is why you almost always want to filter out those calls whose second parameter (context in your code) is equals to FieldChangeListener.PROGRAMMATIC.
A better alternative is to override navigationClick, which also works both for tactile and regular screens.

Answer 4

invokeLater is nevertheless useful, because fieldChanged is invoked at some point within a chain of UI Engine actions, and you'd better let it finish the job before displaying the Dialog. I suspect not using invokeLater is the main reason for the problem.
CONSUME_CLICK is in fact necessary when using fieldChangeListener, otherwise context menu will be invoked. If you switch to using navigationClick within your button field, you can return true to achieve the same behavior (though I prefer using navigationUnclick).