为什么Java SWT Selection侦听器中新创建的元素不会显示/触发绘制事件？

Question

I've got a problem with SWT (standard widget toolkit) and redrawing. Basically I want something to happen when I click a button eg new elements to appear. For the sake of simplicity let it be text here.

However when I click the button nothing happens, no element appears, no PaintEvent is triggered. When I resize the window, suddenly all of the elements are there (because that triggers a paintEvent/redrawing).

The docs and blog posts tell me that a paint event is triggered whenever something needs to be redrawn. I thought that adding a new element to an application would fall into that category.

So the question is: Am I doing something wrong here? Am I supposed to call layout()/redraw() by hand to trigger the redraw? That seems tedious to me and to my understanding was that SWT handles that for me for the most cases.

Oh I'm running Linux x64 with SWT 4.2.2. But we have this on pretty much any platform (and also on the old SWT 3.7).

And here goes my little example code:

import org.eclipse.swt.*;
import org.eclipse.swt.widgets.*;
import org.eclipse.swt.events.*;
import org.eclipse.swt.layout.*;

public class MainExample {

    public static void createText(Shell shell) {
      Text text = new Text(shell, SWT.SINGLE);
      text.setText("here is some fancy text");
    }

    public static void main (String [] args) {
        Display display = new Display ();
        Shell shell = new Shell (display);
        Button button = new Button(shell, SWT.PUSH);
        button.setText("I am a button");

        button.addSelectionListener(new SelectionAdapter() {
            public void widgetSelected(SelectionEvent e) {
                System.out.println("Button Clicked");
                // Why do these texts not show up and not trigger a Paint Event?
                createText(e.display.getActiveShell());
            }
        });

        createText(shell);
        shell.setLayout (new RowLayout());
        shell.addPaintListener(new PaintListener() {
            public void paintControl(PaintEvent e) {
                // just triggered in the beginning and with resizes
                System.out.println("Hooray we got a Paint event");
            }
        });
        shell.open();

        while (!shell.isDisposed()) {
            if (!display.readAndDispatch()) display.sleep();
        }
        display.dispose();
    }

}

Please keep in mind, that this is not purely about making that example just work. This is a simple version of a problem for a much bigger project :-)

Any help is well appreciated!

Answer 1

If you using layout, you should call Composite.layout() (or layout(true)) after add a new child to composite. Layout do not know (so do not places widget) until you call layout().

By the way, if you resize the shell(composite), layout automatically invalidates its internal caches and checks all the children, places them. that's why newly created text appears after you resized the shell.