SWT user control design decision

Hypothetical question:

What about this SWT design decision that to make a custom compound widget I have to subclass Composite? Is that really wise?

Wouldn't it be better if SWT had a UserControl class like in Win Forms or something?

When I subclass Composite my custom widget get the Composite interface, even if it is not intended to be used as a Composite by clients开发者_开发知识库. That's kind of bad. The same is true for some SWT widgets, like Spinner.

Is there a good way around this?

And, most interesting: Do anyone know the motivation for this design decision?

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You could look at the source code for org.eclipse.swt.custom.CCombo to see how SWT deals with this problem internally, as CCombo extends from Composite. Here's how they handle setLayout(), for example:

/**
 * Sets the layout which is associated with the receiver to be
 * the argument which may be null.
 * <p>
 * Note: No Layout can be set on this Control because it already
 * manages the size and position of its children.
 * </p>
 *
 * @param layout the receiver's new layout or null
 *
 * @exception SWTException <ul>
 *    <li>ERROR_WIDGET_DISPOSED - if the receiver has been disposed</li>
 *    <li>ERROR_THREAD_INVALID_ACCESS - if not called from the thread that created the receiver</li>
 * </ul>
 */
public void setLayout (Layout layout) {
    checkWidget ();
    return;
}

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You could create your own NonCompositeUserControl class:

public class NonCompositeUserControl extends Composite {
    @Override
    public void setLayout(Layout layout) {
        throw new UnsupportedOperationException("This control is not really a composite")
    }
    // similarly for other methods
}