Bug 118429 [DataBinding] Validation should use IStatus, not just Strings
In order to use data binding in Workbench or IDE dialogs and preference pages, we need validation in terms of IStatus objects. Using strings is too simplistic. For example, look at the wizard for creating a new class. You will get a warning if the class name does not start with an uppercase letter, or if you don't provide a package name, but you will still be able to click "Finish" to proceed anyway. (Note that IStatus is currently not on the classpath, we need to re-introduce the dependency on org.eclipse.equinox.common, which contains IStatus etc.)
bundles/org.eclipse.core.databinding/src/org/eclipse/core/databinding/validation/IValidator.java
/**
	 * Determines if the given value is partially valid. This method is used to
	 * determine, for example, if keystrokes can still be applied to the value.
	 * 
	 * @param value
	 *            the value to validate
	 * @return the ValidationError, or </code>null</code> if the value is
	 *         partially valid.
	 */
	public ValidationError isPartiallyValid(Object value);
	/**
	 * Determines if the given value is valid, that is if it can successfully
	 * be converted to the target data type.
	 * 
	 * @param value
	 *            the value to validate
	 * @return the ValidationError, or </code>null</code> if the value is valid.
	 */
	public ValidationError isValid(Object value);
}