NullableCheckBox.java

package com.eas.widgets.boxes;

import com.google.gwt.dom.client.Document;
import com.google.gwt.dom.client.Element;
import com.google.gwt.dom.client.InputElement;
import com.google.gwt.dom.client.Style;
import com.google.gwt.dom.client.Style.VerticalAlign;
import com.google.gwt.event.dom.client.ChangeEvent;
import com.google.gwt.event.dom.client.ChangeHandler;
import com.google.gwt.event.logical.shared.ValueChangeEvent;
import com.google.gwt.user.client.DOM;
import com.google.gwt.user.client.ui.CheckBox;

public class NullableCheckBox extends CheckBox {

	protected Element anchor = Document.get().createDivElement();
	
	protected InputElement inputElem;
	protected Boolean value;

	public NullableCheckBox() {
	    this(DOM.createInputCheck());
		getElement().removeClassName("gwt-CheckBox");
		Element child = getElement().getFirstChildElement();
		while(child != null){
			if("input".equalsIgnoreCase(child.getTagName())){
				child.addClassName("check-box");
			}else{
				child.addClassName("check-label");
			}
			child = child.getNextSiblingElement();
		}
	}

	public NullableCheckBox(Element elem) {
		super(elem);
		inputElem = InputElement.as(elem);
		inputElem.setPropertyBoolean("indeterminate", true);		
		inputElem.getStyle().setVerticalAlign(VerticalAlign.MIDDLE);
		anchor.getStyle().setDisplay(Style.Display.INLINE_BLOCK);
		anchor.getStyle().setPosition(Style.Position.RELATIVE);
		anchor.getStyle().setHeight(100, Style.Unit.PCT);
		anchor.getStyle().setVerticalAlign(Style.VerticalAlign.MIDDLE);
		getElement().appendChild(anchor);
	}

	@Override
	public void setValue(Boolean aValue, boolean fireEvents) {
		Boolean oldValue = getValue();
		value = aValue;
		inputElem.setPropertyBoolean("indeterminate", value == null);
		if (value == null ? oldValue != null : !value.equals(oldValue)) {
			inputElem.setChecked(aValue != null ? aValue : false);
			inputElem.setDefaultChecked(aValue != null ? aValue : false);
			if (fireEvents) {
				ValueChangeEvent.fire(this, aValue);
			}
		}
	}

	@Override
	public Boolean getValue() {
		return value;
	}

	protected void ensureDomEventHandlers() {
		addDomHandler(new ChangeHandler() {
			@Override
			public void onChange(ChangeEvent event) {
				setValue(inputElem.isChecked(), true);
			}
		}, ChangeEvent.getType());
	}
}