How to listen for data change event in a readonly doublebox?

<?page title="onchange event demo" contentType="text/html;charset=UTF-8"?>
<window title="onchange event demo" border="normal">
	<grid width="100%" fixedLayout="true">
		<rows>
			<row>
				First doublebox
				<doublebox id="one"
					onChanging='two.text = event.value' />
			</row>
			<row>
				Second doublebox
				<doublebox id="two" readonly="true" onChanging='three.text = two.text'/>
			</row>
			<row>
				Third doublebox
				<doublebox id="three" readonly="true" />
			</row>
		</rows>
	</grid>
</window>