This example demonstrates how data is not loaded until the requested data is needed in view, when scrolling down.

<div apply="org.zkoss.bind.BindComposer"
	viewModel="@id('vm') @init('demo.listbox.load_on_demand.LoadOnDemandViewModel')">

	<listbox id="list" model="@bind(vm.hugeList)" width="200px" height="540px">
		<listhead>
			<listheader label="Load On Demand" sort="auto" />
		</listhead>
	</listbox>

	<style>
		div.z-listbox-body .z-listcell {
			padding: 2px 5px;
		}
	</style>
</div>	

package demo.listbox.load_on_demand;

import org.zkoss.zul.ListModel;

public class LoadOnDemandViewModel {

	private ListModel<String> hugeList = new demo.listbox.load_on_demand.FakeListModel(250000);

	public ListModel<String> getHugeList() {
		return hugeList;
	}
}

package demo.listbox.load_on_demand;

import java.util.Comparator;

import org.zkoss.zul.AbstractListModel;
import org.zkoss.zul.event.ListDataEvent;
import org.zkoss.zul.ext.Sortable;

public class FakeListModel extends AbstractListModel<String> implements Sortable<String> {
	
	private static final long serialVersionUID = -3086046175152725037L;
	
	private int _size;
	private boolean _asc = true;

	public FakeListModel() {
		this(10000);
	}
	public FakeListModel(int size) {
		_size = size;
	}

	// ListModelExt //
	@Override
	public void sort(Comparator<String> cmpr, boolean asc) {
		_asc = asc;
		invalidate();
	}
	
	public void invalidate() {
		fireEvent(ListDataEvent.CONTENTS_CHANGED, -1, -1);
	}

	// AbstractListModel //
	public String getElementAt(int v) {
		String value = "item"+(_asc ? v: _size - v);
		return value;
	}
	
	public int getSize() {
		return _size;
	}
	
	public void setSize(int size){
		_size = size;
	}
	@Override
	public String getSortDirection(Comparator<String> arg0) {
		return null;
	}

}