<vlayout width="100%" height="100%"
    apply="org.zkoss.bind.BindComposer"
    viewModel="@id('vm') @init('bugs.TooManyMarkersViewModel')">
    <hlayout>
        Number of markers:
        <intbox value="@bind(vm.numMarkers)" constraint="no empty, no negative, no zero"/>
        <button label="Generate" onClick="@command('generate')"/>
    </hlayout>
    <gmaps id="map" children="@load(vm.model)" lat="25.0661365" lng="121.48981605" zoom="15" hflex="1" vflex="1">
        <template name="children" var="location">
            <gmarker lat="@load(location.latitude)" lng="@load(location.longitude)"/>       
        </template>
    </gmaps>
</vlayout>

public class TooManyMarkersViewModel {
    
    @Wire
    private Gmaps map;

    public static class Location {
        private double latitude;
        private double longitude;
        
        public Location(double latitude, double longitude) {
            this.latitude = latitude;
            this.longitude = longitude;
        }
        
        public double getLatitude() {
            return this.latitude;
        }
        
        public double getLongitude() {
            return this.longitude;
        }
    }
    
    public static final Random RANDOM = new Random();
    
    private int numMarkers = 1000;
    private ListModelList<Location> model = new ListModelList<Location>();
    
    @AfterCompose
    public void doAfterCompose(@ContextParam(ContextType.VIEW) Component comp) {
        Selectors.wireComponents(comp, this, true);
        
        generateMarkers();
    }
    
    @Command("generate")
    @NotifyChange("model")
    public void generateMarkers() {
        model.clear();
        for (int i = 0; i < numMarkers; i++) {
            double latitude = (RANDOM.nextDouble() * 0.0269384) + 25.0526673;
            double longitude = (RANDOM.nextDouble() * 0.0595021) + 121.460065;
            
            model.add(new Location(latitude, longitude));
        }
        map.invalidate();
    }

    public int getNumMarkers() {
        return numMarkers;
    }
    
    public void setNumMarkers(int numMarkers) {
        this.numMarkers = numMarkers;
    }

    public ListModelList<Location> getModel() {
        return model;
    }
    
}