java - 在 GWT-Openlayers 中显示 WMS 图层

Question

我使用 GWT-Openlayers，我想向我的 map 添加一个 WMS 图层。这是代码:

    ...
    MapOptions defaultMapOptions = new MapOptions();
    defaultMapOptions.setNumZoomLevels(7);

    //Create a MapWidget
    MapWidget mapWidget = new MapWidget("700px", "700px", defaultMapOptions);

    GoogleV3Options gSatelliteOptions = new GoogleV3Options();
    gSatelliteOptions.setIsBaseLayer(true);
    gSatelliteOptions.setType(GoogleV3MapType.G_SATELLITE_MAP);
    GoogleV3 gSatellite = new GoogleV3("Google Satellite", gSatelliteOptions);

    mapWidget.getMap().addLayer(gSatellite);

    //Create a WMS layer as base layer
    WMSParams wmsParams = new WMSParams();
    wmsParams.setFormat("image/png");
    wmsParams.setLayers("topp:tasmania_state_boundaries");
    wmsParams.setStyles("");
    wmsParams.setParameter("transparent", "true");

    //create a WMS layer
    WMSOptions wmsLayerParams = new WMSOptions();
    wmsLayerParams.setUntiled();
    wmsLayerParams.setTransitionEffect(TransitionEffect.RESIZE);
    wmsLayerParams.setIsBaseLayer(false);

    String wmsUrl = "http://demo.opengeo.org/geoserver/wms";

    final WMS wmsLayer = new WMS("Basic WMS", wmsUrl, wmsParams,
                                 wmsLayerParams);

    //Add the WMS to the map
    Map map = mapWidget.getMap();
    map.addLayer(wmsLayer);

    //Lets add some default controls to the map
    map.addControl(new LayerSwitcher()); //+ sign in the upperright corner to display the layer switcher
    map.addControl(new OverviewMap()); //+ sign in the lowerright to display the overviewmap
    map.addControl(new ScaleLine()); //Display the scaleline
    map.addControl(new MousePosition()); 

    //Center and zoom to a location
    LonLat lonLat = new LonLat(146.7, -41.8);
    lonLat.transform(new Projection(ProjectionCode.LONGLAT.getEpsgCode()).getProjectionCode(),
            map.getProjection()); //transform lonlat to OSM coordinate system
    map.setCenter(lonLat, 6);
    ...

我使用了我在这个网站上找到的示例代码: http://demo.gwt-openlayers.org/gwt_ol_showcase/GwtOpenLayersShowcase.html

任何人都可以尝试，因为 wmsUrl 指向演示站点。

问题是 WMS 图层仅在我缩小到最外层缩放级别时出现。比起我尝试放大，它又消失了。有什么想法吗？

Answer 1

我找到了一个设置:

wmsLayerParams.setDisplayOutsideMaxExtent(true);

但我不明白为什么默认值不是真实值...