cesium entity render
cesium 对 entity 的渲染过程大致分为两个分过程：
一、初始化
二、渲染
下面分此两个过程阐述：
一、初始化
1、在Viewer构造函数中初始化_dataSourceDisplay，
new Cesium.Viewer(container, options){
...
dataSourceCollection = new DataSourceCollection();
dataSourceDisplay = new DataSourceDisplay({
scene : scene,
dataSourceCollection : dataSourceCollection
});
this._dataSourceDisplay=dataSourceDisplay ；
}
并定义Entities属性
entities : {
get : function() {
return this._dataSourceDisplay.defaultDataSource.entities;
}
},
2、在DataSourceDisplay构造函数中初始化一个CustomDataSource作为defaultDataSource
new Cesium.DataSourceDisplay(){
var defaultDataSource = new CustomDataSource();
this._onDataSourceAdded(undefined, defaultDataSource);
}
并在其中初始化_entityCollection
new Cesium.CustomDataSource(){
this._entityCollection = new EntityCollection(this);
}
在_onDataSourceAdded中初始化_visualizers
DataSourceDisplay.prototype._onDataSourceAdded = function(dataSourceCollection, dataSource){
dataSource._visualizers = this._visualizersCallback(scene, entityCluster, dataSource);
}
并初始化GeometryVisualizer
DataSourceDisplay.defaultVisualizersCallback = function(scene, entityCluster, dataSource) {
var entities = dataSource.entities;
return [new BillboardVisualizer(entityCluster, entities),
new GeometryVisualizer(scene, entities, dataSource._primitives, dataSource._groundPrimitives),
...
];
}
3、在GeometryVisualizer中对entities进行分组可视化
function GeometryVisualizer(scene, entityCollection, primitives, groundPrimitives) {
this._outlineBatches = new Array(numberOfShadowModes*2);
this._closedColorBatches = new Array(numberOfShadowModes*2);
this._closedMaterialBatches = new Array(numberOfShadowModes*2);
this._openColorBatches = new Array(numberOfShadowModes*2);
this._openMaterialBatches = new Array(numberOfShadowModes*2);
...
for (i = 0; i < numberOfShadowModes; ++i) {
this._closedColorBatches[i] = new StaticGeometryColorBatch(primitives, PerInstanceColorAppearance, undefined, true, i, true);
....
}
...
//是否贴地
//事件绑定
} if (supportsMaterialsforEntitiesOnTerrain) {
for (i = 0; i < numberOfClassificationTypes; ++i) {
groundMaterialBatches.push(new StaticGroundGeometryPerMaterialBatch(groundPrimitives, i, MaterialAppearance));
groundColorBatches[i] = new StaticGroundGeometryPerMaterialBatch(groundPrimitives, i, PerInstanceColorAppearance);
}
} else {
for (i = 0; i < numberOfClassificationTypes; ++i) {
groundColorBatches[i] = new StaticGroundGeometryColorBatch(groundPrimitives, i);
}
}
entityCollection.collectionChanged.addEventListener(GeometryVisualizer.prototype._onCollectionChanged, this);
this._onCollectionChanged(entityCollection, entityCollection.values, emptyArray);
事件处理程序
GeometryVisualizer.prototype._onCollectionChanged = function(entityCollection, added, removed) {
var addedObjects = this._addedObjects;
var removedObjects = this._removedObjects;
var changedObjects = this._changedObjects;
for (i = added.length - 1; i > -1; i--) {
entity = added[i];
id = entity.id;
if (removedObjects.remove(id)) {
changedObjects.set(id, entity);
} else {
addedObjects.set(id, entity);
}
}
}
4、关于事件绑定
在以上三步的默认构造函数中，通过在viewer的dataSourceDisplay的defaultDataSource（customDataSource）中初始化一个entityCollecton，并在viewer中定义该属性，构造一个空的entityCollection。
在EntityCollection里定义一个collectionChanged事件，
function EntityCollection(owner) {
this._collectionChanged = new Event();
}
并在add函数里触发该事件
EntityCollection.prototype.add = function(entity) {
fireChangedEvent(this);}
function fireChangedEvent(collection) {
collection._collectionChanged.raiseEvent(collection, addedArray, removedArray, changedArray);
}
而在第三步的geometryVisualizer里挂接该事件的处理程序，将添加的entity传递到geometryVisualizer的addedObjects数组。这样，无论在何时何地调用viewer.entities.add(entity),都会通过该事件绑定传入geometryVisualizer，从而为entity的渲染准备好数据。
