RenderLayer

RenderLayer是ShipLayer、TrackLayer、IconfontLayer、TyphoonLayer的父类

RenderLayer又继承layer/Vector (opens new window)

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RenderLayer继承属性：

lastExtent:当前视野的范围

currentZoom:当前视野的层级

RenderLayer继承方法：

findById(id){}:根据ID查找要素

clearSelectFeature(){}:清除选中要素