A layer event emitted by a MapViewport

class LayerEvent {
 /// layer has been added
 static const ADDED = 0;
 /// layer has been removed
 static const REMOVED = 1;
 /// layer has been moved
 static const MOVED = 2;

 /// the map viewport emitting the event
 final MapViewport map;
 /// the layer
 final Layer layer;
 /// the type of event (either [ADDED], [REMOVED], or [MOVED])
 final int type;

 /// Creates an event of type [type] emitted by [map] for [layer]
 LayerEvent(this.map, this.layer, this.type);
}