leaflet中的图层与esri中的图层不同，esri中的图层包含多个要素，而leaflet中的每个要素就是一个图层。（可能有误解，后期接触深了再确认是否存在误解）

Layer的基类，定义了方法，继承了来自L.Evented的所有方法、选项和事件

属性

• options:Object 扩展了L.Layer的类都会继承以下选项
• • pane:String，默认overlayPane
    默认情况下，图层将被添加到map的overlay pane，覆盖该选项会将图层添加到其它pane上
• • attribution:String =null
    在attributionControl中展示的字符串，描述了图层数据，通常表示版本等，例如 © OpenStreetMap contributors
• • bubblingMouseEvents:Boolean=true

方法

• addTo(map:Map|LayerGroup):this
  把图层添加到map上或layergroup中
• remove:this
  从map中删除该图层
• removeFrom(map:Map):this
  从给定的map中删除该图层
• getPane(name? : String): HTMLElement
  Returns the HTMLElement representing the named pane on the map ，If name is omitted, returns the pane for this layer.
• addInteractiveTarget(targetEl:HTMLElement):this

addInteractiveTarget:function(targetEl){
    this._map._targets[Util.stamp(targetEl)]=this;
    return this;
  }

添加可交互的目标对象

• removeInteractiveTarget(targetEl:HTMLElement):this
• getAttribution():String
  Used by the attribution control, returns the attribution option

必须要实现的方法

每个layer都应扩展L.Layer，并且重新实现以下方法

• onAdd(map:Map):this
  为图层创建DOM元素，把这些DOM元素添加到所属的map panes中，并且把监听函数关联到相关的地图事件上，Called on map.addLayer(layer)
• onRemove(map:Map):this
  从DOM中删除图层的dom元素，删除 onAdd中添加的监听，Called on map.removeLayer(layer).
• getEvents():Object
  返回一个对象，类似于{ viewreset: this._reset }for addEventListener，自动在map中添加或删除这个对象中的事件处理句柄
• getAttribution():String
  返回一个包含了HTML的字符串，展示在attributionControl中
• beforeAdd(map:Map):this
  可选方法,Called on map.addLayer(layer), before the layer is added to the map,before events are initialized, without waiting until the map is in a usable state. Use for early initialization only.

事件

• @event layeradd: LayerEvent
  Fired when a new layer is added to the map
• @event layerremove: LayerEvent
  Fired when some layer is removed from the map

用于处理图层和控件的方法

• addLayer(layer:Layer):this
• 其它

/**@method removeLayer(layer: Layer): this */
  removeLayer:function(layer){
    var id=Util.stamp(layer);

    //不存在，则返回
    if(!this._layers[id]){return this;}

    if(this._loaded){
      layer.onRemove(this);
    }

    if(layer.getAttribution&&this.attributionControl){
      this.attributionControl.removeAttribution(layer.getAttribution());
    }

    delete this._layers[id];

    if(this._loaded){
      this.fire('layerremove',{layer:layer});
      layer.fire('remove');
    }

    layer._map=layer._mapToAdd= null;

    return this;
  },

  /**@method hasLayer(layer:Layer):this
   * @description 若包含指定的图层，返回true
   */
  hasLayer:function(layer){
    //!!否定之否定表示肯定
    return !!layer&&(Util.stamp(layer) in this._layers);
  }

  /**method eachLayer(fn:Function, context?:Object):this
   * 对map中的每个图层迭代执行指定的函数，context表示函数的上下文
   * * ```
     * map.eachLayer(function(layer){
     *     layer.bindPopup('Hello');
     * });
     * ```
   */
  eachLayer:function(method,context){
    for(var i in this._layers){
      method.call(context,this._layers[i]);
    }
    return this;
  },

  /**@method _addLayers(layers:Array|Object)
   * 添加图层数组或组合图层
   */
  _addLayers:function(layers){
    // 若layers不为空，则判断是图层数组还是组合图层
    // 若为图层数组则返回layers
    // 若为组合图层，则返回只包含该组合图层的数组
    // 若layers为空，则返回一个空的数组
    layers=layers?(Util.isArray(layers)?layers:[layers]):[];
    for(var i=o,len=layers.length;i<len;i++){
      this.addLayer(layers[i]);
    }
  },

  /**@method _addZoomLimit(layer)
   * 添加对缩放的限制
   */
  _addZoomLimit:function(layer){
    // 判断图层的maxZoom和minZoom是否为数字
    // 若maxZoom是数字或minZoom不是数字
    if(isNaN(layer.options.maxZoom)||!isNaN(layer.options.minZoom)){
      this._zoomBoundLayers[Util.stamp(layer)]=layer;
      this._updateZoomLevels();
    }
  },

  /**@method _removeZoomLimit(layer) */
  _removeZoomLimit:function(layer){
    var id=Util.stamp(layer);
    if(this._zoomBoundLayers[id]){
      delete this._zoomBoundLayers[id];
      this._updateZoomLevels();
    }
  },

  /**@method _updateZoomLevels() */
  _updateZoomLevels:function(){
    var minZoom = Infinity,
    maxZoom=-Infinity,
    oldZoomSpan=this._getZoomSpan();

    // 比较获取minZoom和maxZoom
    for(var i in this._zoomBoundLayers){
      var options=this._zoomBoundLayers[i].options;
      minZoom = options.minZoom ===undefined?minZoom:Math.min(minZoom,options.minZoom);
      maxZoom = options.maxZoom === undefined?maxZoom:Math.max(maxZoom,options.maxZoom);
    }
    this._layersMaxZoom=maxZoom===-Infinity?undefined:maxZoom;
    this._layersMinZoom=minZoom===Infinity?undefined:minZoom;

    // @section Map状态变化事件
    //@event zoomlevelschange:Event
    // 当地图的缩放等级变化时触发
    if(oldZoomSpan!===this._getZoomSpan()){
      this.fire('zoomlevelschange');
    }

    if(this.options.maxZoom===undefined&&this._layersMaxZoom&&this.getZoom()>this._layersMaxZoom){
      this.setZoom(this._layersMaxZoom);
    }
    if(this.options.minZoom===undefined&&this._layersMinZoom&&this.getZoom()<this._layersMinZoom){
      this.setZoom(this._layersMinZoom);
    }
  }