add getMinZoom and getMaxZoom in Layer

Author: fuzhenn

src/layer/Layer.js

@@ -160,6 +160,26 @@ class Layer extends JSONAble(Eventable(Renderable(Class))) {
         return this._zIndex || 0;
     }
 
+    /**
+     * Get Layer's minZoom to display
+     * @return {Number}
+     */
+    getMinZoom() {
+        const map = this.getMap();
+        const minZoom = this.options['minZoom'];
+        return map ? Math.max(map.getMinZoom(), minZoom || 0) : minZoom;
+    }
+
+    /**
+     * Get Layer's maxZoom to display
+     * @return {Number}
+     */
+    getMaxZoom() {
+        const map = this.getMap();
+        const maxZoom = this.options['maxZoom'];
+        return map ? Math.min(map.getMaxZoom(), isNil(maxZoom) ? Infinity : maxZoom) : maxZoom;
+    }
+
     /**
      * Get layer's opacity
      * @returns {Number}

test/layer/LayerSpec.js

@@ -84,6 +84,16 @@ describe('Layer.Spec', function () {
         });
     });
 
+    it('#getMinZoom and #getMaxZoom', function () {
+        var layer = new maptalks.VectorLayer('1', {
+            minZoom : 10
+        });
+
+        map.addLayer(layer);
+        expect(layer.getMinZoom()).to.be.eql(10);
+        expect(layer.getMaxZoom()).to.be.eql(map.getMaxZoom());
+    });
+
     describe('zindex of layers', function () {
         it('default zindex is 0', function () {
             var layer1 = new maptalks.TileLayer('1', { renderer:'canvas' });