[LayerGroup] Use eachLayer for iterations over layers (#5809)

* [LayerGroup] use eachLayer method instead of for...in

* [LayerGroup] use eachLayer method in getLayers method

* [LayerGroup] use eachLayer method in onAdd & onRemove methods

* [LayerGroup] invoke method fix

- use `eachLayer` method instead of for...in
- add unit test for `invoke` method

* revert invoke method change

spec/suites/layer/LayerGroupSpec.js

@@ -92,4 +92,20 @@
 			L.geoJson(layerGroup.toGeoJSON());
 		});
 	});
+
+	describe("#invoke", function () {
+		it('should invoke `setOpacity` method on every layer', function () {
+			var layers = [
+				L.marker([0, 0]),
+				L.marker([1, 1])
+			];
+			var lg = L.layerGroup(layers);
+			var opacity = 0.5;
+
+			expect(layers[0].options.opacity).to.not.eql(opacity);
+			lg.invoke('setOpacity', opacity);
+			expect(layers[0].options.opacity).to.eql(opacity);
+			expect(layers[1].options.opacity).to.eql(opacity);
+		});
+	});
 });

src/layer/LayerGroup.js

@@ -77,10 +77,7 @@ export var LayerGroup = Layer.extend({
 	// @method clearLayers(): this
 	// Removes all the layers from the group.
 	clearLayers: function () {
-		for (var i in this._layers) {
+		return this.eachLayer(this.removeLayer, this);
-			this.removeLayer(this._layers[i]);
-		}
-		return this;
 	},
 
 	// @method invoke(methodName: String, …): this
@@ -103,15 +100,11 @@ export var LayerGroup = Layer.extend({
 	},
 
 	onAdd: function (map) {
-		for (var i in this._layers) {
+		this.eachLayer(map.addLayer, map);
-			map.addLayer(this._layers[i]);
-		}
 	},
 
 	onRemove: function (map) {
-		for (var i in this._layers) {
+		this.eachLayer(map.removeLayer, map);
-			map.removeLayer(this._layers[i]);
-		}
 	},
 
 	// @method eachLayer(fn: Function, context?: Object): this
@@ -138,10 +131,7 @@ export var LayerGroup = Layer.extend({
 	// Returns an array of all the layers added to the group.
 	getLayers: function () {
 		var layers = [];
-
+		this.eachLayer(layers.push, layers);
-		for (var i in this._layers) {
-			layers.push(this._layers[i]);
-		}
 		return layers;
 	},