[ol.layer.TileLayer, ol.layer.XYZ] add a getMaxResolution func to TileLayer, and make ol.layer.XYZ be just a projection and a number of zoom levels

src/ol/layer/TileLayer.js

@@ -153,15 +153,44 @@ ol.layer.TileLayer.prototype.getTileOrigin = function() {
     return null;
 };
 
+/**
+ * Get max resolution. Return undefined if the layer has no maxResolution,
+ * and no extent from which maxResolution could be derived.
+ * @return {number|undefined}
+ */
+ol.layer.TileLayer.prototype.getMaxResolution = function() {
+    if (!goog.isDef(this.maxResolution_)) {
+        var extent = this.getExtent();
+        if (!goog.isNull(extent)) {
+            this.maxResolution_ = Math.max(
+                (extent.getMaxX() - extent.getMinX()) / this.tileWidth_,
+                (extent.getMaxY() - extent.getMaxX()) / this.tileHeight_);
+        }
+    }
+    return this.maxResolution_;
+};
+
+/**
+ * Get the number of the zoom levels.
+ * @return {number|undefined}
+ */
+ol.layer.TileLayer.prototype.getNumZoomLevels = function() {
+    return this.numZoomLevels_;
+};
+
 /**
  * Get layer resolutions. Return null if the layer has no resolutions.
  * @return {Array.<number>}
  */
 ol.layer.TileLayer.prototype.getResolutions = function() {
-    if (goog.isNull(this.resolutions_) && goog.isDef(this.maxResolution_)) {
-        this.resolutions_ = [];
-        for (var i = 0; i < this.numZoomLevels_; i++) {
-            this.resolutions_[i] = this.maxResolution_ / Math.pow(2, i);
+    if (goog.isNull(this.resolutions_)) {
+        var maxResolution = this.getMaxResolution(),
+            numZoomLevels = this.getNumZoomLevels();
+        if (goog.isDef(maxResolution) && goog.isDef(numZoomLevels)) {
+            this.resolutions_ = [];
+            for (var i = 0; i < numZoomLevels; i++) {
+                this.resolutions_[i] = maxResolution / Math.pow(2, i);
+            }
         }
     }
     return this.resolutions_;

src/ol/layer/XYZ.js

@@ -18,7 +18,8 @@ ol.layer.XYZ = function(url) {
     goog.base(this);
 
     this.setUrl(url);
-    this.setMaxResolution(156543.03390625);
+    this.setProjection(new ol.Projection("EPSG:3857"));
+    this.setNumZoomLevels(22);
 };
 
 goog.inherits(ol.layer.XYZ, ol.layer.TileLayer);

test/spec/ol/layer/TileLayer.test.js

@@ -134,6 +134,38 @@ describe('ol.layer.TileLayer', function() {
         });
     });
 
+    describe('get max resolution', function() {
+        var layer;
+
+        beforeEach(function() {
+            layer = new ol.layer.TileLayer();
+        });
+
+        describe('with max resolution', function() {
+
+            beforeEach(function() {
+                layer.setMaxResolution(156543.03390625);
+            });
+
+            it('returns the expected maxResolution', function() {
+                var maxResolution = layer.getMaxResolution();
+                expect(maxResolution).toEqual(156543.03390625);
+            });
+        });
+
+        describe('with projection', function() {
+
+            beforeEach(function() {
+                layer.setProjection(new ol.Projection("EPSG:3857"));
+            });
+
+            it('returns the expected maxResolution', function() {
+                var maxResolution = layer.getMaxResolution();
+                expect(maxResolution).toEqual(156543.03390625);
+            });
+        });
+    });
+
     describe('get data from the layer', function() {
         var layer;