Upgrade notes and example update for shortest arc rotation animation

changelog/upgrade-notes.md

@@ -2,6 +2,26 @@
 
 ### Next release
 
+#### `ol.animate` now takes the shortest arc for rotation animation
+
+Usually rotation animations should animate along the shortest arc. There are rare occasions where a spinning animation effect is desired. So if you previously had something like
+```js
+map.getView().animate({
+  rotation: 2 * Math.PI,
+  duration: 2000
+});
+```
+we recommend to split the animation into two parts and use different easing functions. The code below results in the same effect as the snippet above did with previous versions:
+```js
+map.getView().animate({
+  rotation: Math.PI,
+  easing: ol.easing.easeIn
+}, {
+  rotation: 2 * Math.PI,
+  easing: ol.easing.easeOut
+});
+```
+
 ### v4.2.0
 
 #### Return values of two `ol.style.RegularShape` getters have changed

examples/animation.js

@@ -1,5 +1,6 @@
 goog.require('ol.Map');
 goog.require('ol.View');
+goog.require('ol.easing');
 goog.require('ol.layer.Tile');
 goog.require('ol.proj');
 goog.require('ol.source.OSM');
@@ -73,9 +74,16 @@ onClick('rotate-right', function() {
 });
 
 onClick('rotate-around-rome', function() {
+  // Rotation animation takes the shortest arc, so animate in two parts
+  var rotation = view.getRotation();
   view.animate({
-    rotation: view.getRotation() + 2 * Math.PI,
+    rotation: rotation + Math.PI,
-    anchor: rome
+    anchor: rome,
+    easing: ol.easing.easeIn
+  }, {
+    rotation: rotation + 2 * Math.PI,
+    anchor: rome,
+    easing: ol.easing.easeOut
   });
 });
 
@@ -103,10 +111,19 @@ onClick('bounce-to-istanbul', function() {
 });
 
 onClick('spin-to-rome', function() {
+  // Rotation animation takes the shortest arc, so animate in two parts
+  var center = view.getCenter();
   view.animate({
+    center: [
+      center[0] + (rome[0] - center[0]) / 2,
+      center[1] + (rome[1] - center[1]) / 2
+    ],
+    rotation: Math.PI,
+    easing: ol.easing.easeIn
+  }, {
     center: rome,
     rotation: 2 * Math.PI,
-    duration: 2000
+    easing: ol.easing.easeOut
   });
 });