Add constrainRotation option to ol.View2D

src/objectliterals.jsdoc

@@ -91,6 +91,11 @@
  *     The coordinate system for the center is specified with the `projection`
  *     option. Default is `undefined`, and layer sources will not be fetched if
  *     this is not set.
+ * @property {boolean|number|undefined} constrainRotation Rotation constraint.
+ *     `false` means no constraint. `true` means no constraint, but snap to
+ *     zero near zero. A number constraints the rotation to that number of
+ *     values. For example, `4` will constrain the rotation to 0, 90, 180, and
+ *     270 degrees.  The default is `true`.
  * @property {boolean|undefined} enableRotation Enable rotation. Default is
  *     `true`.
  * @property {ol.Extent|undefined} extent The extent that constrains the center,

src/ol/view2d.js

@@ -572,7 +572,17 @@ ol.View2D.createRotationConstraint_ = function(options) {
   var enableRotation = goog.isDef(options.enableRotation) ?
       options.enableRotation : true;
   if (enableRotation) {
-    return ol.RotationConstraint.createSnapToZero();
+    var constrainRotation = options.constrainRotation;
+    if (!goog.isDef(constrainRotation) || constrainRotation === true) {
+      return ol.RotationConstraint.createSnapToZero();
+    } else if (constrainRotation === false) {
+      return ol.RotationConstraint.none;
+    } else if (goog.isNumber(constrainRotation)) {
+      return ol.RotationConstraint.createSnapToN(constrainRotation);
+    } else {
+      goog.asserts.fail();
+      return ol.RotationConstraint.none;
+    }
   } else {
     return ol.RotationConstraint.disable;
   }