Merge pull request #1662 from tschaub/drawing-rings

Add `minPointsPerRing` option to the draw interaction.  By default, polygon drawing cannot be finished until three points have been added.  To allow polygon drawing to finish after only two points have been added, set `minPointsPerRing: 2`.

src/objectliterals.jsdoc

@@ -415,6 +415,8 @@
  *     drawing finish (default is 12).
  * @property {ol.geom.GeometryType} type Drawing type ('Point', 'LineString',
  *     'Polygon', 'MultiPoint', 'MultiLineString', or 'MultiPolygon').
+ * @property {number|undefined} minPointsPerRing The number of points that must
+ *     be drawn before a polygon ring can be finished (default is 3).
  * @property {ol.style.Style|Array.<ol.style.Style>|ol.feature.StyleFunction|undefined} style
  *     Style for sketch features.
  * @todo stability experimental

src/ol/interaction/drawinteraction.js

@@ -89,6 +89,15 @@ ol.interaction.Draw = function(options) {
   this.snapTolerance_ = goog.isDef(options.snapTolerance) ?
       options.snapTolerance : 12;
 
+  /**
+   * The number of points that must be drawn before a polygon ring can be
+   * finished.  The default is 3.
+   * @type {number}
+   * @private
+   */
+  this.minPointsPerRing_ = goog.isDef(options.minPointsPerRing) ?
+      options.minPointsPerRing : 3;
+
   /**
    * Geometry type.
    * @type {ol.geom.GeometryType}
@@ -314,7 +323,8 @@ ol.interaction.Draw.prototype.atFinish_ = function(event) {
       potentiallyDone = geometry.getCoordinates().length > 2;
     } else if (this.mode_ === ol.interaction.DrawMode.POLYGON) {
       goog.asserts.assertInstanceof(geometry, ol.geom.Polygon);
-      potentiallyDone = geometry.getCoordinates()[0].length > 2;
+      potentiallyDone = geometry.getCoordinates()[0].length >
+          this.minPointsPerRing_;
       potentiallyFinishCoordinates = [this.sketchRawPolygon_[0][0],
         this.sketchRawPolygon_[0][this.sketchRawPolygon_[0].length - 2]];
     }