Add method to install/uninstall the event listener

Author: Emmanuel Garcia

iron-scroll-target-behavior.html

@@ -73,12 +73,16 @@
       '_scrollTargetChanged(scrollTarget, isAttached)'
     ],
 
+    /**
+     * True if the event listener should be installed.
+     */
+    _shouldHaveListener: true,
+
     _scrollTargetChanged: function(scrollTarget, isAttached) {
-      var eventTarget;
+      var eventTarget, hasListener = this._shouldHaveListener;
 
       if (this._oldScrollTarget) {
-        eventTarget = this._oldScrollTarget === this._doc ? window : this._oldScrollTarget;
-        eventTarget.removeEventListener('scroll', this._boundScrollHandler);
+        this._enableScrollListener(false, this._oldScrollTarget);
         this._oldScrollTarget = null;
       }
 
@@ -97,11 +101,10 @@
 
       } else if (this._isValidScrollTarget()) {
 
-        eventTarget = scrollTarget === this._doc ? window : scrollTarget;
         this._boundScrollHandler = this._boundScrollHandler || this._scrollHandler.bind(this);
         this._oldScrollTarget = scrollTarget;
+        this._enableScrollListener(hasListener, scrollTarget);
 
-        eventTarget.addEventListener('scroll', this._boundScrollHandler);
       }
     },
 
@@ -228,7 +231,28 @@
      */
     _isValidScrollTarget: function() {
       return this.scrollTarget instanceof HTMLElement;
+    },
+
+    /**
+     * Enables or disables the scroll event listener
+     *
+     * @param {boolean} yes True to add the event, False to remove it
+     * @param {HTMLElement} scrollTarget The scroll target
+     */
+    _enableScrollListener: function(yes, scrollTarget) {
+      if (!this._boundScrollHandler) {
+        return;
+      }
+      var eventTarget = scrollTarget === this._doc ? window : scrollTarget;
+
+      if (yes) {
+        eventTarget.addEventListener('scroll', this._boundScrollHandler);
+      } else {
+        eventTarget.removeEventListener('scroll', this._boundScrollHandler);
+      }
+      this._shouldHaveListener = yes;
     }
+
   };
 
 </script>

test/basic.html

@@ -94,6 +94,25 @@
           done();
         });
       });
+
+      test('_enableScrollListener', function(done) {
+        flush(function() {
+          xScroll._scrollHandler = sinon.spy();
+          xScroll.scrollTarget = 'temporaryScrollingRegion';
+          xScroll.scroll(0, 100);
+          xScroll._enableScrollListener(true, xScroll.scrollTarget);
+          setTimeout(function() {
+            assert.isTrue(xScroll._scrollHandler.called, '_scrollHandler should be called');
+            xScroll._scrollHandler.reset();
+            xScroll._enableScrollListener(false, xScroll.scrollTarget);
+            xScroll.scroll(0, 200);
+            setTimeout(function() {
+              assert.isFalse(xScroll._scrollHandler.called, '_scrollHandler should not be called');
+              done();
+            }, 100);
+          }, 100);
+        });
+      });
     });
 
     suite('scrolling region', function() {