Fixes #3065: Add dom-repeat.renderedItemCount property

src/lib/template/dom-repeat.html

@@ -191,6 +191,17 @@ Then the `observe` property should be configured as follows:
        */
       delay: Number,
 
+      /**
+       * Count of currently rendered items after `filter` (if any) has been applied.
+       * If "chunking mode" is enabled, `renderedItemCount` is updated each time a 
+       * set of template instances is rendered.
+       * 
+       */
+      renderedItemCount: {
+        type: Number,
+        notify: true
+      },
+
       /**
        * Defines an initial count of template instances to render after setting
        * the `items` array, before the next paint, and puts the `dom-repeat`
@@ -438,6 +449,8 @@ Then the `observe` property should be configured as follows:
       // `item` may not be sufficient if the pooled instance happens to be
       // the same item.
       this._pool.length = 0;
+      // Set rendered item count 
+      this.renderedItemCount = this._instances.length;
       // Notify users
       this.fire('dom-change');
       // Check to see if we need to render more items

test/unit/dom-repeat.html

@@ -115,6 +115,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       test('basic rendering, downward item binding', function() {
         var stamped = Polymer.dom(configured.root).querySelectorAll('*:not(template)');
         assert.equal(stamped.length, 3 + 3*3 + 3*3*3, 'total stamped count incorrect');
+        assert.equal(configured.$.repeater.renderedItemCount, 3, 'rendered item count incorrect');
         assert.equal(stamped[0].itemaProp, 'prop-1');
         assert.equal(stamped[0].computeda, 'prop-1+itemForComputedA');
         assert.equal(stamped[0].indexa, 0);
@@ -282,6 +283,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
         CustomElements.takeRecords();
         var stamped = Polymer.dom(configured.root).querySelectorAll('*:not(template)');
         assert.equal(stamped.length, 2 + 2*3 + 2*3*3, 'total stamped count incorrect');
+        assert.equal(configured.$.repeater.renderedItemCount, 2, 'rendered item count incorrect');
         assert.equal(stamped[0].itemaProp, 'prop-1');
         assert.equal(stamped[0].indexa, 0);
         assert.equal(stamped[13].itemaProp, 'prop-3');