Merge pull request #3066 from Polymer/arthure-rendered-item-count

Fixes #3065: Add dom-repeat.renderedItemCount property

src/lib/template/dom-repeat.html

@@ -191,6 +191,18 @@ 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,
+        readOnly: true
+      },
+
       /**
        * Defines an initial count of template instances to render after setting
        * the `items` array, before the next paint, and puts the `dom-repeat`
@@ -439,6 +451,8 @@ 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._setRenderedItemCount(this._instances.length);
       // Notify users
       this.fire('dom-change');
       // Check to see if we need to render more items

test/unit/dom-repeat.html

@@ -119,6 +119,7 @@ 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.$.repeater.renderedItemCount, 3, 'rendered item count incorrect');
         assert.equal(stamped[0].itemaProp, 'prop-1');
         assert.equal(stamped[0].computeda, 'prop-1+itemForComputedA');
         assert.equal(stamped[0].indexa, 0);
@@ -286,6 +287,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
         CustomElements.takeRecords();
         var stamped = Polymer.dom(configured.root).querySelectorAll('*:not(template)');
         assert.equal(stamped.length, 2 + 2*3 + 2*3*3, 'total stamped count incorrect');
+        assert.equal(configured.$.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');
@@ -3313,6 +3315,23 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
         assert.equal(repeater3.keyForElement(stamped1[4]), coll3.getKey(items3[2]));
       });
 
+      test('renderedItemCount', function() {
+         var repeater1 = primitive.$.repeater1;
+         primitive.items = [ 'a', 'b', 'c', 'd', 'e' ];
+         repeater1.render();
+         assert.equal(repeater1.renderedItemCount, 5, 'renderedItemCount is incorrect');
+         repeater1.renderedItemCount = 0;
+         assert.equal(repeater1.renderedItemCount, 5, 'renderedItemCount is writable');
+         repeater1.filter = function(item) {
+           return (item != 'a' && item != 'e');
+         }
+         repeater1.render();
+         assert.equal(repeater1.renderedItemCount, 3, 'renderedItemCount incorrect after filter');
+         // reset repeater
+         repeater1.filter = undefined;
+         repeater1.render();
+      });
+
       test('__hideTemplateChildren__', function() {
         // Initially all showing
         var stamped1 = Polymer.dom(primitive.$.container1).querySelectorAll('*:not(template)');