Further work on Loz’ solution

src/ORM/DataList.php

@@ -33,7 +33,7 @@ use LogicException;
  *
  * Subclasses of DataList may add other methods that have the same effect.
  */
-class DataList extends ViewableData implements SS_List, Filterable, Sortable, Limitable, TemplateIterator
+class DataList extends ViewableData implements SS_List, Filterable, Sortable, Limitable
 {
 
     /**
@@ -883,30 +883,13 @@ class DataList extends ViewableData implements SS_List, Filterable, Sortable, Li
         }
     }
 
-    /**
-     * @return Generator|DataObject[]
-     */
-    public function getTemplateIterator()
-    {
-        foreach ($this->getFinalisedQuery() as $row) {
-            yield $this->createDataObject($row);
-        }
-    }
-
-    /**
-     * @return int
-     */
-    public function getTemplateIteratorCount()
-    {
-        return $this->getFinalisedQuery()->numRecords();
-    }
-
     /**
      * Returns the Query result for this DataList. Repeated calls will return
      * a cached result, unless the DataQuery underlying this list has been
      * modified
      *
      * @return SilverStripe\ORM\Connect\Query
+     * @internal This API may change in minor releases
      */
     protected function getFinalisedQuery()
     {

src/View/SSViewer_Scope.php

@@ -290,27 +290,28 @@ class SSViewer_Scope
         }
 
         if (!$this->itemIterator) {
-            // TemplateIterator provides methods for extracting the count and iterator directly
-            if ($this->item instanceof TemplateIterator) {
-                $this->itemIterator = $this->item->getTemplateIterator();
-                $this->itemIteratorTotal = $this->item->getTemplateIteratorCount();
-            } else {
+            // Note: it is important that getIterator() is called before count() as implemenations may rely on
+            // this to efficiency get both the number of records and an iterator (e.g. DataList does this)
+
             // Item may be an array or a regular IteratorAggregate
             if (is_array($this->item)) {
                 $this->itemIterator = new ArrayIterator($this->item);
             } else {
                 $this->itemIterator = $this->item->getIterator();
+
+                // This will execute code in a generator up to the first yield. For example, this ensures that
+                // DataList::getIterator() is called before Datalist::count()
+                $this->itemIterator->rewind();
             }
 
             // If the item implements Countable, use that to fetch the count, otherwise we have to inspect the
-                // iterator and then rewind it
+            // iterator and then rewind it.
             if ($this->item instanceof Countable) {
                 $this->itemIteratorTotal = count($this->item);
             } else {
                 $this->itemIteratorTotal = iterator_count($this->itemIterator);
                 $this->itemIterator->rewind();
             }
-            }
 
             $this->itemStack[$this->localIndex][SSViewer_Scope::ITEM_ITERATOR] = $this->itemIterator;
             $this->itemStack[$this->localIndex][SSViewer_Scope::ITEM_ITERATOR_TOTAL] = $this->itemIteratorTotal;

src/View/TemplateIterator.php

@@ -1,24 +0,0 @@
-<?php
-
-namespace SilverStripe\View;
-
-use IteratorAggregate;
-
-/**
- * A special iterator type used by the template engine. The template engine
- * needs to know the total number of items before iterating (to save rewinding
- * database queries), which is impossible with a natural Generator, so this
- * interface allows implementors to pre-compute the total number of items
- */
-interface TemplateIterator extends IteratorAggregate
-{
-    /**
-     * @return Iterator|Generator
-     */
-    public function getTemplateIterator();
-
-    /**
-     * @return int
-     */
-    public function getTemplateIteratorCount();
-}