Use a generator for Cells::getAllCacheKeys

Using a generator reduces memory usage and improves performance
when loading large spreadsheets.

Closes #822

CHANGELOG.md

@@ -20,6 +20,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 - Fix VLOOKUP with exact matches - [#809](https://github.com/PHPOffice/PhpSpreadsheet/pull/809)
 - Support COUNTIFS multiple arguments - [#830](https://github.com/PHPOffice/PhpSpreadsheet/pull/830)
 - Change `libxml_disable_entity_loader()` as shortly as possible - [#819](https://github.com/PHPOffice/PhpSpreadsheet/pull/819)
+- Improved memory usage and performance when loading large spreadsheets - [#822](https://github.com/PHPOffice/PhpSpreadsheet/pull/822)
 
 ## [1.5.2] - 2018-11-25
 

src/PhpSpreadsheet/Collection/Cells.php

@@ -495,15 +495,12 @@ class Cells
     /**
      * Returns all known cache keys.
      *
-     * @return string[]
+     * @return \Generator|string[]
      */
     private function getAllCacheKeys()
     {
-        $keys = [];
         foreach ($this->getCoordinates() as $coordinate) {
-            $keys[] = $this->cachePrefix . $coordinate;
+            yield $this->cachePrefix . $coordinate;
         }
-
-        return $keys;
     }
 }