API CHANGE: Added DataList::filterByCallback() to allow filtering by PHP code.

model/DataList.php

@@ -256,6 +256,24 @@ class DataList extends ViewableData implements SS_List, SS_Filterable, SS_Sortab
 		return $this;
 	}
 
+	/**
+	 * Filter this DataList by a callback function.
+	 * The function will be passed each record of the DataList in turn, and must return true for the record to be included.
+	 * Returns the filtered list.
+	 * 
+	 * Note that, in the current implementation, the filtered list will be an ArrayList, but this may change in a future
+	 * implementation.
+	 */
+	public function filterByCallback($callback) {
+		if(!is_callable($callback)) throw new LogicException("DataList::filterByCallback() must be passed something callable.");
+		
+		$output = new ArrayList;
+		foreach($this as $item) {
+			if($callback($item)) $output->push($item);
+		}
+		return $output;
+	}
+
 	/**
 	 * Translates a Object relation name to a Database name and apply the relation join to 
 	 * the query.  Throws an InvalidArgumentException if the $field doesn't correspond to a relation