API Singleton method allowing type inference

This pattern improves over the current usage of singleton by allowing type inference.
This also better supports refactor, code usage detection, and auto-completion of classes.

core/Object.php

@@ -142,6 +142,27 @@ abstract class Object {
 		return Injector::inst()->createWithArgs($class, $args);
 	}
 
+	/**
+	 * Creates a class instance by the "singleton" design pattern.
+	 * It will always return the same instance for this class,
+	 * which can be used for performance reasons and as a simple
+	 * way to access instance methods which don't rely on instance
+	 * data (e.g. the custom SilverStripe static handling).
+	 *
+	 * @param string $className Optional classname (if called on Object directly)
+	 * @return static The singleton instance
+	 */
+	public static function singleton() {
+		$args = func_get_args();
+
+		// Singleton to create should be the calling class if not Object,
+		// otherwise the first parameter
+		$class = get_called_class();
+		if($class === 'Object') $class = array_shift($args);
+
+		return Injector::inst()->get($class);
+	}
+
 	private static $_cache_inst_args = array();
 
 	/**

docs/en/changelogs/3.2.0.md

@@ -36,6 +36,7 @@
  * `SQLConditionalExpression/SQLQuery` `select()`, `limit()`, `orderby()`, `groupby()`, `having()`, `from()`, `leftjoin()`, `innerjoin()`, `where()` and `whereAny()` removed.
  Use `set*()` and `add*()` methods instead.
  * Template `<% control $MyList %>` syntax removed. Use `<% loop $MyList %>` instead.
+ * Object::singleton() method for better type-friendly singleton generation
 
 ### CMS
 

tests/core/ObjectTest.php

@@ -150,6 +150,16 @@ class ObjectTest extends SapphireTest {
 		$this->assertTrue($obj3_2 instanceof ObjectTest_CreateTest3);
 	}
 
+	/**
+	 * Tests {@link Object::singleton()}
+	 */
+	public function testSingleton() {
+		$inst = Controller::singleton();
+		$this->assertInstanceOf('Controller', $inst);
+		$inst2 = Controller::singleton();
+		$this->assertSame($inst2, $inst);
+	}
+
 	public function testGetExtensions() {
 		$this->assertEquals(
 			Object::get_extensions('ObjectTest_ExtensionTest'),