Finished guide about DI container and service locator.

docs/guide/concept-service-locator.md

 Service Locator
 ===============
 
-> Note: This chapter needs cleanup.
-
-Both service locator and dependency injection are popular design patterns that allow building software
-in a loosely-coupled fashion. Yii uses service locator and dependency injection extensively,
-even though you may not be aware of them. In this tutorial, we will explore their implementation
-and support to help you write code more consciously. We also highly recommend you to read
-[Martin's article](http://martinfowler.com/articles/injection.html) to get a deeper understanding of
-service locator and dependency injection.
-
 A service locator is an object that knows how to provide all sorts of services (or components) that an application
 might need. Within a service locator, each component has only a single instance which is uniquely identified by an ID.
-You use the ID to retrieve a component from the service locator. In Yii, a service locator is simply an instance 
-of [[yii\di\ServiceLocator]] or its child class.
+You use the ID to retrieve a component from the service locator.
+
+In Yii, a service locator is simply an instance of [[yii\di\ServiceLocator]] or its child class.
 
 The most commonly used service locator in Yii is the *application* object which can be accessed through
-`\Yii::$app`. The services it provides are called *application components*, such as `request`, `response`,
-`urlManager`. You may configure these components or replace them with your own implementations easily
+`\Yii::$app`. The services it provides are called *application components*, such as the `request`, `response`,
+`urlManager` components. You may configure these components or even replace them with your own implementations easily
 through functionality provided the service locator.
 
 Besides the application object, each module object is also a service locator.
@@ -26,7 +18,10 @@ To use a service locator, the first step is to register components. A component
 via [[yii\di\ServiceLocator::set()]]. The following code shows different ways of registering components:
 
 ```php
-$locator = new \yii\di\ServiceLocator;
+use yii\di\ServiceLocator;
+use yii\caching\FileCache;
+
+$locator = new ServiceLocator;
 
 // register "cache" using a class name that can be used to create a component
 $locator->set('cache', 'yii\caching\ApcCache');
@@ -43,6 +38,9 @@ $locator->set('db', [
 $locator->set('search', function () {
     return new app\components\SolrService;
 });
+
+// register "pageCache" using a component
+$locator->set('pageCache', new FileCache);
 ```
 
 Once a component is registered, you can access it using its ID in one of the following two ways:
@@ -62,11 +60,10 @@ You may use [[yii\di\ServiceLocator::has()]] to check if a component ID has alre
 If you call [[yii\di\ServiceLocator::get()]] with an invalid ID, an exception will be thrown.
 
 
-Because service locators are often being configured using configuration arrays, a method named
-[[yii\di\ServiceLocator::setComponents()]] is provided to allow registering components in configuration arrays.
-The method is a setter which defines a writable property `components` that can be configured.
-The following code shows a configuration array that can be used to configure an application and register
-the "db", "cache" and "search" components:
+Because service locators are often being created with [configurations](concept-configurations.md),
+a writable property named [[yii\di\ServiceLocator::setComponents()|components]] is provided so that
+you can configure it and register multiple components at once. The following code shows a configuration array
+that can be used to configure an application and register the "db", "cache" and "search" components:
 
 ```php
 return [

tests/unit/framework/di/ContainerTest.php

@@ -36,6 +36,8 @@ class ContainerTest extends TestCase
         $this->assertTrue($foo instanceof $Foo);
         $this->assertTrue($foo->bar instanceof $Bar);
         $this->assertTrue($foo->bar->qux instanceof $Qux);
+        $foo2 = $container->get($Foo);
+        $this->assertFalse($foo === $foo2);
 
         // full wiring
         $container = new Container;
@@ -80,5 +82,12 @@ class ContainerTest extends TestCase
         $this->assertTrue($foo instanceof $Foo);
         $this->assertTrue($foo->bar instanceof $Bar);
         $this->assertTrue($foo->bar->qux instanceof $Qux);
+
+        // wiring by closure
+        $container = new Container;
+        $container->set('qux', new Qux);
+        $qux1 = $container->get('qux');
+        $qux2 = $container->get('qux');
+        $this->assertTrue($qux1 === $qux2);
     }
 }