Service locator

Instead of manually creating instances of different shared services (application components) we can get them from a special global object, which contains configurations and instances of all components.

A service locator is a global object that contains a list of components or definitions, uniquely identified by an ID, and allow us to retrieve any needed instance by its ID. The locator creates a single instance of the component on-the-fly at the first call and returns a previous instance at the subsequent calls.

In this recipe, we will create a shopping cart component and will write a cart controller for working with it.

Getting ready

Create a new application by using the Composer package manager, as described in the official guide at http://www.yiiframework.com/doc-2.0/guide-start-installation.html.

How to do it…

Carry out the following steps to create a shopping cart component:

Create a shopping cart component. It will store selected items in a user session:

<?php
namespace app\components;

use Yii;
use yii\base\Component;

class ShoppingCart extends Component
{
    public $sessionKey = 'cart';

    private $_items = [];

    public function add($id, $amount)
    {
        $this->loadItems();
        if (array_key_exists($id, $this->_items)) {
            $this->_items[$id]['amount'] += $amount;
        } else {
            $this->_items[$id] = [
                'id' => $id,
                'amount' => $amount,
            ];
        }
       $this->saveItems();
    }

    public function remove($id)
    {
        $this->loadItems();
        $this->_items = array_diff_key($this->_items, [$id => []]);
        $this->saveItems();
    }

    public function clear()
    {
        $this->_items = [];
        $this->saveItems();
    }

    public function getItems()
    {
        $this->loadItems();
        return $this->_items;
    }

    private function loadItems()
    {
        $this->_items = Yii::$app->session->get($this->sessionKey, []);
    }

    private function saveItems()
    {
        Yii::$app->session->set($this->sessionKey, $this->_items);
    }
}

'components' => [
    …
    'cart => [
        'class' => 'app\components\ShoppingCart',
        'sessionKey' => 'primary-cart',
    ],
]

Create a cart controller:

<?php
namespace app\controllers;

use app\models\CartAddForm;
use Yii;
use yii\data\ArrayDataProvider;
use yii\filters\VerbFilter;
use yii\web\Controller;

class CartController extends Controller
{
    public function behaviors()
    {
        return [
            'verbs' => [
                'class' => VerbFilter::className(),
                'actions' => [
                    'delete' => ['post'],
                ],
            ],
        ];
    }

    public function actionIndex()
    {
        $dataProvider = new ArrayDataProvider([
            'allModels' => Yii::$app->cart->getItems(),
        ]);

        return $this->render('index', [
            'dataProvider' => $dataProvider,
        ]);
    }

    public function actionAdd()
    {
        $form = new CartAddForm();

        if ($form->load(Yii::$app->request->post()) && $form->validate()) {
            Yii::$app->cart->add($form->productId, $form->amount);
            return $this->redirect(['index']);
        }

        return $this->render('add', [
            'model' => $form,
        ]);
    }

    public function actionDelete($id)
    {
        Yii::$app->cart->remove($id);

        return $this->redirect(['index']);
    }
}

Create a form:

<?php
namespace app\models;

use yii\base\Model;

class CartAddForm extends Model
{
    public $productId;
    public $amount;

    public function rules()
    {
        return [
            [['productId', 'amount'], 'required'],
            [['amount'], 'integer', 'min' => 1],
        ];
    }
}

Create the views/cart/index.php view:

<?php
use yii\grid\ActionColumn;
use yii\grid\GridView;
use yii\grid\SerialColumn;
use yii\helpers\Html;

/* @var $this yii\web\View */
/* @var $dataProvider yii\data\ArrayDataProvider */

$this->title = 'Cart';
$this->params['breadcrumbs'][] = $this->title;
?>
<div class="site-contact">
    <h1><?= Html::encode($this->title) ?></h1>

    <p><?= Html::a('Add Item', ['add'], ['class' => 'btn btn-success']) ?></p>

    <?= GridView::widget([
        'dataProvider' => $dataProvider,
        'columns' => [
            ['class' => SerialColumn::className()],

            'id:text:Product ID',
            'amount:text:Amount',

            [
               'class' => ActionColumn::className(),
               'template' => '{delete}',
            ]
        ],
    ]) ?>
</div>

Create the views/cart/add.php view:

<?php
use yii\helpers\Html;
use yii\bootstrap\ActiveForm;

/* @var $this yii\web\View */
/* @var $form yii\bootstrap\ActiveForm */
/* @var $model app\models\CartAddForm */

$this->title = 'Add item';
$this->params['breadcrumbs'][] = ['label' => 'Cart', 'url' => ['index']];
$this->params['breadcrumbs'][] = $this->title;
?>
<div class="site-contact">
    <h1><?= Html::encode($this->title) ?></h1>

    <?php $form = ActiveForm::begin(['id' => 'contact-form']); ?>
        <?= $form->field($model, 'productId') ?>
        <?= $form->field($model, 'amount') ?>
        <div class="form-group">
            <?= Html::submitButton('Add', ['class' => 'btn btn-primary']) ?>
        </div>
    <?php ActiveForm::end(); ?>
</div>

Add a link item into the main menu:

['label' => 'Home', 'url' => ['/site/index']],
['label' => 'Cart', 'url' => ['/cart/index']],
['label' => 'About', 'url' => ['/site/about']],
// …

Open the cart page and try to add rows:

How it works…

First of all we created our own class with a public sessionKey option:

<?php
namespace app\components;
use yii\base\Component;

class ShoppingCart extends Component
{
    public $sessionKey = 'cart';

    // …
}

Secondly, we added the component definition into the components section of the configuration file:

'components' => [
    …
    'cart => [
        'class' => 'app\components\ShoppingCart',
        'sessionKey' => 'primary-cart',
    ],
]

Right now we can retrieve the component instance in two ways:

$cart = Yii::$app->cart;
$cart = Yii::$app->get('cart');

And we can use this object in our own controllers, widgets, and other places.

When we call any component such as cart:

Yii::$app->cart

We call the virtual property of the Application class instance in the Yii::$app static variable. But the yii\base\Application class extends the yii\base\Module class, which extends the yii\di\ServiceLocator class with the __get magic method. This magic method just calls the get() method of the yii\di\ServiceLocator class:

namespace yii\di;

class ServiceLocator extends Component
{
    private $_components = [];
    private $_definitions = [];

    public function __get($name)
    {
        if ($this->has($name)) {
            return $this->get($name);
        } else {
            return parent::__get($name);
        }
    }
    // …
}

As a result it is an alternative to directly calling the service via the get method:

Yii::$app->get('cart);

When we get a component from the get method of service locator, the locator finds needed definition in its _definitions list and if successful it creates a new object by the definition on the fly, registers it in its own list of complete instances _components and returns the object.

If we get some component, multiplying the locator will always return the previous saved instance again and again:

$cart1 = Yii::$app->cart;
$cart2 = Yii::$app->cart;
var_dump($cart1 === $cart2); // bool(true)

It allows us to use the shared single cart instance Yii::$app->cart or single database connection Yii::$app->db instead of creating one large set from scratch again and again.

Yii2 Application Development Cookbook - Third Edition

By : Andrew Bogdanov, Eliseev

Yii2 Application Development Cookbook

By: Andrew Bogdanov, Eliseev

Overview of this book

Service locator

Getting ready

How to do it…

How it works…

See also

Yii2 Application Development Cookbook - Third Edition

By : Andrew Bogdanov, Eliseev

Yii2 Application Development Cookbook

By: Andrew Bogdanov, Eliseev

Overview of this book

Service locator

Getting ready

How to do it…

How it works…

See also

Confirmation

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