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Managing data

At the end of In-app Navigation, the online store application has a product catalog with two views: a product list and product details. Users can click on a product name from the list to see details in a new view, with a distinct URL, or route.

This page guides you through creating the shopping cart in three phases:

Update the product details view to include a "Buy" button, which adds the current product to a list of products that a cart service manages.
Add a cart component, which displays the items in the cart.
Add a shipping component, which retrieves shipping prices for the items in the cart by using Angular's HttpClient to retrieve shipping data from a .json file.

{@a services}

Services

Services are an integral part of Angular applications. In Angular, a service is an instance of a class that you can make available to any part of your application using Angular's dependency injection system.

Services are the place where you share data between parts of your application. For the online store, the cart service is where you store your cart data and methods.

{@a create-cart-service}

Create the shopping cart service

Up to this point, users can view product information, and simulate sharing and being notified about product changes. They cannot, however, buy products.

In this section, you add a "Buy" button to the product details view and set up a cart service to store information about products in the cart.

A later part of this tutorial, Use forms for user input, guides you through accessing this cart service from the view where the user checks out.

{@a generate-cart-service}

Define a cart service

To generate a cart service, right click on the app folder, choose Angular Generator, and choose Service. Name the new service cart.
```
 <code-example header="src/app/cart.service.ts" path="getting-started/src/app/cart.service.1.ts"></code-example>
```
The StackBlitz generator might provide the cart service in app.module.ts by default. That differs from the example, which uses a bundle-optimization technique, an @Injectable() decorator with the { providedIn: 'root' } statement. For more information about services, see Introduction to Services and Dependency Injection.
In the CartService class, define an items property to store the array of the current products in the cart.
Define methods to add items to the cart, return cart items, and clear the cart items:
- The addToCart() method appends a product to an array of items.
- The getItems() method collects the items users add to the cart and returns each item with its associated quantity.
- The clearCart() method returns an empty array of items.

{@a product-details-use-cart-service}

Use the cart service

This section walks you through using the cart service to add a product to the cart with a "Buy" button.

Open product-details.component.ts.
Configure the component to use the cart service.
1. Import the cart service.
2. Inject the cart service by adding it to the constructor().
Define the addToCart() method, which adds the current product to the cart.

The addToCart() method does the following three things:
- Receives the current product.
- Uses the cart service's addToCart() method to add the product the cart.
- Displays a message that you've added a product to the cart.
Update the product details template with a "Buy" button that adds the current product to the cart.
1. Open product-details.component.html.
2. Add a button with the label "Buy", and bind the click() event to the addToCart() method:
The line, <h4>{{ product.price | currency }}</h4>, uses the currency pipe to transform product.price from a number to a currency string. A pipe is a way you can transform data in your HTML template. For more information about Angular pipes, see Pipes.
To see the new "Buy" button, refresh the application and click on a product's name to display its details.
Click the "Buy" button to add the product to the stored list of items in the cart and display a confirmation message.

Create the cart view

At this point, users can put items in the cart by clicking "Buy", but they can't yet see their cart.

Create the cart view in two steps:

Create a cart component and configure routing to the new component. At this point, the cart view has only default text.
Display the cart items.

Set up the component

To create the cart view, begin by following the same steps you did to create the product details component and configure routing for the new component.

Generate a cart component, named cart.

Reminder: In the file list, right-click the app folder, choose Angular Generator and Component.
Add routing (a URL pattern) for the cart component.

Open app.module.ts and add a route for the component CartComponent, with a path of cart:
Update the "Checkout" button so that it routes to the /cart url.

Open top-bar.component.html and add a routerLink directive pointing to /cart.
To see the new cart component, click the "Checkout" button. You can see the "cart works!" default text, and the URL has the pattern https://getting-started.stackblitz.io/cart, where getting-started.stackblitz.io may be different for your StackBlitz project.

Display the cart items

You can use services to share data across components:

The product details component already uses the cart service to add products to the cart.
This section shows you how to use the cart service to display the products in the cart.

Open cart.component.ts.
Configure the component to use the cart service.
1. Import the CartService from the cart.service.ts file.
2. Inject the CartService so that the cart component can use it.
Define the items property to store the products in the cart.
Set the items using the cart service's getItems() method. Recall that you defined this method when you generated cart.service.ts.

The resulting CartComponent class is as follows:
Update the template with a header, and use a <div> with an *ngFor to display each of the cart items with its name and price.

The resulting CartComponent template is as follows:
Test your cart component.
1. Click on "My Store" to go to the product list view.
2. Click on a product name to display its details.
3. Click "Buy" to add the product to the cart.
4. Click "Checkout" to see the cart.
5. To add another product, click "My Store" to return to the product list.

Repeat to add more items to the cart.

<div class="lightbox">
  <img src='generated/images/guide/start/cart-page-full.png' alt="Cart view with products added">
</div>

StackBlitz tip: Any time the preview refreshes, the cart is cleared. If you make changes to the app, the page refreshes, so you'll need to buy products again to populate the cart.

For more information about services, see Introduction to Services and Dependency Injection.

Retrieve shipping prices

Servers often return data in the form of a stream. Streams are useful because they make it easy to transform the returned data and make modifications to the way you request that data. The Angular HTTP client, HttpClient, is a built-in way to fetch data from external APIs and provide them to your app as a stream.

This section shows you how to use the HTTP client to retrieve shipping prices from an external file.

Predefined shipping data

The application that StackBlitz generates for this guide comes with predefined shipping data in assets/shipping.json. Use this data to add shipping prices for items in the cart.

Use `HttpClient` in the `AppModule`

Before you can use Angular's HTTP client, you must configure your app to use HttpClientModule.

Angular's HttpClientModule registers the providers your app needs to use a single instance of the HttpClient service throughout your app.

Open app.module.ts.

This file contains imports and functionality that is available to the entire app.

Import HttpClientModule from the @angular/common/http package at the top of the file with the other imports. As there are a number of other imports, this code snippet omits them for brevity. Be sure to leave the existing imports in place.
Add HttpClientModule to the AppModule @NgModule() imports array to register Angular's HttpClient providers globally.

Use `HttpClient` in the cart service

Now that the AppModule imports the HttpClientModule, the next step is to inject the HttpClient service into your service so your app can fetch data and interact with external APIs and resources.

Open cart.service.ts.
Import HttpClient from the @angular/common/http package.
Inject HttpClient into the CartService constructor:

Define the `get()` method

Multiple components can leverage the same service. Later in this tutorial, the shipping component uses the cart service to retrieve shipping data via HTTP from the shipping.json file. First, define a get() method.

Continue working in cart.service.ts.
Below the clearCart() method, define a new getShippingPrices() method that uses the HttpClient get() method to retrieve the shipping data.

For more information about Angular's HttpClient, see the Client-Server Interaction guide.

Define the shipping view

Now that your app can retrieve shipping data, create a shipping component and template.

Generate a new component named shipping.

Reminder: In the file list, right-click the app folder, choose Angular Generator and Component.
In app.module.ts, add a route for shipping. Specify a path of shipping and a component of ShippingComponent.

There's no link to the new shipping component yet, but you can see its template in the preview pane by entering the URL its route specifies. The URL has the pattern: https://getting-started.stackblitz.io/shipping where the getting-started.stackblitz.io part may be different for your StackBlitz project.
Modify the shipping component so that it uses the cart service to retrieve shipping data via HTTP from the shipping.json file.
1. Import the cart service.
2. Define a shippingCosts property.
3. Inject the cart service in the ShippingComponent constructor:
4. Set the shippingCosts property using the getShippingPrices() method from the cart service.
Update the shipping component's template to display the shipping types and prices using the async pipe:

The async pipe returns the latest value from a stream of data and continues to do so for the life of a given component. When Angular destroys that component, the async pipe automatically stops. For detailed information about the async pipe, see the AsyncPipe API documentation.
Add a link from the cart view to the shipping view:
Test your shipping prices feature:

Click the "Checkout" button to see the updated cart. Remember that changing the app causes the preview to refresh, which empties the cart.

Click on the link to navigate to the shipping prices.

Next steps

Congratulations! You have an online store application with a product catalog and shopping cart. You can also look up and display shipping prices.

To continue exploring Angular, choose either of the following options:

Continue to the "Forms" section to finish the app by adding the shopping cart view and a checkout form.
Skip ahead to the "Deployment" section to move to local development, or deploy your app to Firebase or your own server.

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