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Create a Blazor-based frontend web shop

Now that we have an API to provide catalog items, we can start the process of building a frontend web shop that customers can use to browse the catalog. We'll use Blazor to create this web app.

Review the changes to the eShop solution

The provided starting point for this lab is based on the work you completed in the previous lab, however the Catalog API has been extended with more endpoints and the code refactored for better organization.

Open the eShop.lab2.sln in Visual Studio or VS Code.
Note there are some changes to the Catalog.API project:
- The Apis directory contains the CatalogApi.cs file now, rather than it being in the project root
- There is an Extension directory containing a HostingExtensions.cs file, in which an extension method is defined called AddApplicationServices. This method is called from the Program.cs file to setup the application's services. If any other extension methods are required, they can be placed in classes defined in this directory.
- There is a Pics directory containing product photos for the items in the catalog.
- There is a CatalogOptions class defined in CatalogOptions.cs file in the project root. This class is used to support configurable values used in the project.
- The Apis/CatalogApi.cs file now defines many other endpoints for the Catalog API which will be used to support the frontend experience. Take a moment to read the additional endpoint definitions and implementations.
The eShop.ServiceDefaults project has some changes too, including more extension methods to help with reading required configuration values, and configuring OpenAPI documents from the configuration. Take a moment to read the contents of the new files.
Launch the AppHost project and navigate to the Swagger UI for the Catalog API to see the additional endpoints that are now available.

Add the WebApp project to the AppHost project

The provided starting point for this lab includes a Blazor web project for the frontend of our eShop that we're going to make updates to, but it hasn't yet been composed into the AppHost project.

Add a project reference from the eShop.AppHost project to the WebApp project.
Open the Program.cs file in the eShop.AppHost project and add a line to add the WebApp project to the app model, with a reference to the Catalog.API project:
```
builder.AddProject<WebApp>("webapp")
    .WithReference(catalogApi);
```
Run the AppHost project and verify that the WebApp project is now included in the list of running resources on the dashboard.
Click the first hyperlink for the WebApp project in the Endpoints column and verify you see the home page of the eShop website.

Create a client service for the Catalog API

To communicate with the Catalog API from the web app, we'll create a service class that uses an HttpClient instance managed by the IHttpClientFactory features.

In the WebApp project, open the Services directory and inside it create a new file CatalogService.cs
Declare a class in this file called CatalogService with a primary constructor that accepts a single HttpClient httpClient parameter, and a field to store the common base path of the Catalog API endpoints "api/v1/catalog":
```
namespace eShop.WebApp.Services;

public class CatalogService(HttpClient httpClient)
{
    private readonly string remoteServiceBaseUrl = "api/v1/catalog/";


}
```
We need a method that will be called to retrieve a page of catalog items from the API. Add an async method called GetCatalogItems that accepts appropriate parameters for the page size, page index, and optional item brand and item type filters, and returns CatalogResult (this type is already defined in the project), e.g.:
```
public async Task<CatalogResult> GetCatalogItems(int pageIndex, int pageSize, int? brand, int? type)
{
    
}
```

The Catalog API represents the catalog items in the URL space categorized optionally by brand and type, e.g. /items, /items/type/all, /items/type/123/brand/456, etc. Additionally the paging parameters are passed via the query string. This means the URL called by our service needs to be dynamically constructed based on the values of the parameters passed to this method. Create a new method to handle this job called GetCatalogItemsUri, e.g.:

private static string GetAllCatalogItemsUri(string baseUri, int pageIndex, int pageSize, int? brand, int? type)
{
    // Build URLs like:
    //   [base]/items
    //   [base]/items/type/all
    //   [base]/items/type/123/brand/456
    //   [base]/items/type/123/brand/456?pageSize=9&pageIndex=2
    string filterPath;

    if (type.HasValue)
    {
        var brandPath = brand.HasValue ? brand.Value.ToString() : string.Empty;
        filterPath = $"/type/{type.Value}/brand/{brandPath}";

    }
    else if (brand.HasValue)
    {
        var brandPath = brand.HasValue ? brand.Value.ToString() : string.Empty;
        filterPath = $"/type/all/brand/{brandPath}";
    }
    else
    {
        filterPath = string.Empty;
    }

    return $"{baseUri}items{filterPath}?pageIndex={pageIndex}&pageSize={pageSize}";
}

Using this method, update the body of the GetCatalogItems method to retrieve the items from the Catalog API using the httpClient constructor parameter:

public async Task<CatalogResult> GetCatalogItems(int pageIndex, int pageSize, int? brand, int? type)
{
    var uri = GetAllCatalogItemsUri(remoteServiceBaseUrl, pageIndex, pageSize, brand, type);
    var result = await httpClient.GetFromJsonAsync<CatalogResult>(uri);
    return result ?? new(0, 0, 0, []);
}

Register the CatalogService with the application's DI container and configure its base address to point at the catalog-api resource by adding a line to the Extensions/HostingExtensions.cs file. Be sure that the host name set in the URI BaseAddress:
```
// HTTP and gRPC client registrations
builder.Services.AddHttpClient<CatalogService>(o => o.BaseAddress = new("http://catalog-api"));
```
Ensure the WebApp project builds successfully before continuing.

Update the home page to display catalog items

Now that we have a service we can use to easily retrieve the catalog items from the Catalog API, let's update the site's home page to use the service and display some catalog items.

In the WebApp project, open the Components/Pages/Catalog.razor file. Note that this page is configured to be the home page (served from the / path) via the @page "/" directive at the top of the file.
In order to get an instance of the CatalogService class in the Catalog.razor file from the application's DI container, add a line using the @inject directive near the top of the file (make sure it's after the @page directive). Note that the format of this is a bit like a property or field in C#, where the first part is the type name and the second part the member name (in this case the property name is the same as the type name but you could use any valid property name):
```
@inject CatalogService CatalogService
```
Locate the @code block at the bottom of the Catalog.razor file. This is where the page's fields, parameters, and methods can be defined. Define a constant value for the page size of 9 (while the API supports different page sizes, the UI we're going to build assumes a fixed page size):
```
const int PageSize = 9;
```
Razor components (including pages) execute according to a pre-defined lifecycle, represented by methods you can override to perform the desired logic needed to implement the component's behavior. Add code to use the injected CatalogService instance to load catalog items and store them in a field when the component is initializing. Just pass fixed values for the parameters other than pageSize for now:
```
CatalogResult? catalogResult;

protected override async Task OnInitializedAsync()
{
    catalogResult = await CatalogService.GetCatalogItems(0, PageSize, null, null);
}
```

Back in the markup section of the file, add some HTML and use Razor expressions to display the items in the catalogResult field. Note that you'll need to check the value is not null first to avoid a compiler error. Use the existing CatalogListItem component defined in the project to handle rendering each item inside the <div class="catalog> element:

<div class="catalog">
    @if (catalogResult is not null)
    {
        <div>
            <div class="catalog-items">
                @foreach (var item in catalogResult.Data)
                {
                    <CatalogListItem Item="@item" />
                }
            </div>
        </div>
    }
</div>

Run the AppHost project and navigate to the eShop front page. You should see catalog items now but their images aren't rendering correctly:
Open the browser developer tools (F12) and navigate to the Network pane then refresh the page. From the failed requests log we can see the images are trying to be loaded from paths like /product-images/99, where 99 is the product ID but the site doesn't have these files or any endpoint configured to serve them. Like the product details, the product images are served by the Catalog API.
YARP is a package for ASP.NET Core applications that provides highly customizable reverse proxying capabilities. We'll use YARP to proxy the product image requests to the frontend site on to the Catalog API. Add a reference to the Microsoft.Extensions.ServiceDiscovery.Yarp package, version 8.0.1. You can use the dotnet CLI, or Visual Studio NuGet Package Manager, or edit the WebApp.csproj directly:
```
<PackageReference Include="Microsoft.Extensions.ServiceDiscovery.Yarp" Version="9.0.0" />
```
To setup the proxying behavior, first add a line to the AddApplicationServices method in the HostingExtensions.cs file, to add the require services to the application's DI container:
```
builder.Services.AddHttpForwarderWithServiceDiscovery();
```
To setup the proxying endpoint, add a line in Program.cs, just before the call to app.Run() at the end of the file, to map a forwarder endpoint from the incoming path to the Catalog API. The first argument is the pattern for the incoming request path, the second argument is the address of the server to forward the request to, and the last argument is the target path on the forwarded server (i.e. the Catalog API):
```
app.MapForwarder("/product-images/{id}", "http://catalog-api", "/api/v1/catalog/items/{id}/pic");
```
Run the AppHost project again and navigate to the store's front page. This time the product images are loaded correctly:
Navigate to the Traces page of the dashboard and locate a trace row for one of the product image requests, then click on the View link in the Details column to open the trace details page. Notice the information that is displayed in the waterfall diagram of the various spans that make up the traced operation, including the request from the browser to the webapp resource, the forwarded request from webapp to catalog-api (both the outgoing and incoming spans), and finally the database call by catalog-api to the CatalogDB PostgreSQL resource. Click on any individual span to see all the details included in the trace. See if you can find the actual SQL query that was executed against the database as part of serving the image:

Improve the user experience while the catalog page is loading

You may have noticed that, when visiting the catalog page, there is a delay before the page is rendered in the browser. This delay is more noticeable when first loading the page after launching the AppHost project, as it takes a bit of time before the various resources involved are fully started. Let's improve this by utilizing the streaming rendering feature in Blazor to display a loading message while waiting for the catalog items to be returned by the backend service.

In the Catalog.razor file, update the markup to render a simple "Loading..." message if the catalogResult field is null, instead of rendering nothing:

<div class="catalog">
    @if (catalogResult is null)
    {
        <p>Loading...</p>
    }
    else
    {
        <div>
            <div class="catalog-items">
                @foreach (var item in catalogResult.Data)
                {
                    <CatalogListItem Item="@item" />
                }
            </div>
        </div>
    }
</div>

If you try loading the page again at this point, you'll see no change. This is because by default, Blazor doesn't send any response to the browser until the component has completely finished executing, including asynchronous operations started in component lifecycle events, e.g. in the OnInitializedAsync method.

Add a directive at the top of the page (after the @page directive) to add the StreamRendering attribute to the page. This will instruct Blazor to stream the response to the browser as asynchronous operations started by the component complete:
```
@attribute [StreamRendering]
```
Load the page again and notice that the loading message is displayed briefly before the catalog items are rendered.
Try forcing a longer delay to make the message easier to see when running locally, by adding a call to await Task.Delay(1000) before the call to CatalogService.GetCatalogItems in the OnInitializedAsync method:
```
protected override async Task OnInitializedAsync()
{
    await Task.Delay(1000);
    catalogResult = await CatalogService.GetCatalogItems(0, PageSize, null, null);
}
```

Add support for multiple pages of catalog items

The Catalog API supports returning a subset of matching catalog items to allow for building a page-based UI. Let's update the Catalog.razor page to display navigating through the paged results.

Under the PageSize field, declare a new component parameter to store the current page number being viewed. The value for this will be bound from the querystring of the request URL, e.g. /?page=2, by using the SupplyParameterFromQuery attribute:
```
[SupplyParameterFromQuery]
public int? Page { get; set; }
```
Update the call to CatalogService.GetCatalogItems to use the value of the Page parameter if it's set (use GetValueOrDefault to help with this). Note that the API accepts the page as a zero-based index, but as this page is viewed by humans, we'll want to offset that by +1 so that the first page is page 1, the second is page 2, and so on:
```
protected override async Task OnInitializedAsync()
{
    catalogResult = await CatalogService.GetCatalogItems(Page.GetValueOrDefault(1) - 1, PageSize, null, null);
}
```
To help with rendering a set of hyperlinks for navigating to other pages, add a method called GetVisiblePageIndexes that generates a list of page numbers based on the CatalogResult returned from the call to the Catalog API:
```
static IEnumerable<int> GetVisiblePageIndexes(CatalogResult result)
=> Enumerable.Range(1, (int)Math.Ceiling(1.0 * result.Count / PageSize));
```

Add markup to render the page links by looping over the page indexes returned from the GetVisiblePageIndexes method and render a hyperlink using the built-in NavLink component. The CSS defined in Catalog.razor.css already includes a styles targeting classes page-links and active-page so use those to style the links. Add this markup just after the closing </div> tag of the catalog-items div:

<div class="catalog">
    @if (catalogResult is null)
    {
        <p>Loading...</p>
    }
    else
    {
        <div>
            <div class="catalog-items">
                @foreach (var item in catalogResult.Data)
                {
                    <CatalogListItem Item="@item" />
                }
            </div>

            <div class="page-links">
                @foreach (var pageIndex in GetVisiblePageIndexes(catalogResult))
                {
                    <NavLink ActiveClass="active-page" Match="@NavLinkMatch.All" href="@Nav.GetUriWithQueryParameter("page", pageIndex == 1 ? null : pageIndex)">@pageIndex</NavLink>
                }
            </div>
        </div>
    }
</div>

Reload the home page and navigate through the pages of the catalog using the links rendered under the items. Note that the application must be fully restarted for this to work as the SupplyParameterFromQuery attribute is not recognized after a hot reload:

Add filtering ability to the catalog

The final piece of catalog functionality to add is the ability to filter the items by type and/or brand. The Catalog API already has endpoints to facilitate this so we just need to update CatalogService to call them and consume that from Catalog.razor with some appropriate UI.

In the CatalogService.cs file, add two new methods to allow retrieving catalog item types and brands from the Catalog API:

public async Task<IEnumerable<CatalogBrand>> GetBrands()
{
    var uri = $"{remoteServiceBaseUrl}catalogBrands";
    var result = await httpClient.GetFromJsonAsync<CatalogBrand[]>(uri);
    return result ?? [];
}

public async Task<IEnumerable<CatalogItemType>> GetTypes()
{
    var uri = $"{remoteServiceBaseUrl}catalogTypes";
    var result = await httpClient.GetFromJsonAsync<CatalogItemType[]>(uri);
    return result ?? [];
}

In Catalog.razor, declare two new component parameters to capture the brand ID and item type ID from the querystring:

[SupplyParameterFromQuery(Name = "brand")]
public int? BrandId { get; set; }

[SupplyParameterFromQuery(Name = "type")]
public int? ItemTypeId { get; set; }

Update the OnInitializedAsync method to pass in the values from the BrandId and ItemTypeId parameters, replacing the null values being passed before:

protected override async Task OnInitializedAsync()
{
    catalogResult = await CatalogService.GetCatalogItems(
        Page.GetValueOrDefault(1) - 1,
        PageSize,
        BrandId,
        ItemTypeId);
}

In the markup, add an instance of the CatalogSearch component (already defined in this project) immediately after the opening <div class="catalog"> tag, passing through the values of the BrandId and ItemTypeId parameters:
```
<div class="catalog">
    <CatalogSearch BrandId="@BrandId" ItemTypeId="@ItemTypeId" />
```
Locate and open the /Components/Catalog/CatalogSearch.razor file. Update this file so that it populates the catalogBrands and catalogItemTypes fields by calling the methods you just added to the CatalogService class. Reminder, you'll need to use the @inject directive to get an instance of the CatalogService before you can update the OnInitializedAsync method.

For an extra challenge, try calling the methods in parallel before awaiting their results (the Task.WhenAll method might be helpful here).
Reload the home page and see now that the filter UI is displayed on the left-hand side. Click on the various filter options to verify that the catalog UI functions correctly. Notice how the URL changes as you click through the various filter options and paging links.

Challenge: Update the item details page

If you have time and would like a challenge, try using what you've learned so far and update the Item.razor page to display an item's details. This page is linked to from each item on the catalog page.

Hints:

The Catalog API provides an endpoint at /items/{id} to retrieve an item's details via a GET request.
You can get image URLs for items from the IProductImageUrlProvider service which is already registered in the application's DI container (@inject...).
The Item.razor.css file already defines some styles targeting class names like item-details and description that should help with styling the content.
Think about what should happen if the item ID in the querystring doesn't have a matching catalog item. Displaying a "not found" message and changing the HTTP response status code to 404 might be things to consider.

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README.md

Create a Blazor-based frontend web shop

Review the changes to the eShop solution

Add the WebApp project to the AppHost project

Create a client service for the Catalog API

Update the home page to display catalog items

Improve the user experience while the catalog page is loading

Add support for multiple pages of catalog items

Add filtering ability to the catalog

Challenge: Update the item details page

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README.md

Create a Blazor-based frontend web shop

Review the changes to the eShop solution

Add the WebApp project to the AppHost project

Create a client service for the Catalog API

Update the home page to display catalog items

Improve the user experience while the catalog page is loading

Add support for multiple pages of catalog items

Add filtering ability to the catalog

Challenge: Update the item details page