Getting and Saving Data

Our stakeholders appreciate our progress. Now they want to get the hero data from a server, let users add, edit, and delete heroes, and save these changes back to the server.

In this chapter we teach our application to make the corresponding HTTP calls to a remote server's web API.

Run the for this part.

Where We Left Off

In the previous chapter, we learned to navigate between the dashboard and the fixed heroes list, editing a selected hero along the way. That's our starting point for this chapter.

Keep the app transpiling and running

Open a terminal/console window and enter the following command to start the TypeScript compiler, start the server, and watch for changes:

The application runs and updates automatically as we continue to build the Tour of Heroes.

Providing HTTP Services

The HttpModule is not a core Angular module. It's Angular's optional approach to web access and it exists as a separate add-on module called @angular/http, shipped in a separate script file as part of the Angular npm package.

Fortunately we're ready to import from @angular/http because systemjs.config configured SystemJS to load that library when we need it.

Register for HTTP services

Our app will depend upon the Angular http service which itself depends upon other supporting services. The HttpModule from @angular/http library holds providers for a complete set of HTTP services.

We should be able to access these services from anywhere in the application. So we register them all by adding HttpModule to the imports list of the AppModule where we bootstrap the application and its root AppComponent.

Notice that we supply HttpModule as part of the imports array in root NgModule AppModule.

Simulating the web API

We recommend registering application-wide services in the root AppModule providers. Here we're registering in main for a special reason.

Our application is in the early stages of development and far from ready for production. We don't even have a web server that can handle requests for heroes. Until we do, we'll have to fake it.

We're going to trick the HTTP client into fetching and saving data from a mock service, the in-memory web API. The application itself doesn't need to know and shouldn't know about this. So we'll slip the in-memory web API into the configuration above the AppComponent.

Here is a version of app/app.module.ts that performs this trick:

We're importing the InMemoryWebApiModule and adding it to the module imports. The InMemoryWebApiModule replaces the default Http client backend — the supporting service that talks to the remote server — with an in-memory web API alternative service.

The forRoot configuration method takes an InMemoryDataService class that primes the in-memory database as follows:

This file replaces the mock-heroes.ts which is now safe to delete.

Heroes and HTTP

Look at our current HeroService implementation

We returned a Promise resolved with mock heroes. It may have seemed like overkill at the time, but we were anticipating the day when we fetched heroes with an HTTP client and we knew that would have to be an asynchronous operation.

That day has arrived! Let's convert getHeroes() to use HTTP.

Our updated import statements are now:

Refresh the browser, and the hero data should be successfully loaded from the mock server.

HTTP Promise

We're still returning a Promise but we're creating it differently.

The Angular http.get returns an RxJS Observable. Observables are a powerful way to manage asynchronous data flows. We'll learn about Observables later in this chapter.

For now we get back on familiar ground by immediately converting that Observable to a Promise using the toPromise operator.

Unfortunately, the Angular Observable doesn't have a toPromise operator ... not out of the box. The Angular Observable is a bare-bones implementation.

There are scores of operators like toPromise that extend Observable with useful capabilities. If we want those capabilities, we have to add the operators ourselves. That's as easy as importing them from the RxJS library like this:

Extracting the data in the then callback

In the promise's then callback we call the json method of the HTTP Response to extract the data within the response.

That response JSON has a single data property. The data property holds the array of heroes that the caller really wants. So we grab that array and return it as the resolved Promise value.

The caller is unaware of these machinations. It receives a Promise of heroes just as it did before. It has no idea that we fetched the heroes from the (mock) server. It knows nothing of the twists and turns required to convert the HTTP response into heroes. Such is the beauty and purpose of delegating data access to a service like this HeroService.

Error Handling

At the end of getHeroes() we catch server failures and pass them to an error handler:

This is a critical step! We must anticipate HTTP failures as they happen frequently for reasons beyond our control.

In this demo service we log the error to the console; we would do better in real life.

We've also decided to return a user friendly form of the error to the caller in a rejected promise so that the caller can display a proper error message to the user.

Unchanged getHeroes API

Although we made significant internal changes to getHeroes(), the public signature did not change. We still return a Promise. We won't have to update any of the components that call getHeroes().

Our stakeholders are thrilled with the added flexibility from the API integration. Now they want the ability to create and delete heroes.

Let's see first what happens when we try to update a hero's details.

Update hero details

We can edit a hero's name already in the hero detail view. Go ahead and try it. As we type, the hero name is updated in the view heading. But when we hit the Back button, the changes are lost!

Save hero details

Let's ensure that edits to a hero's name aren't lost. Start by adding, to the end of the hero detail template, a save button with a click event binding that invokes a new component method named save:

The save method persists hero name changes using the hero service update method and then navigates back to the previous view:

Hero service update method

The overall structure of the update method is similar to that of getHeroes, although we'll use an HTTP put to persist changes server-side:

We identify which hero the server should update by encoding the hero id in the URL. The put body is the JSON string encoding of the hero, obtained by calling JSON.stringify. We identify the body content type (application/json) in the request header.

Refresh the browser and give it a try. Changes to hero names should now persist.

Add a hero

To add a new hero we need to know the hero's name. Let's use an input element for that, paired with an add button.

Insert the following into the heroes component HTML, first thing after the heading:

In response to a click event, we call the component's click handler and then clear the input field so that it will be ready to use for another name.

When the given name is non-blank, the handler delegates creation of the named hero to the hero service, and then adds the new hero to our array.

Finally, we implement the create method in the HeroService class.

Refresh the browser and create some new heroes!

Delete a hero

Too many heroes? Let's add a delete button to each hero in the heroes view.

Add this button element to the heroes component HTML, right after the hero name in the repeated <li> tag:

The <li> element should now look like this:

In addition to calling the component's delete method, the delete button click handling code stops the propagation of the click event — we don't want the <li> click handler to be triggered because that would select the hero that we are going to delete!

The logic of the delete handler is a bit trickier:

Of course, we delegate hero deletion to the hero service, but the component is still responsible for updating the display: it removes the deleted hero from the array and resets the selected hero if necessary.

We want our delete button to be placed at the far right of the hero entry. This extra CSS accomplishes that:

Hero service delete method

The hero service's delete method uses the delete HTTP method to remove the hero from the server:

Refresh the browser and try the new delete functionality.

Observables

Each Http service method returns an Observable of HTTP Response objects.

Our HeroService converts that Observable into a Promise and returns the promise to the caller. In this section we learn to return the Observable directly and discuss when and why that might be a good thing to do.

Background

An observable is a stream of events that we can process with array-like operators.

Angular core has basic support for observables. We developers augment that support with operators and extensions from the RxJS Observables library. We'll see how shortly.

Recall that our HeroService quickly chained the toPromise operator to the Observable result of http.get. That operator converted the Observable into a Promise and we passed that promise back to the caller.

Converting to a promise is often a good choice. We typically ask http.get to fetch a single chunk of data. When we receive the data, we're done. A single result in the form of a promise is easy for the calling component to consume and it helps that promises are widely understood by JavaScript programmers.

But requests aren't always "one and done". We may start one request, then cancel it, and make a different request before the server has responded to the first request. Such a request-cancel-new-request sequence is difficult to implement with Promises. It's easy with Observables as we'll see.

Search-by-name

We're going to add a hero search feature to the Tour of Heroes. As the user types a name into a search box, we'll make repeated HTTP requests for heroes filtered by that name.

We start by creating HeroSearchService that sends search queries to our server's web api.

The http.get() call in HeroSearchService is similar to the one in the HeroService, although the URL now has a query string. Another notable difference: we no longer call toPromise, we simply return the observable instead.

HeroSearchComponent

Let's create a new HeroSearchComponent that calls this new HeroSearchService.

The component template is simple — just a text box and a list of matching search results.

We'll also want to add styles for the new component.

As the user types in the search box, a keyup event binding calls the component's search method with the new search box value.

The *ngFor repeats hero objects from the component's heroes property. No surprise there.

But, as we'll soon see, the heroes property is now an Observable of hero arrays, rather than just a hero array. The *ngFor can't do anything with an Observable until we flow it through the async pipe (AsyncPipe). The async pipe subscribes to the Observable and produces the array of heroes to *ngFor.

Time to create the HeroSearchComponent class and metadata.

Search terms

Let's focus on the searchTerms:

A Subject is a producer of an observable event stream; searchTerms produces an Observable of strings, the filter criteria for the name search.

Each call to search puts a new string into this subject's observable stream by calling next.

Initialize the heroes property (ngOnInit)

A Subject is also an Observable. We're going to turn the stream of search terms into a stream of Hero arrays and assign the result to the heroes property.

If we passed every user keystroke directly to the HeroSearchService, we'd unleash a storm of HTTP requests. Bad idea. We don't want to tax our server resources and burn through our cellular network data plan.

Fortunately, we can chain Observable operators to the string Observable that reduce the request flow. We'll make fewer calls to the HeroSearchService and still get timely results. Here's how:

• debounceTime(300) waits until the flow of new string events pauses for 300 milliseconds before passing along the latest string. We'll never make requests more frequently than 300ms.

• distinctUntilChanged ensures that we only send a request if the filter text changed. There's no point in repeating a request for the same search term.

• switchMap calls our search service for each search term that makes it through the debounce and distinctUntilChanged gauntlet. It cancels and discards previous search observables, returning only the latest search service observable.

• catch intercepts a failed observable. Our simple example prints the error to the console; a real life application should do better. Then we return an observable containing an empty array to clear the search result.

Import RxJS operators

The RxJS operators are not available in Angular's base Observable implementation. We have to extend Observable by importing them.

We could extend Observable with just the operators we need here by including the pertinent import statements at the top of this file.

We take a different approach in this example. We combine all of the RxJS Observable extensions that our entire app requires into a single RxJS imports file.

We load them all at once by importing rxjs-extensions at the top of AppModule.

Add the search component to the dashboard

We add the hero search HTML element to the bottom of the DashboardComponent template.

Finally, we import HeroSearchComponent from hero-search.component.ts and add it to the declarations array:

Run the app again, go to the Dashboard, and enter some text in the search box. At some point it might look like this.

Application structure and code

Review the sample source code in the for this chapter. Verify that we have the following structure:

Home Stretch

We are at the end of our journey for now, but we have accomplished a lot.

• We added the necessary dependencies to use HTTP in our application.
• We refactored HeroService to load heroes from a web API.
• We extended HeroService to support post, put and delete methods.
• We updated our components to allow adding, editing and deleting of heroes.
• We configured an in-memory web API.
• We learned how to use Observables.

Here are the files we added or changed in this chapter.