Creating A Reactive Data Source in Angular 4 – Part 2

WRITTEN BY GARETH DUNNE @JSDIARIES

In this part of the tutorial we will be focusing on our API calls and how they interact with the brewery API.

Again, if your looking to expand your Angular and Typescript knowledge I highly recommend this Angular & TypeScript book by Yakov Fain.

You can see the final version of what my beer app is here. I took the designs and styles of this application from Coursetro’s MEAN stack course and was able to implement a service for interactions with the data and the Brewery API.

Proxy

First up, in order to get data from the Brewery API we want to enable CORS (Cross Origin Resource Sharing) in our browser.

To accomplish this we have two choices:

The easiest option here is to install a CORS Chrome extension here.

This will enable you to perform cross origin shares in Chrome without having to setup a local proxy. This is handy for testing an application locally but would obviously fall short if pushed to a production environment or hosted externally.

The second option, which is something I would encourage you to do is to setup a local proxy. This will allow you to easily enable cross origin resource sharing for all your browsers and doesn’t take long to create.

To do this lets first create a proxy.config.json file in the root of our project directory.

Proxy file location
Proxy file location

This is our proxy API json object. It contains properties that allows our calls to send and receive data to the Brewery API.

However, it won’t work without running the proxy file alongside our application. So we will need to execute this command to start it alongside our application.

Beer Model

Next we need to create a beer view model. This will basically act as an interface for our retrieved data. Properties such as name, description etc will feature here.

In your app folder create a file called beer.ts and put in the following.

We can now import our Beer class to any of our components to reference data retrieved from the Beer API. This is

Get Beers

We now want to implement a method that retrieves all beers from our Beers API. Open up beer.service.ts and change it to the following.

So we now have a getBeers() method and a handleError method for our beers API call.

In our if condition we first check if this.result variable already has a value. This is an important step because our service shares permanent data throughout our application so if the application has already been loaded then there is no point in making the getBeers API call again as the data is already cached in the our browser/service to be used.

We then take the _http variable of type HTTP that was created in our constructor and then invoke the get method using our beer API url details as the parameters for it.

The line .map((res: Response) => res.json()) takes the response from the Beer API server and maps it into an JSON array for us.

.publishReplay(1) will allow the data to be reactive. We will come back to this.

.refCount(1) has a similar function. It allows the Observable returned by the HTTP get request to keep its connection open, this will allow the data source to be reactive.

catch(this.handleError) this will catch any error from our request and run the this.handleError function. All you really need to know about this function is that it will filter down through the error message to the point where it will usually be the most informative. Usually, a detailed HTTP code error or associated.

If you noticed some of the variables declared above the constructor you should see

These are both core Rxjs features that will allows our data to be reactive. We will come back to these but for now just think of them as reactive variables that we will using later on in our home.component.ts

This is the basic version of our service set up with the functionality to retrieve an initial list of beers. It gives a good overview of how our service will share data. However, this on its own will not do anything without injecting it into one of your components.

We will be doing this in part 3. For now think of its as a piece of functionality that will share data throughout the application, the data will be made reactive by using Rxjs Subjects and Observables which are declared at the top of the file.