Observables are the way to stream data in Angular2. Here’s how to get it working.
This article has one focus: providing example code that demonstrates how to create an observable, and then consume it. So you can start out by cloning the Github repository below. Once you’ve done that, you can run the example code locally and see it in action.
Making the HTTP request
In the following example, we will make an HTTP request, and then stream the return data to anyone who has subscribed to it.
Example # 1
In Example # 1, we have the code for the PackageService service. This service makes the HTTP request for the JSON data. I’m using myjson.com so that we don’t have to spend time with implementation details about serving up the JSON. We just want to make the request and then talk about how we can share that data across our application via an RXJS stream.
So let’s talk about this line:
packageData: Subject = new BehaviorSubject();
Here we create the packageData property. Although it will be an array, it will be a BehaviorSubject instance. So when we define the property, we specify that it is of type: Subject. Then we instantiate the BehaviorSubject class, passing it an empty array. The reason for the empty array is that we don’t want to stream “undefined“. Somewhere else in our code, there is a consumer who will want to subscribe to this stream. That consumer expects an array. It’s fine if the array is empty at first. We just don’t want the consumer to get “undefined“.
Later in Example # 1, we call the http.get method. We map that result to JSON, and then we subscribe to that JSON. I don’t want to get into too many implementation details about the last sentence, as I promised that this would take “five minutes.” So let’s focus on the next line: subscribe. By subscribing to this JSON, we are saying “Hey, any time there is a change in this JSON, I want to know about it.” And when that change occurs, then the function you see passed to the subscribe method is executed.
That function will receive the updated JSON data as its first argument. The very next line is critical: this.packageData.next(data). What’s happening here is: the packageData object that we created earlier has a “next” method. That method takes an argument, which can be anything. In our case, it is the JSON data. So, anyone who has subscribed to our packageData property will receive a notification that there is new data and will be passed that data.
Example # 2
In Example # 2, we have our PackagesComponent component. So let’s zero-in on the critical part: the ngOnInit method. Well, as you probably guessed by the name, this method is executed when our component is initialized. Take a look at this line: this.packageService .packageData .subscribe. There we are subscribing to the packageData property that we created in the packageService service. Because that is an instance of BehaviorSubject, subscribing to it gives us a live connection to anything it streams out. In our case, the http.get request fetched some JSON data. And in that service, the JSON data is streamed out via the line: this.packageData.next(data). The JSON data comes to the subscribe callback via the variable: “packages.” So we set this.destinations = packages. Therefore, at that point, the UI is updated and we see the list of travel packages in the page.
I promised that this would take less than five minutes. Hopefully, it did. RXJS is a deep subject that takes some time to get familiar with. I wrote this article because the first time I needed to stream the result of an HTTP request in an Angular2 application, it was a giant pain in the rump. But here, I have tried to provide an easy example of how to sketch out this scenario and get it working quickly. I hope it was helpful!