New Data Architecture using Reactive Programming and Observables

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Angular introduces many innovations: performance improvements, the Component Router, sharpened Dependency Injection (DI), lazy loading, async templating, Server rendering (aka Angular Universal), orchestrated animations, mobile development with Native Script; all topped with a solid tooling (thanks to TypeScript and the new Angular CLI) and excellent testing support.

In this post we will focus on the new Angular HTTP Module. We are going to look into:

New Angular Data Architecture using RxJS 5

Setting up angular/http

Creating a simple Http Service (using http.get)

Basic Authentication scenario (using http.post)

Differences between $http and angular/http

Angular uses Reactive Programming as its core building block. This is based on Asynchronous Data Streams aka Observables.

Demo | Source

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New Angular Data Architecture using RxJS 5

Reactive Extensions for JavaScript (RxJS) is a reactive streams library that allows you to work with Observables. RxJS combines Observables, Operators and Schedulers so we can subscribe to streams and react to changes using composable operations.

Angular team decided to use RxJS 5 instead of RxJS 4. This is a rewrite focused on improving performance and reducing the API complexity for easier adoption. It is understood version 5 will supersede version 4 by the time Angular is released.

🐒 Angular uses RxJS 5. Take this into account when looking into specific operators and documentation.

TypeScript

We are going to use TypeScript in this post. This is a superset of ES6. Read an introduction to ES6 to familiarise with common ES6 features.

We are going to use basic types, optional parameters, interfaces and Generics. Follow the links to learn more about these features.

Setting up angular/http

In order to use the new Http module in our components we have to import Http (see ES6 import). After that, we can just inject it via Dependency Injection in the constructor. Find an example below.

Besides the changes to our components using Http, we also need to include HttpModule during bootstrap.

In our App, we made HttpModule available during bootstrap, using an Array (line 8) for the property imports within ngModule. Note we imported it from: angular/http (line 4). For Angular, Http is no longer included in the core but as a separate file.

Creating a simple Http Service

Let’s create a simple example using a Service that reads some data from a JSON file.

New Dependency Injection

In order to make our service available for DI we added the @Injectable annotation (line 5). This indicates the Dependency Injector that the class has dependencies that should be injected into the constructor when creating an instance of the class.

Annotations allow us to declaratively add metadata to classes and properties for Angular to use. Common annotations are: @Component, @Injectable, @Pipe or @NgModule.

We defined a simple service using a class (line 6). We also preceded it with export to make it accessible to other components. Find below the syntax you use to inject Http into the constructor’s http argument.

import {Http} from ‘@angular/http’; constructor(private http:Http){ ... }

Http GET

On the constructor we are setting a public property this.http and calling http.get. Let’s take a look at its definition.



: Observable< http.get(url: string, options?: RequestOptionsArgs : Observable< Response

url?: string;

method?: string |

search?: string |

headers?:

body?: string;

} interface RequestOptionsArgs {url?: string;method?: string | RequestMethods search?: string | URLSearchParams headers?: Headers body?: string;

The http.get signature above is written in TypeScript and it reads as follows. We need to provide a url and optionally an options object, after the colons we find the corresponding types.

Type information can be used by IDEs to provide features similar to IntelliSense in Visual Studio and type checking.

When providing an options object it will follow the RequestOptionsArgs interface. In our example we did not use the options argument. Finally the last bit indicates that http.get returns an Observable emitting Response objects.

The first thing you will notice is that instead of returning a promise http.get returns an Observable. Finally we used map to parse the result into a JSON object. This code is using an arrow function (in ES6) or fat arrow (aka lambda syntax in TypeScript). Let’s see how it would translate to ES5 (line 8):

// TypeScript/ES6

.map(response => response.json()) // ES5

.map(function(response){ return response.json(); })

The result of map is also an Observable that emits a JSON object containing an Array of people.

Using a Service returning an Observable

Let’s take a look at how we would use the new PeopleService we just created to display a list of people. See a simplified version of the code below:

We imported PeopleService (line 1). Following, in the constructor, we injected the service using the same syntax we saw before (line 8).

During App instantiation, peopleService.getPeople() will return an Observable. We want to display a list of people so we can use subscribe to set our local people variable when the data is emitted. In this case, we just set the resulting Array into this.people.

Explore the examples above using this Plunker.

Basic Authentication scenario

Let’s see how a common authentication scenario would be implemented using Angular. We will use a simplified example from Auth0 and go over the code below.

Http POST

Let’s imagine we received the username and password from a form the user submitted. We would call authenticate to log in the user. Once the user is logged in we will proceed to store the token so we can include it in following requests.

The http.post signature above reads as follows. We need to provide a url and a body, both strings and then optionally an options object. In our example we are passing the modified headers property. http.post returns an Observable, we use map to extract the JSON object from the response and subscribe. This will setup our stream as soon as it emits the result. Finally We call this.storeToken with the corresponding user token.

Note we are passing the password as clear text. In a real scenario, We would have applied more strict options and would have used https.

Differences between $http and angular/http

Angular Http by default returns an Observable opposed to a Promise ($q module) in $http. This allows us to use more flexible and powerful RxJS operators like switchMap (flatMapLatest in version 4), retry, buffer, debounce, merge or zip.

By using Observables we improve readability and maintenance of our application as they can respond gracefully to more complex scenarios involving multiple emitted values opposed to only a one-off single value.

Observables vs Promises

When used with Http, both implementations provide an easy API for handling requests, but there are some key differences that make Observables a superior alternative:

Promises only accept one value unless we compose multiple promises (Eg: $q.all).

Promises can’t be cancelled.

That’s all folks! Hope this post provided you with some insights. Thanks for reading! Have any questions? Ping me on Twitter at @gerardsans

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