Hypermedia REST API client for AngularJS applications

README.md

Hypermedia REST API client for AngularJS applications

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A hypermedia client for AngularJS applications. Supports relations in HTTP Link headers, JSON properties and JSON HAL, and resource profiles.

An extension module angular-hypermedia-offline is available that adds offline caching of resources.

Installation

Install using Bower.

bower install angular-hypermedia --save

Then include it (and its dependencies) in your HTML page.

<script src="bower_components/angular-hypermedia/dist/hypermedia.js"></script>
<script src="bower_components/linkheader-parser/dist/linkheader-parser-browser.js"></script>
<script src="bower_components/mediatype-parser/dist/mediatype-parser-browser.js"></script>
<script src="bower_components/uri-templates/uri-templates.js"></script>

Alternatively, install using NPM

npm install angular-hypermedia --save

Webpack integration

You can use Webpack ProvidePlugin to integrate dependencies as follows:

new webpack.ProvidePlugin({
  mediaTypeParser: 'mediatype-parser',
  linkHeaderParser: 'linkheader-parser',
  UriTemplate: 'uri-templates'
}),

Since mediatype-parser and linkheader-parser releases v0.1.2 you also need to add aliases to your Webpack configuration:

resolve: {
  ...
  alias: {
    'mediatype-parser': 'mediatype-parser/dist/mediatype-parser-node.js',
    'linkheader-parser': 'linkheader-parser/dist/linkheader-parser-node.js'
  }

Quickstart

Consider a controller that lists all GitHub notifications for the current user. It can use the next and prev links provided by the GitHub API for pagination.

This could be an implementation of the controller:

angular.module('myGitHubBrowser', ['hypermedia'])

  .controller('NotificationsController', function (ResourceContext) {
    $scope.page = null;

    new ResourceContext().get('https://api.github.com/notifications').then(function (page) {
      $scope.page = page;
    });

    $scope.followRel = function (rel) {
      $scope.page.$linkRel(rel).$get().then(function (page) {
        $scope.page = page;
      })
    };

    $scope.hasRel = function (rel) {
      return rel in $scope.page.$links;
    };
  });

The accompanying HTML template:

<div>
  <!-- List the notifications on the current page -->

  <ul class="pagination">
    <li>
      <a ng-click="{{ followRel('prev') }}" ng-class="{disabled: !hasRel('prev')}">&laquo;</a>
    </li>
    <li>
      <a ng-click="{{ followRel('next') }}" ng-class="{disabled: !hasRel('next')}">&raquo;</a>
    </li>
  </ul>
</div>

Provided services

To use this module, import hypermedia in your Angular module and inject any of the exported services.

Example:

angular.module('myApp', ['hypermedia'])
  .factory('MyController', function (ResourceContext, Resource, HalContext, BlobContext) {
    ...
  });

Resources and contexts

This module assumes that a hypermedia API client often interacts with multiple related resources for the functionality provided by a page. The ResourceContext is responsible for keeping together resources that are being used together. Resources are bound to a single context.

Resources are represented by a Resource or one of its subclasses. A resource is a unit of data that can be synchronized with its authoritative source using HTTP requests. In this way, it is similar to a AngularJS $resource instance.

Example:

var context = new ResourceContext();
var person = context.get('http://example.com/composer/john');
expect(person.$uri).toBe('http://example.com/composer/john');

The context acts like an identity map, in the sense that calling context.get with the same URI returns the same Resource object.

If a subclass of Resource is required, a second argument may be used.

Example:

var movie = context.get('http://example.com/movie/jaws', HalResource);

If you are using an API that is based on a media type for which a Resource subclass exists (JSON HAL, for example) it is useful to create a context with a default factory.

Example:

var context2 = new ResourceContext(HalResource);
var movie2 = context.get('http://example.com/movie/jaws');

GET, PUT, DELETE requests: state synchronization

Resources are synchronized using GET, PUT and DELETE requests. The methods on the resource object are $get, $put and $delete respectively. These methods return a promise that is resolved with the Resource when the request completes successfully.

Example:

context.get('http://example.com/composer/john').$get().then(function (composer) {
  expect(composer.firstName).toBe('John');
  expect(composer.lastName).toBe('Williams');
});

person.email = 'john@example.com';
person.favoriteMovie = 'Close Encounters';
person.$put().then(function () {
  console.log('success!');
});

PATCH requests: synchronization using JSON Merge Patch

The PATCH request method updates a resources by only sending a "diff" of the state. Resource uses JSON Merge Patch. It is a very simple JSON patch format suitable for describing modifications to JSON documents that primarily use objects for their structure and do not make use of explicit null values. Subclasses of Resource may choose to support other formats by overriding the $patchRequest method.

The $patch method accepts a mapping of (new or existing) properties to updated values; mapping a property to null will delete the property. Objects are merged recursively, but arrays are replaced. The algorithm is specified in the RFC. The changes are applied to the Resource object after the HTTP request has completed successfully; if it fails the object remains unchanged.

Example:

person.$patch({email: 'johnwilliams@example.com', favoriteMovie: null}).then(function () {
  console.log('email changed to ' + person.email);
}, function () {
  console.log('request failed, email is still ' + person.email);
});

POST requests

A POST request is used to "operate on" data instead of synchronizing it. What the "operate" means is up to the server, and depends on the resource. It is often used to create new resources. The $post method accepts as arguments the data to be sent in the body and a mapping of headers.

Example:

person.$post({password: 'secret'}, {'Content-Type': 'text/plain'}).then(function () {
  console.log('password changed');
});

Relations

The essence of hypermedia is the linking of resources. In its simplest form, a resource can link to another resource by including its URI as a property. Because a reference to another resource is a hypermedia reference, such a property is sometimes called an "href".

Note: a relation can be a string or an array of string.

Example:

person.carHref = 'http://example.com/car/mercedes-sedan';
person.friendHrefs = [
  'http://example.com/director/george',
  'http://example.com/director/steven'
];

Of course, it is possible to look up URIs in the context, but Resource has the convenience method $propRel for getting related resources. If the property value is an array of URIs then an array of resources is returned.

Example:

var car = person.$propRel('carHref');
var friends = person.$propRel('friendHrefs');

If the target resource is not created using the default context factory, you can add the factory as the last parameter.

Example:

car.manufacturerHref = 'http://example.com/hal/companies/mercedes';
var manufacturer = car.$propRel('manufacturerHref', HalResource)

URI Templates

A reference can also be a URI Template, containing parameters that need to be substituted before it can be resolved. The $propRel accepts a second argument of variables (a mapping of names to values) to resolve a URI Template reference.

Example:

person.appointmentsHref = 'http://example.com/appointments/john/{date}'
var todaysAppointments = person.$propRel('appointmentsHref', {date: '2015-03-05'});

URI Template variables and resource factory can be specified at the same time.

Example:

manufacturer.modelsHref = 'http://example.com/hal/companies/mercedes/models{?current}'
var currentModels = manufacturer.$propRel('modelsHref', {current: true}, HalResource);

Links

Instead of referencing other resources in properties, it is also possible to use links. Links are returned by the server as Link headers.

The $links property is a mapping of relations to link objects. A link object has an href property containing the relation target URI. Other properties are link attributes as listed in the RFC.

Relations are either keywords from the IANA list or URIs. (These URIs are used as references, but may point to documentation that describes the relationship.)

Example:

car.$links['http://example.com/rels/owner'] = {
  href: 'http://example.com/composer/john',
  title: 'Owner'
};

Link relations are followed in much the same way as property relations, using the $linkRel method.

Example:

expect(car.$linkRel('http://example.com/rels/owner')).toBe(person);

Self links and URI aliases

The self relation identifies the resource itself. If a HTTP GET response contains a self link to a URI different from resource URI, that URI is added to the context as an alias. In other words, the resource will be available under both the original URI and the self href.

Example:

var movie = context.get('http://example.com/movie/jaws-4');
movie.$get().then(function () {
  // GET response contains self link to "http://example.com/movie/jaws-the-revenge"
  expect(context.get('http://example.com/movie/jaws-the-revenge')).toBe(movie);
});

This behavior can be disabled by setting context.enableAliases to false, or globally by setting ResourceContext.defaultEnableAliases. If aliases are disabled, trying to update a resource with a self link different to the resource URI will throw an error.

Profiles

Resources can often be said to be of a certain type, in the sense that in the examples, the resource referenced by http://example.com/composer/john "is a person". This is called a profile. Profiles are identified by a URI. (As with relations, they may double as a pointer to the profile documentation.) Resources have a $profile property containing the profile URI.

It is possible to add functionality to resources of specific profiles by registering properties. Setting $profile immediately applies the properties registered with that profile. (The properties are set on a per-resource prototype, so they do not interfere with the resource data and are removed when the profile is removed.)

Note: if using an array, adding profiles to the array after setting $profile will not update the properties.

Profiles are registered using Resource.registerProfile(profile, properties) or Resource.registerProfiles(profileProperties). Properties are applied to resources using Object.defineProperties.

Example:

Resource.registerProfiles({
  'http://example.com/profiles/person': {
    fullName: {get: function () {
      return this.firstName + ' ' + this.lastName;
    }},

    car: {get: function () {
      return this.propRel('carHref');
    }}
  }
});

person.$profile = 'http://example.com/profiles/person';

expect(person.fullName).toBe('John Williams');
expect(person.car.brand).toBe('Mercedes');

The profile is automatically set if the response of a GET request contains either a profile link or the profile parameter in the Content-Type header.

Loading and refreshing resources

Because different relations may point to the same URI, just calling $get on all followed resources risks issuing GET requests for the same resource multiple times. By using $load instead of $get a GET request will only be issued if the resource was not already synchronized with the server.

Example:

person.$links['http://example.com/rels/artistic-works'] = 'http://example.com/composers/john/works'
person.$linkRel('http://example.com/rels/artistic-works').$load().then(function (works) {
  console.log("John's works: " + works.map(function (work) { return work.title; }).join(', '));
});

When using resources in Angular views, it is important that all information needed to render the template has been loaded. Often, this means loading all resources that are reached by following a specific path through the resource relations. The $loadPaths method loads all resources reached by follow relation paths. The argument is a nested object hierarchy where the keys represent link or property relations, or computed properties that return other resources directly (such as the car profile property in the examples).

Example:

person.$loadPaths({
  car: {},
  friendHrefs: {
    car: {}
  },
  'http://example.com/rels/artistic-works': {}
});

Loading related resources is usually done in resolve functions of a URL route.

Example:

$routeProvider.when('/composers', {
  templateUrl: 'composers.html',
  controller: 'ComposersController',
  resolve: {
    composers: function (ResourceContext) {
      var context = new ResourceContext();
      return context.get('http://example.com/composers').$loadPaths({
        item: {
          car: {},
          friendHrefs: {
            car: {}
          }
        }
      });
    }
  }
});

It is often useful to make sure the resource data is not too old. You can pass a timestamp to the $load and $loadPaths methods to issue a GET request if the last synchronization was before that time. The $refresh and $refreshPaths methods work similarly, but use Date.now() as a default timestamp.

Example:

var oneHourAgo = Date.now() - 60*60*1000;
movie.$load(oneHourAgo);
person.$loadPaths({car: {}}, oneHourAgo);

car.$refresh();
manufacturer.$refreshPaths({modelsHref: {}})

JSON HAL

The JSON Hypertext Application Language is a JSON-based media type that reserves properties to include links and embedded resources. HalResource is a subclass of Resource that understands these properties. It accepts the application/hal+json media type, but will use application/json for PUT requests. The idea is that links are API wiring, and not application state.

On GET requests, links are copied from the _links property and embedded resources are extracted from _embedded and added to the context. Both properties are then deleted.

Using $load and $loadPaths makes sense especially with HAL, as this makes the client robust with regard to the presence or absence of embedded resources.

Example:

var root = new ResourceContext(HalResource).get('http://example.com/hal');
root.$loadPaths({
  'ex:manufacturers': {
    'items': {
      'ex:models: {
        'items': {}
      },
      'ex:subsidiaries': {
        'items': {}
      }
    }
  }
});

Blob resources

A BlobResource can be used to represent binary data. The data received from the server will be stored as a Blob in the data property of the object.

Example:

person.profilePhotoHref = 'http://example.com/photos/johnwilliams.jpg';
person.$propRel('profilePhotoHref', BlobResource).$load().then(function (photo) {
  $scope.photoImgSrc = $window.URL.createObjectURL(resource.data);
});

Error handlers

Many APIs will use the body of a 4xx or 5xx response to inform the client of the type of error. An error media type, such as vnd.error, can be used as a formal description of the problem. The ResourceContext HTTP methods can automatically convert such responses to an error property on the response result.

The vnd.error media type is supported automatically. You can register handlers for other media types:

Example:

ResourceContext.registerErrorHandler('text/plain', function (response) {
  return {message: response.data};
});

A handler must return an object with a message property containing a human-readable error message. It may add other properties. For example, the handler for vnd.error will add the following properties:

  • message: the error message
  • logref: an error identifier
  • path: a pointer to the JSON field relevant to the error
  • $links: hyperlinks to error metadata
  • $nested: embedded error objects

The context will return the error object as the error property of the rejection response. If no response body is returned or the media type has not been registered, response.error.message is set to the HTTP response status message.

Example:

person.$get().catch(function (response) {
  console.log('Error: ' + response.error.message);
  console.log('Logref: ' + response.error.logref);
  console.log('Path: ' + response.error.path);
  if (response.error.$nested) {
    response.error.$nested.forEach(function (error) {
      console.log('Nested error: ' + error.message;
    });
  }
});