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This chapter is from the book

This chapter is from the book

Understanding AngularJS

AngularJS provides a very structured framework based on an MVC (Model View Controller) model. This framework enables you to build structured applications that are robust and easily understood and maintained. If you are not familiar with the MVC model, the following paragraph provides a quick synopsis to help you understand the basics. It is by no means complete and only intended to give you enough reference to see how AngularJS applies MVC principles. The Wikipedia website is a great resource if you want additional information about MVC in general.

In MVC, there are three components: the Model is the data source, View is the rendered webpage, and the Controller handles the interaction between the two. A major purpose of MVC is to separate out responsibilities in your JavaScript code to keep it clean and easy to follow. AngularJS is one of the best MVC frameworks available because it makes it very easy to implement MVC.

To get started with AngularJS, you first need to understand the various components that you will be implementing and how they interact with each other. The following sections discuss the various components involved in an AngularJS application, their purpose, and what each is responsible for.

Modules

AngularJS introduces the concept of a module representing components in an application. The module provides a namespace that enables you to reference directives, scopes, and other components based on model name. This makes it easier to package and reuse parts of an application.

Each view or web page in AngularJS has a single module assigned to it via the ng-app directive. (Directives are discussed later in this chapter.) However, you can add other modules to the main module as dependencies, which provides a very structured and componentized application. The main AngularJS module acts similar to the root namespace in C# and Java.

Scopes and the Data Model

AngularJS introduces the concept of a scope. A scope is really just a JavaScript representation of data used to populate a view presented on a web page. The data can come from any source, such as a database, a remote web service, or the client-side AngularJS code, or it can be dynamically generated by the web server.

A great feature of scopes is that they are just plain JavaScript objects, which means you can manipulate them as needed in your AngularJS code with ease. Also, you can nest scopes to organize your data to match the context that they are being used in.

Views with Templates and Directives

HTML web pages are based on a DOM in which each HTML element is represented by a DOM object. A web browser reads the properties of a DOM object and knows how to render the HTML element on the web page, based on the DOM object’s properties.

Most dynamic web applications use direct JavaScript or a JavaScript-based library such as jQuery to manipulate a DOM object to change the behavior and appearance of the rendered HTML element in the user view.

AngularJS introduces a new concept of combining templates that contain directives which extend the HTML tags and attributes directly with JavaScript code in the background to extend the capability of HTML. Directives have two parts. The first part is extra attributes, elements, and CSS classes that are added to an HTML template. The second part is JavaScript code that extends the normal behavior of the DOM.

The advantage of using directives is that the intended logic for visual elements is indicated by the HTML template such that it is easy to follow and is not hidden within a mass of JavaScript code. One of the best features of AngularJS is that the built-in AngularJS directives handle most of the necessary DOM manipulation functionality that you need in order to bind the data in the scope directly to the HTML elements in the view.

You can also create your own AngularJS directives to implement any necessary custom functionality you need in a web application. In fact, you should use your own custom directives to do any direct DOM manipulation that a web application needs.

Expressions

A great feature of AngularJS is the capability to add expressions inside the HTML template. AngularJS evaluates expressions and then dynamically adds the result to a web page. Because expressions are linked to the scope, you can have an expression that utilizes values in the scope, and as the model changes, so does the value of the expression.

Controllers

AngularJS completes the MVC framework through the implementation of controllers. Controllers augment the scope by setting up the initial state or values in the scope and by adding behavior to the scope. For example, you can add a function that sums values in a scope to provide a total such that if the model data behind the scope changes, the total value always changes.

You add controllers to HTML elements by using a directive and then implement them as JavaScript code in the background.

Data Binding

One of the best features of AngularJS is the built-in data binding. Data binding is the process of linking data from the model with what is displayed in a web page. AngularJS provides a very clean interface to link the model data to elements in a web page.

In AngularJS data binding is a two-way process: When data is changed on a web page, the model is updated, and when data is changed in the model, the web page is automatically updated. This way, the model is always the only source for data represented to the user, and the view is just a projection of the model.

Services

Services are the major workhorses in the AngularJS environment. Services are singleton objects that provide functionality for a web app. For example, a common task of web applications is to perform AJAX requests to a web server. AngularJS provides an HTTP service that houses all the functionality to access a web server.

The service functionality is completely independent of context or state, so it can be easily consumed from the components of an application. AngularJS provides a lot of built-in service components for basic uses, such as HTTP requests, logging, parsing, and animation. You can also create your own services and reuse them throughout your code.

Dependency Injection

Dependency injection is a process in which a code component defines dependencies on other components. When the code is initialized, the dependent component is made available for access within the component. AngularJS applications make heavy use of dependency injection.

A common use for dependency injection is consuming services. For example, if you are defining a module that requires access to the web server via HTTP requests, you can inject the HTTP service into the module, and the functionality is available in the module code. In addition, one AngularJS module consumes the functionality of another via dependency.

Compiler

AngularJS provides an HTML complier that will discover directives in the AngularJS template and use the JavaScript directive code to build out extended HTML elements. The AngularJS compiler is loaded into the browser when the AngularJS library is bootstrapped. When loaded, the compiler will search through the HTML DOM in the browser and link in any back-end JavaScript code to the HTML elements, and then the final application view will be rendered to the user.

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