Improve this DocHTML CompilerNote: this guide is targeted towards developers who are already familiar with AngularJS basics. If you're just getting started, we recommend the tutorial first. If you just want to create custom directives, we recommend the directives guide. If you want a deeper look into Angular's compilation process, you're in the right place.
Angular's HTML compiler allows the developer to teach the browser new HTML syntax. The compiler allows you to attach behavior to any HTML element or attribute and even create new HTML elements or attributes with custom behavior. Angular calls these behavior extensions directives.
HTML has a lot of constructs for formatting the HTML for static documents in a declarative fashion. For example if something needs to be centered, there is no need to provide instructions to the browser how the window size needs to be divided in half so that the center is found, and that this center needs to be aligned with the text's center. Simply add an
align="center" attribute to any element to achieve the desired behavior. Such is the power of declarative language.
However, the declarative language is also limited, as it does not allow you to teach the browser new syntax. For example, there is no easy way to get the browser to align the text at 1/3 the position instead of 1/2. What is needed is a way to teach the browser new HTML syntax.
Angular comes pre-bundled with common directives which are useful for building any app. We also expect that you will create directives that are specific to your app. These extensions become a Domain Specific Language for building your application.
All of this compilation takes place in the web browser; no server side or pre-compilation step is involved.
Compiler is an Angular service which traverses the DOM looking for attributes. The compilation process happens in two phases.
Compile: traverse the DOM and collect all of the directives. The result is a linking function.
Link: combine the directives with a scope and produce a live view. Any changes in the scope model are reflected in the view, and any user interactions with the view are reflected in the scope model. This makes the scope model the single source of truth.
Some directives such as
ng-repeat clone DOM elements once for each item in a collection. Having a compile and link phase improves performance since the cloned template only needs to be compiled once, and then linked once for each clone instance.
A directive is a behavior which should be triggered when specific HTML constructs are encountered during the compilation process. The directives can be placed in element names, attributes, class names, as well as comments. Here are some equivalent examples of invoking the
<span ng-bind="exp"></span> <span class="ng-bind: exp;"></span> <ng-bind></ng-bind> <!-- directive: ng-bind exp -->
A directive is just a function which executes when the compiler encounters it in the DOM. See directive API for in-depth documentation on how to write directives.
Here is a directive which makes any element draggable. Notice the
draggable attribute on the
The presence of the
draggable attribute on any element gives the element new behavior. We extended the vocabulary of the browser in a way which is natural to anyone who is familiar with the principles of HTML.
Most other templating systems consume a static string template and combine it with data, resulting in a new string. The resulting text is then
innerHTMLed into an element.