@ngdoc overview
@name Developer Guide: HTML Compiler
@description

# Overview

Angular's {@link api/ng.$compile HTML compiler} allows the developer to teach the
browser new HTML syntax. The compiler allows you to attach behavior to any HTML element or attribute
and even create new HTML elements or attributes with custom behavior. Angular calls these behavior
extensions {@link api/ng.$compileProvider#directive 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.

But the declarative language is also limited, since 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

Compiler is an angular service which traverses the DOM looking for attributes. The compilation
process happens in two phases.

  1. **Compile:** traverse the DOM and collect all of the directives. The result is a linking
  function.

  2. **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 {@link api/ng.directive:ngRepeat `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.


# Directive

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 {@link
api/ng.directive:ngBind `ng-bind`} directive.

<pre>
  <span ng-bind="exp"></span>
  <span class="ng-bind: exp;"></span>
  <ng-bind></ng-bind>
  <!-- directive: ng-bind exp -->
</pre>

A directive is just a function which executes when the compiler encounters it in the DOM. See {@link
api/ng.$compileProvider#directive 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
`<span>` element.

<example module="drag">
  <file name="script.js">
    angular.module('drag', []).
      directive('draggable', function($document) {
        return function(scope, element, attr) {
          var startX = 0, startY = 0, x = 0, y = 0;
          element.css({
           position: 'relative',
           border: '1px solid red',
           backgroundColor: 'lightgrey',
           cursor: 'pointer'
          });
          element.on('mousedown', function(event) {
            // Prevent default dragging of selected content
            event.preventDefault();
            startX = event.screenX - x;
            startY = event.screenY - y;
            $document.on('mousemove', mousemove);
            $document.on('mouseup', mouseup);
          });

          function mousemove(event) {
            y = event.screenY - startY;
            x = event.screenX - startX;
            element.css({
              top: y + 'px',
              left:  x + 'px'
            });
          }

          function mouseup() {
            $document.unbind('mousemove', mousemove);
            $document.unbind('mouseup', mouseup);
          }
        }
      });
  </file>
  <file name="index.html">
    <span draggable>Drag ME</span>
  </file>
</example>


The presence of the `draggable` attribute on any element gives the element new behavior. The beauty of
this approach is that we have taught the browser a new trick. We have extended the vocabulary of
what the browser understands in a way which is natural to anyone who is familiar with HTML
principles.


# Understanding View

There are many templating systems out there. Most of them consume a static string template and
combine it with data, resulting in a new string. The resulting text is then `innerHTML`ed into
an element.

<img src="img/One_Way_Data_Binding.png">

This means that any changes to the data need to be re-merged with the template and then
`innerHTML`ed into the DOM. Some of the issues with this approach are: reading user input and merging it with data,
clobbering user input by overwriting it, managing the whole update process, and lack of behavior
expressiveness.

Angular is different. The Angular compiler consumes the DOM with directives, not string templates.
The result is a linking function, which when combined with a scope model results in a live view. The
view and scope model bindings are transparent. No action from the developer is needed to update
the view. And because no `innerHTML` is used there are no issues of clobbering user input.
Furthermore, Angular directives can contain not just text bindings, but behavioral constructs as
well.

<img src="img/Two_Way_Data_Binding.png">

The Angular approach produces a stable DOM. This means that the DOM element instance bound to a model
item instance does not change for the lifetime of the binding. This means that the code can get
hold of the elements and register event handlers and know that the reference will not be destroyed
by template data merge.
