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

<div class="alert alert-warning">
**Note:** this guide is targeted towards developers who are already familiar with AngularJS basics.

If you're just getting started, we recommend the {@link tutorial/ tutorial} first.
If you just want to create custom directives, we recommend the {@link guide/directive directives guide}.
If you want a deeper look into Angular's compilation process, you're in the right place.
</div>


# 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#methods_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#methods_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.
We extended the vocabulary of the browser in a way which is natural to anyone who is familiar with the principles of HTML.


## Understanding View

Most other templating systems 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:

1. reading user input and merging it with data
2. clobbering user input by overwriting it
3. managing the whole update process
4. lack of behavior expressiveness

Angular is different. The Angular compiler consumes the DOM, 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. The developer does not need to make any special calls to update
the view. And because `innerHTML` is not used, you won't accidentally clobber 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. 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.



## How directives are compiled

It's important to note that Angular operates on DOM nodes rather than strings. Usually, you don't
notice this restriction because when a page loads, the web browser parses HTML into the DOM automatically.

However it's important to keep this in mind when calling `$compile` yourself, because passing it a string
will fail. Instead, use `angular.element` to convert a string to DOM before passing elements into
Angular's `$compile` service.

HTML compilation happens in three phases:

  1. {@link api/ng.$compile `$compile`} traverses the DOM and matches directives.

  If the compiler finds than an element matches a directive, then the directive is added to the list of
  directives that match the DOM element. A single element may match multiple directives.

  2. Once all directives matching a DOM element have been identified, the compiler sorts the directives
  by their `priority`.

  Each directive's `compile` functions are executed. Each `compile` function has a chance to
  modify the DOM. Each `compile` function returns a `link` function. These functions are composed into
  a "combined" link function, which invokes each directive's returned `link` function.

  3. `$compile` links the template with the scope by calling the combined linking function from the previous step.
  This in turn will call the linking function of the individual directives, registering listeners on the elements
  and setting up {@link api/ng.$rootScope.Scope#methods_$watch `$watch`s} with the {@link api/ng.$rootScope.Scope `scope`}
  as each directive is configured to do.

The result of this is a live binding between the scope and the DOM. So at this point, a change in
a model on the compiled scope will be reflected in the DOM.

Below is the corresponding code using the `$compile` service.
This should help give you an idea of what Angular does internally.

<pre>
  var $compile = ...; // injected into your code
  var scope = ...;

  var html = '<div ng-bind="exp"></div>';

  // Step 1: parse HTML into DOM element
  var template = angular.element(html);

  // Step 2: compile the template
  var linkFn = $compile(template);

  // Step 3: link the compiled template with the scope.
  linkFn(scope);
</pre>

### The difference between Compile and Link

At this point you may wonder why the compile process has separate compile and link phases. The
short answer is that compile and link separation is needed any time a change in a model causes
a change in the **structure** of the DOM.

It's rare for directives to have a **compile function**, since most directives are concerned with
working with a specific DOM element instance rather than changing its overall structure.

Directives often have a **link function**. A link function allows the directive to register
listeners to the specific cloned DOM element instance as well as to copy content into the DOM
from the scope.

<div class="alert alert-success">
**Best Practice:** Any operation which can be shared among the instance of directives should be
moved to the compile function for performance reasons.
</div>

#### An Example of "Compile" Versus "Link"

To understand, let's look at a real-world example with `ngRepeat`:

<pre>
Hello {{user}}, you have these actions:
<ul>
  <li ng-repeat="action in user.actions">
    {{action.description}}
  </li>
</ul>
</pre>

When the above example is compiled, the compiler visits every node and looks for directives.

`{{user}}` matches the {@link api/ng.$interpolate interpolation directive}
and `ng-repeat` matches the {@link api/ng.directive:ngRepeat `ngRepeat` directive}.

But {@link api/ng.directive:ngRepeat ngRepeat} has a dilemma.

It needs to be able to clone new `<li>` elements for every `action` in `user.actions`.
This initially seems trivial, but it becomes more complicated when you consider that `user.actions`
might have items added to it later. This means that it needs to save a clean copy of the `<li>`
element for cloning purposes.

As new `action`s are inserted, the template `<li>` element needs to be cloned and inserted into `ul`.
But cloning the `<li>` element is not enough. It also needs to compile the `<li>` so that its
directives, like `{{action.description}}`, evaluate against the right {@link api/ng.$rootScope.Scope scope}.


A naive approach to solving this problem would be to simply insert a copy of the `<li>` element and
then compile it.
The problem with this approach is that compiling on every `<li>` element that we clone would duplicate
a lot of the work. Specifically, we'd be traversing `<li>` each time before cloning it to find the
directives. This would cause the compilation process to be slower, in turn making applications
less responsive when inserting new nodes.

The solution is to break the compilation process into two phases:

the **compile phase** where all of the directives are identified and sorted by priority,
and a **linking phase** where any work which "links" a specific instance of the
{@link api/ng.$rootScope.Scope scope} and the specific instance of an `<li>` is performed.

<div class="alert alert-warning">
**Note:** *Link* means setting up listeners on the DOM and setting up `$watch` on the Scope to
keep the two in sync.
</div>

{@link api/ng.directive:ngRepeat `ngRepeat`} works by preventing the compilation process from
descending into the `<li>` element so it can make a clone of the original and handle inserting
and removing DOM nodes itself.

Instead the {@link api/ng.directive:ngRepeat `ngRepeat`} directive compiles `<li>` separately.
The result of the `<li>` element compilation is a linking function which contains all of the
directives contained in the `<li>` element, ready to be attached to a specific clone of the `<li>`
element.

At runtime the {@link api/ng.directive:ngRepeat `ngRepeat`} watches the expression and as items
are added to the array it clones the `<li>` element, creates a new
{@link api/ng.$rootScope.Scope scope} for the cloned `<li>` element and calls the link function
on the cloned `<li>`.



### Understanding How Scopes Work with Transclused Directives

One of the most common use cases for directives is to create reusable components.

Below is a pseudo code showing how a simplified dialog component may work.

<pre>
<div>
  <button ng-click="show=true">show</button>

  <dialog title="Hello {{username}}."
          visible="show"
          on-cancel="show = false"
          on-ok="show = false; doSomething()">
     Body goes here: {{username}} is {{title}}.
  </dialog>
</div>
</pre>

Clicking on the "show" button will open the dialog. The dialog will have a title, which is
data bound to `username`, and it will also have a body which we would like to transclude
into the dialog.

Here is an example of what the template definition for the `dialog` widget may look like.

<pre>
<div ng-show="visible">
  <h3>{{title}}</h3>
  <div class="body" ng-transclude></div>
  <div class="footer">
    <button ng-click="onOk()">Save changes</button>
    <button ng-click="onCancel()">Close</button>
  </div>
</div>
</pre>

This will not render properly, unless we do some scope magic.

The first issue we have to solve is that the dialog box template expects `title` to be defined, but
the place of instantiation would like to bind to `username`. Furthermore the buttons expect the
`onOk` and `onCancel` functions to be present in the scope. This limits the usefulness of the
widget. To solve the mapping issue we use the `locals` to create local variables which the template
expects as follows:

<pre>
  scope: {
    title: '@',             // the title uses the data-binding from the parent scope
    onOk: '&',              // create a delegate onOk function
    onCancel: '&',          // create a delegate onCancel function
    visible: '='            // set up visible to accept data-binding
  }
</pre>

Creating local properties on widget scope creates two problems:

  1. isolation - if the user forgets to set `title` attribute of the dialog widget the dialog
     template will bind to parent scope property. This is unpredictable and undesirable.

  2. transclusion - the transcluded DOM can see the widget locals, which may overwrite the
     properties which the transclusion needs for data-binding. In our example the `title`
     property of the widget clobbers the `title` property of the transclusion.


To solve the issue of lack of isolation, the directive declares a new `isolated` scope. An
isolated scope does not prototypically inherit from the child scope, and therefore we don't have
to worry about accidentally clobbering any properties.

However `isolated` scope creates a new problem: if a transcluded DOM is a child of the widget
isolated scope then it will not be able to bind to anything. For this reason the transcluded scope
is a child of the original scope, before the widget created an isolated scope for its local
variables. This makes the transcluded and widget isolated scope siblings.

This may seem to be unexpected complexity, but it gives the widget user and developer the least
surprise.

Therefore the final directive definition looks something like this:

<pre>
transclude: true,
scope: {
    title: '@',             // the title uses the data-binding from the parent scope
    onOk: '&',              // create a delegate onOk function
    onCancel: '&',          // create a delegate onCancel function
    visible: '='            // set up visible to accept data-binding
},
restrict: 'E',
replace: true
</pre>