@ngdoc overview
@name Directives
@description

Directives are a way to teach HTML new tricks. During DOM compilation directives are matched
against the HTML and executed. This allows directives to register behavior, or transform the DOM.

Angular comes with a built in set of directives which are useful for building web applications but
can be extended such that HTML can be turned into a declarative domain specific language (DSL).

# Invoking directives from HTML

Directives have camel cased names such as 'ngBind'. The directive can be invoked by translating
the camel case name into snake case with these special characters `:`, `-`, or `_`. Optionally the
directive can be prefixed with `x-`, or `data-` to make it HTML validator compliant. Here is a
list of some of the possible directive names: `ng:bind`, `ng-bind`, `ng_bind`, `x-ng-bind` and
`data-ng-bind`.

The directives can be placed in element names, attributes, class names, as well as comments. Here
are some equivalent examples of invoking `myDir`. (However, most directives are restricted to
attribute only.)

<pre>
  <span my-dir="exp"></span>
  <span class="my-dir: exp;"></span>
  <my-dir></my-dir>
  <!-- directive: my-dir exp -->
</pre>

Directives can be invoked in many different ways, but are equivalent in the end result as shown in
the following example.

<doc:example>
  <doc:source >
   <script>
     function Ctrl1($scope) {
       $scope.name = 'angular';
     }
   </script>
   <div ng-controller="Ctrl1">
     Hello <input ng-model='name'> <hr/>
     &ltspan ng:bind="name"&gt <span ng:bind="name"></span> <br/>
     &ltspan ng_bind="name"&gt <span ng_bind="name"></span> <br/>
     &ltspan ng-bind="name"&gt <span ng-bind="name"></span> <br/>
     &ltspan data-ng-bind="name"&gt <span data-ng-bind="name"></span> <br/>
     &ltspan x-ng-bind="name"&gt <span x-ng-bind="name"></span> <br/>
   </div>
  </doc:source>
  <doc:scenario>
    it('should show off bindings', function() {
      expect(element('div[ng-controller="Ctrl1"] span[ng-bind]').text()).toBe('angular');
    });
  </doc:scenario>
</doc:example>

# String interpolation

During the compilation process the {@link api/angular.module.ng.$compile compiler} matches text and
attributes using the {@link api/angular.module.ng.$interpolate $interpolate} service to see if they
contain embedded expressions. These expressions are registered as {@link
api/angular.module.ng.$rootScope.Scope#$watch watches} and will update as part of normal {@link
api/angular.module.ng.$rootScope.Scope#$digest digest} cycle. An example of interpolation is shown
here:

<pre>
<img src="img/{{username}}.jpg">Hello {{username}}!</img>
</pre>

# Compilation process, and directive matching

Compilation of HTML happens in three phases:

  1. First the HTML is parsed into DOM using the standard browser API. This is important to
  realize because the templates must be parsable HTML. This is in contrast to most templating
  systems that operate on strings, rather then on DOM elements.

  2. The compilation of the DOM is performed by the call to {@link api/angular.module.ng.$compile
  $compile()} method. The method traverses the DOM and matches the directives. If a match is found
  it is added to the list of directives associated with the given DOM element. Once all directives
  for a given DOM element have been identified they are sorted by priority and their `compile()`
  functions are executed. The directive compile function has a chance to modify the DOM structure
  and is responsible for producing a `link()` function explained next. The {@link
  api/angular.module.ng.$compile $compile()} method returns a combined linking function, which is a
  collection of all of the linking functions returned from the individual directive compile
  functions.

  3. Link the template with scope by calling the linking function returned from the previous step.
  This in turn will call the linking function of the individual directives allowing them to
  register any listeners on the elements and set up any {@link
  api/angular.module.ng.$rootScope.Scope#$watch watches} with the {@link
  api/angular.module.ng.$rootScope.Scope scope}. The result of this is a live binding between the
  scope and the DOM. A change in the scope is reflected in the DOM.

<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>

## Reasons behind the compile/link separation

At this point you may wonder why is the compile process broken down to a compile and link phase.
To understand this, lets look at a real world example with repeater:

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

The short answer is that compile and link separation is needed any time a change in model causes
a change in DOM structure such as in repeaters.

When the above example is compiled, the compiler visits every node and looks for directives. The
`{{user}}` is an example of {@link api/angular.module.ng.$interpolate interpolation} directive. {@link
api/angular.module.ng.$compileProvider.directive.ngRepeat ngRepeat} is another directive. But {@link
api/angular.module.ng.$compileProvider.directive.ngRepeat ngRepeat} has a dilemma. It needs to be
able to quickly stamp out new `li`s for every `action` in `user.actions`. This means that it needs
to save a clean copy of the `li` element for cloning purposes and as new `action`s are inserted,
the template `li` element needs to be cloned and inserted into `ul`. But cloning the `li` element
is not enough. It also needs to compile the `li` so that its directives such as
`{{action.descriptions}}` evaluate against the right {@link api/angular.module.ng.$rootScope.Scope
scope}. A naive method would be to simply insert a copy of the `li` element and then compile it.
But compiling on every `li` element clone would be slow, since the compilation requires that we
traverse the DOM tree and look for directives and execute them. If we put the compilation inside a
repeater which needs to unroll 100 items we would quickly run into performance problems.

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/angular.module.ng.$rootScope.Scope scope} and the specific
instance of an `li` is performed.

{@link api/angular.module.ng.$compileProvider.directive.ngRepeat ngRepeat} works by preventing the
compilation process form descending into `li` element. Instead the {@link
api/angular.module.ng.$compileProvider.directive.ngRepeat ngRepeat} directive compiles `li`
seperatly. The result of 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 `li`
element. At runtime the {@link api/angular.module.ng.$compileProvider.directive.ngRepeat ngRepeat}
watches the expression and as items are added to the array it clones the `li` element, creates a
new {@link api/angular.module.ng.$rootScope.Scope scope} for the cloned `li` element and calls the
link function on the cloned `li`.

Summary:

  * *compile function* - The compile function is relatively rare in directives, since most
    directives are concerned with working with a specific DOM element instance rather then
    transforming the template DOM element. Any operation which can be shared among the instance of
    directives should be moved to the compile function for performance reasons.

  * *link function* - It is rare for the directive not to have a link function. 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.


# Writing directives (short version)

In this example we will build a directive which displays the current time.

<doc:example module="time">
  <doc:source>
   <script>
     function Ctrl2($scope) {
       $scope.format = 'M/d/yy h:mm:ss a';
     }

     angular.module('time', [])
       // Register the 'myCurrentTime' directive factory method.
       // We inject $defer and dateFilter service since the factory method is DI.
       .directive('myCurrentTime', function($defer, dateFilter) {
         // return the directive link function. (compile function not needed)
         return function(scope, element, attrs) {
           var format,  // date format
               deferId; // deferId, so that we can cancel the time updates

           // used to update the UI
           function updateTime() {
             element.text(dateFilter(new Date(), format));
           }

           // watch the expression, and update the UI on change.
           scope.$watch(attrs.myCurrentTime, function(value) {
             format = value;
             updateTime();
           });

           // schedule update in one second
           function updateLater() {
             // save the deferId for canceling
             deferId = $defer(function() {
               updateTime(); // update DOM
               updateLater(); // schedule another update
             }, 1000);
           }

           // listen on DOM destroy (removal) event, and cancel the next UI update
           // to prevent updating time ofter the DOM element was removed.
           element.bind('$destroy', function() {
             $defer.cancel(deferId);
           });

           updateLater(); // kick of the UI update process.
         }
       });
   </script>
   <div ng-controller="Ctrl2">
     Date format: <input ng-model='format'> <hr/>
     Current time is: <span my-current-time="format"></span
   </div>
  </doc:source>
  <doc:scenario>
  </doc:scenario>
</doc:example>


# Writing directives (long version)

An example skeleton of the directive is shown here, for the complete list see below.

<pre>
  var myModule = angular.module(...);

  myModule.directive('directiveName', function factory(injectables) {
    var directiveDefinitionObject = {
      priority: 0,
      template: '<div></div>',
      templateUrl: 'directive.html',
      replace: false,
      transclude: false,
      restrict: 'A',
      scope: false,
      compile: function compile(tElement, tAttrs, transclude) {
        return {
          pre: function preLink(scope, iElement, iAttrs, controller) { ... },
          post: function postLink(scope, iElement, iAttrs, controller) { ... }
        }
      },
      link: function postLink(scope, iElement, iAttrs) { ... }
    };
    return directiveDefinitionObject;
  });
</pre>

In most cases you will not need such fine control and so the above can be simplified. All of the
different parts of this skeleton are explained in following sections. In this section we are
interested only isomers of this skeleton.

The first step in simplyfing the code is to rely on the deafult values. Therefore the above can be
simplified as:

<pre>
  var myModule = angular.module(...);

  myModule.directive('directiveName', function factory(injectables) {
    var directiveDefinitionObject = {
      compile: function compile(tElement, tAttrs) {
        return function postLink(scope, iElement, iAttrs) { ... }
      }
    };
    return directiveDefinitionObject;
  });
</pre>

Most directives concern themselves only with instances not with template transformations allowing
further simplification:

<pre>
  var myModule = angular.module(...);

  myModule.directive('directiveName', function factory(injectables) {
    return function postLink(scope, iElement, iAttrs) { ... }
  });
</pre>


## Factory method

The factory method is responsible for creating the directive. It is invoked only once, when the
{@link api/angular.module.ng.$compile compiler} matches the directive for the first time. You can
perform any initialization work here. The method is invoked using the {@link
http://localhost:8000/build/docs/api/angular.module.AUTO.$injector#invoke $injector.invoke} which
makes it injectable following all of the rules of injection annotation.

## Directive Definition Object

The directive definition object provides instructions to the {@link api/angular.module.ng.$compile
compiler}. The attributes are:

  * `name` - Name of the current scope. Optional defaults to the name at registration.

  * `priority` - When there are multiple directives defined on a single DOM element, sometimes it
    is necessary to specify the order in which the directives are applied. The `priority` is used
    to sort the directives before their `compile` functions get called. Higher `priority` goes
    first. The order of directives within the same priority is undefined.

  * `terminal` - If set to true then the current `priority` will be the last set of directives
    which will execute (any directives at the current priority will still execute
    as the order of execution on same `priority` is undefined).

  * `scope` - If set to:

    * `true` - then a new scope will be created for this directive. If multiple directives on the
      same element request new scope, only one new scope is created. The new scope rule does not
      apply for the root of the template since the root of the template always gets a new scope.

    * `{}` (object hash) - then a new 'isolate' scope is created. The 'isolate' scope differs from
      normal scope that it does not prototypically inherit from the parent scope. This is useful
      when creating reusable components, which should not accidentally read or modify data in
      parent scope. <br/>
      The 'isolate' scope takes an object hash which defines a set of local scope properties
      derived from the parent scope. These local properties are useful for aliasing values for
      templates. Locals definition is a hash of local scope property to its source:

      * `@` or `@attr` - bind a local scope property to the DOM attribute. The result is always a
        string since DOM attributes are strings. If no `attr` name is specified then the local name
        and attribute name are same. Given `<widget my-attr="hello {{name}}">` and widget definition
        of `scope: { localName:'@myAttr' }`, then widget scope property `localName` will reflect
        the interpolated value of `hello {{name}}`. As the `name` attribute changes so will the
        `localName` property on the widget scope. The `name` is read from the parent scope (not
        component scope).

      * `=` or `=expression` - set up bi-directional binding between a local scope property and the
        parent scope property. If no `attr` name is specified then the local name and attribute
        name are same. Given `<widget my-attr="parentModel">` and widget definition of
        `scope: { localModel:'=myAttr' }`, then widget scope property `localName` will reflect the
        value of `parentModel` on the parent scope. Any changes to `parentModel` will be reflected
        in `localModel` and any changes in `localModel` will reflect in `parentModel`.

      * `&` or `&attr` - provides a way to execute an expression in the context of the parent scope.
        If no `attr` name is specified then the local name and attribute name are same.
        Given `<widget my-attr="count = count + value">` and widget definition of
        `scope: { localFn:'increment()' }`, then isolate scope property `localFn` will point to
        a function wrapper for the `increment()` expression. Often it's desirable to pass data from
        the isolate scope via an expression and to the parent scope, this can be done by passing a
        map of local variable names and values into the expression wrapper fn. For example if the
        expression is `increment(amount)` then we can specify the amount value by calling the
        `localFn` as `localFn({amount: 22})`.

  * `controller` - Controller constructor function. The controller is instantiated before the
    pre-linking phase and it is shared with other directives if they request it by name (see
    `require` attribute). This allows the directives to communicate with each other and augment
    each other behavior. The controller is injectable with the following locals:

    * `$scope` - Current scope associated with the element
    * `$element` - Current element
    * `$attrs` - Current attributes obeject for the element
    * `$transclude` - A transclude linking function pre-bound to the correct transclusion scope:
      `function(cloneLinkingFn)`.

  * `require` - Require another controller be passed into current directive linking function. The
    `require` takes a name of the directive controller to pass in. If no such controller can be
    found an error is raised. The name can be prefixed with:

    * `?` - Don't raise an error. This makes the require dependency optional.
    * `^` - Look for the controller on parent elements as well.


  * `restrict` - String of subset of `EACM` which restricts the directive to a specific directive
    declaration style. If omitted directives are allowed on attributes only.

    * `E` - Element name: `<my-directive></my-directive>`
    * `A` - Attribute: `<div my-directive="exp"></div>`
    * `C` - Class: `<div class="my-directive: exp;"></div>`
    * `M` - Comment: `<!-- directive: my-directive exp -->`

  * `template` - replace the current element with the contents of the HTML. The replacement process
    migrates all of the attributes / classes from the old element to the new one. See Creating
    Widgets section below for more information.

  * `templateUrl` - Same as `template` but the template is loaded from the specified URL. Because
    the template loading is asynchronous the compilation/linking is suspended until the template
    is loaded.

  * `replace` - if set to `true` then the template will replace the current element, rather then
    append the template to the element.

  * `transclude` - compile the content of the element and make it available to the directive.
    Typically used with {@link api/angular.module.ng.$compileProvider.directive.ngTransclude
    ngTransclude}. The advantage of transclusion is that the linking function receives a
    transclusion function which is pre-bound to the correct scope. In a typical setup the widget
    creates an `isolate` scope, but the transclusion is not a child, but a sibling of the `isolate`
    scope. This makes it possible for the widget to have private state, and the transclusion to
    be bound to the parent (pre-`isolate`) scope.

    * `true` - transclude the content of the directive.
    * `'element'` - transclude the whole element including any directives defined at lower priority.


  * `compile`: This is the compile function described in the section below.

  * `link`: This is the link function described in the section below. This property is used only
    if the `compile` property is not defined.

## Compile function

<pre>
  function compile(tElement, tAttrs, transclude) { ... }
</pre>

Compile function deals with transforming the template DOM. Since most directives do not do
template transformation, it is not used often. Examples which require compile functions are
directives which transform template DOM such as {@link
api/angular.module.ng.$compileProvider.directive.ngRepeat ngRepeat} or load the contents
asynchronously such as {@link api/angular.module.ng.$compileProvider.directive.ngView ngView}. The
compile functions takes the following arguments.

  * `tElement` - template element - The element where the directive has been declared. It is
    safe to do template transformation on the element and child elements only.

  * `tAttrs` - template attributes - Normalized list of attributes declared on this element shared
    between all directive compile functions. See {@link
    #Attributes Attributes}

  * `transclude` - A transclude linking function: `function(scope, cloneLinkingFn)`.

NOTE: The template instance and the link instance may not be the same objects if the template has
been cloned. For this reason it is not safe in the compile function to do anything other the DOM
transformation that applies to all DOM clones. Specifically, DOM listener registration should be
done in a linking function rather than in a compile function.

A compile function can have a return value which can be either a function or an object.

* returning a function - is equivalent to registering the linking function via the `link` property
  of the config object when the compile function is empty.

* returning an object with function(s) registered via `pre` and `post` properties - allows you to
  control when a linking function should be called during the linking phase. See info about
  pre-linking and post-linking functions below.


## Linking function

<pre>
  function link(scope, iElement, iAttrs, controller) { ... }
</pre>

Link function is responsible for registering DOM listeners as well as updating the DOM. It is
executed after the template has been cloned. This is where most of the directive logic will be
put.

  * `scope` - {@link api/angular.module.ng.$rootScope.Scope Scope} - The scope to be used by the
    directive for registering {@link api/angular.module.ng.$rootScope.Scope#$watch watches}.

  * `iElement` - instance element - The element where the directive is to be used. It is safe to
    manipulate the children of the element only in `postLink` function since the children have
    already been linked.

  * `iAttrs` - instance attributes - Normalized list of attributes declared on this element shared
    between all directive linking functions. See {@link #Attributes Attributes}

  * `controller` - a controller instance - A controller instance if at least one directive on the
    element defines a controller. The controller is shared among all the directives, which allows
    the directives to use the controllers as a communication channel.



### Pre-linking function

Executed before the child elements are linked. Not safe to do DOM transformation since the
compiler linking function will fail to locate the correct elements for linking.

### Post-linking function

Executed after the child elements are linked. Safe to do DOM transformation in here.

<a name="Attributes"></a>
## Attributes

The attributes object - passed as a parameter in the link() or compile() functions - is a way of
accessing:

  * *normalized attribute names:* Since a directive such as 'ngBind' can be expressed in many ways
    sucha s as 'ng:bind', or 'x-ng-bind', the attributes object allows for a normalize accessed to
    the attributes.

  * *directive inter-communication:* All directives share the same instance of the attributes
    object which allows the directives to use the attributes object as inter directive
    communication.

  * *supports interpolation:* Interpolation attributes are assigned to the attribute object
    allowing other directives to read the interpolated value.

  * *observing interpolated attributes:* Use `$observe` to observe the value changes of attributes
    that contain interpolation (e.g. `src="{{bar}}"`). Not only is this very efficient but it's also
    the only way to easily get the actual value because during the linking phase the interpolation
    hasn't been evaluated yet and so the value is at this time set to `undefined`.

<pre>
function linkingFn(scope, elm, attrs, ctrl) {
  // get the attribute value
  console.log(attrs.ngModel);

  // change the attribute
  attrs.$set('ngModel', 'new value');

  // observe changes to interpolated attribute
  attrs.$observe('ngModel', function(value) {
    console.log('ngModel has changed value to ' + value);
  });
}
</pre>


# Understanding Transclusion and Scopes

It is often desirable to have 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>
</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="show()">
    <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 expect `title` to be defined, but
the place of instantiation would like to bind to `username`. Furthermore the buttons expect `onOk`
as well as `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: 'bind',          // set up title to accept data-binding
    onOk: 'expression',     // create a delegate onOk function
    onCancel: 'expression', // create a delegate onCancel function
    show: 'accessor'        // create a getter/setter function for visibility.
  }
</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 as 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: 'bind',          // set up title to accept data-binding
  onOk: 'expression',     // create a delegate onOk function
  onCancel: 'expression', // create a delegate onCancel function
  show: 'accessor'        // create a getter/setter function for visibility.
}
</pre>

# Creating Components

It is often desirable to replace a single directive with a more complex DOM structure. This
allows the directives to become a short hand for reusable components from which applications
can be built.

Following is an example of building a reusable widget.


<doc:example module="zippyModule">
  <doc:source>
   <script>
     function Ctrl3($scope) {
       $scope.title = 'Lorem Ipsum';
       $scope.text = 'Neque porro quisquam est qui dolorem ipsum quia dolor...';
     }

     angular.module('zippyModule', [])
       .directive('zippy', function(){
         return {
           restrict: 'C',
           // This HTML will replace the zippy directive.
           replace: true,
           transclude: true,
           scope: { title:'@zippyTitle' },
           template: '<div>' +
                       '<div class="title">{{title}}</div>' +
                       '<div class="body" ng-transclude></div>' +
                     '</div>',
           // The linking function will add behavior to the template
           link: function(scope, element, attrs) {
                 // Title element
             var title = angular.element(element.children()[0]),
                 // Opened / closed state
                 opened = true;

             // Clicking on title should open/close the zippy
             title.bind('click', toggle);

             // Toggle the closed/opened state
             function toggle() {
               opened = !opened;
               element.removeClass(opened ? 'closed' : 'opened');
               element.addClass(opened ? 'opened' : 'closed');
             }

             // initialize the zippy
             toggle();
           }
         }
       });
   </script>
   <style>
     .zippy {
       border: 1px solid black;
       display: inline-block;
       width: 250px;
     }
     .zippy.opened > .title:before { content: '▼ '; }
     .zippy.opened > .body { display: block; }
     .zippy.closed > .title:before { content: '► '; }
     .zippy.closed > .body { display: none; }
     .zippy > .title {
       background-color: black;
       color: white;
       padding: .1em .3em;
       cursor: pointer;
     }
     .zippy > .body {
       padding: .1em .3em;
     }
   </style>
   <div ng-controller="Ctrl3">
     Title: <input ng-model="title"> <br>
     Text: <textarea ng-model="text"></textarea>
     <hr>
     <div class="zippy" zippy-title="Details: {{title}}...">{{text}}</div>
   </div>
  </doc:source>
  <doc:scenario>
    it('should bind and open / close', function() {
      input('title').enter('TITLE');
      input('text').enter('TEXT');
      expect(element('.title').text()).toEqual('Details: TITLE...');
      expect(binding('text')).toEqual('TEXT');

      expect(element('.zippy').prop('className')).toMatch(/closed/);
      element('.zippy > .title').click();
      expect(element('.zippy').prop('className')).toMatch(/opened/);
    });
  </doc:scenario>
</doc:example>
