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+@ngdoc overview
+@name angular.module.ng.$compileProvider.directive
+@description
+
+Directives are a way to teach HTML new tricks. During DOM compilation directives are matched
+against the HTML and executed. This allows directives to register behavior, or transform the DOM.
+
+Angular comes with a built in set of directives which are useful for building web applications but
+can be extended such that HTML can be turned into a declarative domain specific language (DSL).
+
+# Invoking directives from HTML
+
+Directives have camel cased names such as 'ngBind'. The directive can be invoked by translating
+the camel case name into snake case with these special characters `:`, `-`, or `_`. Optionally the
+directive can be prefixed with `x-`, or `data-` to make it HTML validator compliant. Here is a
+list of some of the possible directive names: `ng:bind`, `ng-bind`, `ng_bind`, `x-ng-bind` and
+`data-ng-bind`.
+
+The directives can be placed in element names, attributes, class names, as well as comments. Here
+are some equivalent examples of invoking `ngBind`.
+
+<pre>
+  <span ng-bind="exp"></span>
+  <span class="ng-bind: exp;"></span>
+  <ng-bind></ng-bind>
+  <!-- directive: ng-bind exp --!>
+</pre>
+
+Directives can be invoked in many different ways, but are equivalent in the end result as shown in
+the following example.
+
+<doc:example>
+  <doc:source >
+   <script>
+     function Ctrl1($scope) {
+       $scope.name = 'angular';
+     }
+   </script>
+   <div ng-controller="Ctrl1">
+     Hello <input ng-model='name'> <hr/>
+     &ltspan ng:bind="name"&gt <span ng:bind="name"></span> <br/>
+     &ltspan ng_bind="name"&gt <span ng_bind="name"></span> <br/>
+     &ltspan ng-bind="name"&gt <span ng-bind="name"></span> <br/>
+     &ltspan data-ng-bind="name"&gt <span data-ng-bind="name"></span> <br/>
+     &ltspan x-ng-bind="name"&gt <span x-ng-bind="name"></span> <br/>
+     &ltspan class="ng-bind: name;"&gt <span class="ng-bind: name;"></span> <br/>
+   </div>
+  </doc:source>
+  <doc:scenario>
+    it('should load template1.html', function() {
+      expect(element('div[ng-controller="Ctrl1"] span[ng-bind]').text()).toBe('angular');
+    });
+  </doc:scenario>
+</doc:example>
+
+# String interpolation
+
+During the compilation process the {@link angular.module.ng.$compile compiler} matches text and
+attributes using the {@link angular.module.ng.$interpolate $interpolate} service to see if they
+contain embedded expressions. These expressions are registered as {@link
+angular.module.ng.$rootScope.Scope#.watch watches} and will update as part of normal {@link
+angular.module.ng.$rootScope.Scope#.digest digest} cycle. An example of interpolation is shown
+here:
+
+<pre>
+<img src="img/{{username}}.jpg">Hello {{username}}!</img>
+</pre>
+
+# Compilation process, and directive matching
+
+Compilation of HTML happens in three phases:
+
+  1. First the HTML is parsed into DOM using the standard browser API. This is important to
+  realize because the templates must be parsable HTML. This is in contrast to most templating
+  systems that operate on strings, rather then on DOM elements.
+
+  2. The compilation of the DOM is performed by the call to {@link angular.module.ng.$compile
+  $compile()} method. The method traverses the DOM and matches the directives. If a match is found
+  it is added to the list of directives associated with the given DOM element. Once all directives
+  for a given DOM element have been identified they are sorted by priority and their `compile()`
+  functions are executed. The directive compile function has a chance to modify the DOM structure
+  and is responsible for producing a `link()` function explained next. The {@link
+  angular.module.ng.$compile $compile()} method returns a combined linking function, which is a
+  collection of all of the linking functions returned from the individual directive compile
+  functions.
+
+  3. Link the template with scope by calling the liking function returned from the previous step.
+  This in turn will call the linking function of the individual directives allowing them to
+  register any listeners on the elements and set up any {@link
+  angular.module.ng.$rootScope.Scope#.watch watches} with the {@link
+  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 = $compile(template);
+
+  // Step 3: link the compiled template with the scope.
+  linkFn(scope);
+</pre>
+
+## Reasons behind the compile/link separation
+
+At this point you may wonder why is the compile process broken down to a compile and link phase.
+To understand this, lets look at a real world example with repeater:
+
+<pre>
+  Hello {{user}}, you have these actions:
+  <ul>
+    <li ng:repeat="action in user.actions">
+      {{action.description}}
+    </li>
+  </ul>
+</pre>
+
+The short answer is that compile and link separation is needed any time a change in model causes
+a change in DOM structure such as in repeaters.
+
+When the above example is compiled, the compiler visits every node and looks for directives. The
+`{{user}}` is an example of {@link angular.module.ng.$interpolate interpolation} directive. {@link
+angular.module.ng.$compileProvider.directive.ng:repeat ng:repeat} is another directive. But {@link
+angular.module.ng.$compileProvider.directive.ng:repeat ng:repeat} 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 angular.module.ng.$rootScope.Scope
+scope}. A naive method would be to simply insert a copy of the `li` elemnt and then compile it.
+But compiling on every `li` element clone would be slow, since the compilation requires that we
+traverse the DOM tree and look for directives and execute them. If we put the compilation inside a
+repeater which needs to unroll 100 items we would quickly run into performance problem.
+
+The solution is to break the compilation process into two phases the compile phase where all of
+the directives are identified and sorted by priority, and a linking phase where any work which
+links a specific instance of the {@link angular.module.ng.$rootScope.Scope scope} and the specific
+instance of an `li` is performed.
+
+{@link angular.module.ng.$compileProvider.directive.ng:repeat ng:repeat} works by preventing the
+compilation process form descending into `li` element. Instead the {@link
+angular.module.ng.$compileProvider.directive.ng:repeat ng:repeat} 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 angular.module.ng.$compileProvider.directive.ng:repeat ng:repeat}
+watches the expression and as items are added to the array it clones the `li` element, creates a
+new {@link angular.module.ng.$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) {
+       $scope.format = 'M/d/yy h:mm:ss a';
+     }
+
+     angular.module('time', [], function($compileProvider) {
+       // Register the 'myCurrentTime' directive factory method.
+       // We inject $defer and dateFilter service since the factory method is DI.
+       $compileProvider.directive('myCurrentTime', function($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)
+
+The full skeleton of the directive is shown here:
+
+<pre>
+  var $compileProvider = ...;
+
+  $compileProvider.directive('directiveName', function factory(injectables) {
+    var directiveDefinitionObject = {
+      priority: 0,
+      template: '<div></div>',
+      templateUrl: 'directive.html',
+      restrict: 'EACM',
+      scope: false,
+      compile: function compile(tElement, tAttrs) {
+        return {
+          pre: function preLink(scope, iElement, iAttrs) { ... },
+          post: function postLink(scope, iElement, iAttrs) { ... }
+        }
+      },
+      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.
+
+It is rare that you need `preLink` method since most directives use the `postLink` method.
+Therefore the above can be simplified as:
+
+<pre>
+  var $compileProvider = ...;
+
+  $compileProvider.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 $compileProvider = ...;
+
+  $compileProvider.directive('directiveName', function factory(injectables) {
+    return function postLink(scope, iElement, iAttrs) { ... }
+  });
+</pre>
+
+
+## Factory method
+
+The factory method is responsible for creating the directive. It is invoked only once, when the
+{@link angular.module.ng.$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 $injector.invoke} which
+makes it injectable following all of the rules of injection annotation.
+
+## Directive Definition Object
+
+The directive definition object provides instructions to the {@link angular.module.ng.$compile
+compiler}. The attributes are:
+
+  * `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 (this means that 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. It is an error
+    to have two directives on the same element both requesting new scope. The new scope rule does
+    not apply for the root of the template since the root of the template always gets a new scope.
+
+  * `restrict` - String of subset of `EACM` which restricts the directive to a specific directive
+    declaration style.
+
+    * `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 HTML may have
+    `<<content>>` string embedded in itself, in which case the current element content
+    will be transferred there. 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.
+
+  * `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) { ... }
+</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
+angular.module.ng.$compileProvider.directive.ng:repeat ng:repeat} or load the contents
+asynchronously such as {@link angular.module.ng.$compileProvider.directive.ng:view ng:view}. 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
+    angular.module.ng.$compileProvider.directive.Attributes Attributes}
+
+NOTE: The template instance and the link instance may not be the same objects if the template has
+been cloned. For this reason it is not safe to do anything other the DOM transformation.
+Specifically listener registration as not allowed inside the compile function.
+
+## Link function
+
+<pre>
+  function link(scope, iElement, iAttrs) { ... }
+</pre>
+
+Compile function is responsible for registering DOM listeners as well as updating the DOM. It is
+executed after the template has been cloned. This is where most of the directive logic will be
+put.
+
+  * `scope` - {@link angular.module.ng.$rootScope.Scope Scope} - The scope to be used be the
+    directive for registering {@link angular.module.ng.$rootScope.Scope#.watch watches}.
+
+  * `iElement` - instance element - The element where the directive is to be used. It is safe to
+    manipulate the children of the element only in `postLink` function since the children have
+    already been linked.
+
+  * `iAttrs` - instance attributes - Normalized list of attributes declared on this element shared
+    between all directive linking functions. See {@link
+    angular.module.ng.$compileProvider.directive.Attributes Attributes}
+
+
+### Pre link 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 link function
+
+Executed after the child elements are linked. Safe to do DOM transformation in here.
+
+## Attributes
+
+Attributes object is a way of accessing element attributes which:
+
+  * *normalize 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.
+
+
+# Creating Widgets
+
+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';
+       $scope.text = 'Neque porro quisquam est qui dolorem ipsum quia dolor...';
+     }
+
+     angular.module('zippyModule', [], function($compileProvider) {
+       $compileProvider.directive('zippy', function(){
+         return {
+           // This HTML will replace the zippy directive.
+           replace: true,
+           template: '<div>' +
+                       '<div class="title"></div>' +
+                       '<div class="body"><<content>></div>' +
+                     '</div>',
+           // The linking function will add behavior to the template
+           link: function(scope, element, attrs) {
+                 // Title element
+             var title = angular.element(element.children()[0]),
+                 // Opened / closed state
+                 opened = true;
+
+             // Watch the zippy-title attribute, copy changes to title element
+             scope.$watch(
+               function(){ return attrs.zippyTitle; },
+               function(value) { return title.text(value); }
+             );
+
+             // 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>
+
+