# directive

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.)

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

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

## Demo Source Code

## String interpolation

During the compilation process the `compiler` matches text and attributes using the `$interpolate` service to see if they contain embedded expressions. These expressions are registered as `watches` and will update as part of normal `digest` cycle. An example of interpolation is shown here:

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

## 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 `$compile()` method. The method traverses the DOM and matches the directives. If a match is found it is added to the list of directives associated with the given DOM element. Once all directives for a given DOM element have been identified they are sorted by priority and their `compile()` functions are executed. The directive compile function has a chance to modify the DOM structure and is responsible for producing a `link()` function explained next. The `$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 `watches` with the `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.

      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);

### 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:

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

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 `interpolation` directive. `ngRepeat` is another directive. But `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 `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 `scope` and the specific instance of an `li` is performed.

`ngRepeat` works by preventing the compilation process form descending into `li` element. Instead the `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 `ngRepeat` watches the expression and as items are added to the array it clones the `li` element, creates a new `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.

## Demo Source Code

## Writing directives (long version)

The full skeleton of the directive is shown here:

      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;
      });

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:

      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;
      });

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

      var myModule = angular.module(...);

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

### Factory method

The factory method is responsible for creating the directive. It is invoked only once, when the `compiler` matches the directive for the first time. You can perform any initialization work here. The method is invoked using the $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 `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.
    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 normalized element attribute name to their corresponding binding strategy. Valid binding strategies are:

    - `attribute` - one time read of element attribute value and save it to widget scope.
      Given `<widget my-attr='abc'>` and widget definition of `scope: {myAttr:'attribute'}`, then widget scope property `myAttr` will be `"abc"`.

    - `evaluate` - one time evaluation of expression stored in the attribute.
      Given `<widget my-attr='name'>` and widget definition of `scope: {myAttr:'evaluate'}`, and parent scope `{name:'angular'}` then widget scope property `myAttr` will be `"angular"`.

    - `bind` - Set up one way binding from the element attribute to the widget scope.
      Given `<widget my-attr='{{name}}'>` and widget definition of `scope: {myAttr:'bind'}`, and parent scope `{name:'angular'}` then widget scope property `myAttr` will be `"angular"`, but any changes in the parent scope will be reflected in the widget scope.

    - `accessor` - Set up getter/setter function for the expression in the widget element attribute to the widget scope.
      Given `<widget my-attr='name'>` and widget definition of `scope: {myAttr:'prop'}`, and parent scope `{name:'angular'}` then widget scope property `myAttr` will be a function such that `myAttr()` will return `"angular"` and `myAttr('new value')` will update the parent scope `name` property. This is useful for treating the element as a data-model for reading/writing.

    - `expression` - Treat element attribute as an expression to be executed on the parent scope.
      Given `<widget my-attr='doSomething()'>` and widget definition of `scope: {myAttr:'expression'}`, and parent scope `{doSomething:function() {}}` then calling the widget scope function `myAttr` will execute the expression against the parent scope.

- `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.

- `inject` (object hash) - Specifies a way to inject bindings into a controller. Injection definition is a hash of normalized element attribute names to their corresponding binding strategy. Valid binding strategies are:

  - `attribute` - inject attribute value.
    Given `<widget my-attr='abc'>` and widget definition of `inject: {myAttr:'attribute'}`, then `myAttr` will inject `"abc"`.

  - `evaluate` - inject one time evaluation of expression stored in the attribute.
    Given `<widget my-attr='name'>` and widget definition of `inject: {myAttr:'evaluate'}`, and parent scope `{name:'angular'}` then `myAttr` will inject `"angular"`.

  - `accessor` - inject a getter/setter function for the expression in the widget element attribute to the widget scope.
    Given `<widget my-attr='name'>` and widget definition of `inject: {myAttr:'prop'}`, and parent scope `{name:'angular'}` then injecting `myAttr` will inject a function such that `myAttr()` will return `"angular"` and `myAttr('new value')` will update the parent scope `name` property. This is usefull for treating the element as a data-model for reading/writing.

  - `expression` - Inject expression function.
    Given `<widget my-attr='doSomething()'>` and widget definition of `inject: {myAttr:'expression'}`, and parent scope `{doSomething:function() {}}` then injecting `myAttr` will inject a function which when called will execute the expression against the parent scope.

- `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 `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

      function compile(tElement, tAttrs, transclude) { ... }

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 `ngRepeat` or load the contents asynchronously such as `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 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

      function link(scope, iElement, iAttrs, controller) { ... }

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` - `Scope` - The scope to be used by the directive for registering `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 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.

### 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`.

    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);
      });
    }

## 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.

      <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>

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.

      <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>

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:

      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.
      }

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:

    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.
    }

## 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.

## Demo Source Code