# Directives

Directives are classes which get instantiated as a respones to a particular DOM strcture. By controlling the DOM stracture, what directives are imported, and their selectors, the developer can use the [composition pattern](http://en.wikipedia.org/wiki/Object_composition) to get desirable application behavior.

Directives are the cornerstone af Angular application. We use Directives to break complex problems into smaller more reusable components. Directives, allow the devolper turn HTML into a DSL and than controll the application assembly process.

Angular applications do not have a main method. Instead they have a root Component. Dependency Injection than assembles the directives into a working Angular application.

There are three different kinds of directives (described in mored detailed in later sections). 

1. *Decorators*: can be placed on any DOM element and can be combined with other directives.
2. *Components*: Components have encapsulated view and can configure injectors.
3. *Instantiator*: Is responsible for adding or removing child views in parent view. (i.e. ng-repeat, ng-if)



## CSS Selectors

Decorators are instantiated whenever the decorator CSS selector matches the DOM structure. 

Angular supports these CSS selector constructs:
* Element name: `name`
* Attribute: `[attribute]`
* Attribute has value: `[attribute=value]`
* Attribute contains value: `[attribute*=value]`
* Class: `.class`
* AND operation: `name[attribute]`
* OR operation: `name,.class`

Angular does not support these (and any CSS selector which crosses element boundries):
* Descendant: `body div`
* Direct descendant: `body > div` 
* Adjascent: `div + table`
* Sibling: `div ~ table`
* Wildcard: `*`
* ID: `#id`
* Pseudo selectors: `:pseudo`



Given this DOM:

```<input type="text" required class="primary">```

These CSS selectors will match:
* `input`: Triggers whenever element name is `input`.
* `[required]`: Triggers whenever element contains a required attribute.
* `[type=text]`: Triggers whenever element contains attribute `type` whose value is `text`.
* `.primary`: Triggers whenever element class contains `primary`.

CSS Selectors can be combined:
* `input[type=text]`: Triggers on element name `input` which is of `type` `text`.
* `input[type=text], textarea`: triggers on element name `input` which is of `type` `text` or element name `textarea`



## Decorators

The simplest kind of directive is a decorator. Directives are usefull for encapsulating behavior.

* Multiple decorators can be placed on a single element.
* Decorators do not introduce new evaluation context.
* Decorators are registered througt the `@Decorator` meta-data annotation.

Here is a triavial example of tooltip decorator. The directive will log a tooltip into the console on every time mouse enters a region:

```
@Decorator({
  selector: '[tooltip]', // CSS Selector which triggers the decorator
  bind: {                // List which properties need to be bound
    tooltip: 'text'      //  - DOM element tooltip property should be 
  },                     //    mapped to the directive text property.
  event: {               // List which events need to be mapped.
    mouseover: 'show'    //  - Invoke the show() method ever time 
  }                      //    the mouseover event is fired.
})
class Form {             // Directive controller class, instantiated
                         // when CSS matches.
  text:string;           // text property on the Decorator Controller.

  show(event) {          // Show method which implements the show action.
    console.log(this.text);
  }
}
```

Example of usage:

```<span tooltip="Tooltip text goes here.">Some text here.</span>```

The developer of an applacation can now freely use the `tooltip` attribute wherever the behavior is needed. The code above has tought the browser a new reusable and declarative bahavior.

Notice that databinding will work with this decorator with no further effort as show below.

```<span tooltip="Greetings {{user}}!">Some text here.</span>```



## Components

Component is a directive which uses shadow DOM to create encapsulate visual behavior. Components are tipically used to create UI widgets or to break up the application into smaller components.

* Only one component can be present per DOM element.
* Components CSS selectors usualy trigger on element names. (Best practice)
* Component has its own shadow view which is attached to the element as a Shadow DOM.
* Shadow view context is the component instance. (i.e. template expressions are evaluated against the component instance.)

>> TODO(misko): Configuring the injector

Example of a component:

```
@Component({                      | Component annotation
  selector: 'pane',               | CSS selector on <pane> element
  bind: {                         | List which property need to be bound
    'title': 'title',             |  - title mapped to component title
    'open': 'open'                |  - open mapped to component title
  },                              |
  template: new TemplateConfig({  | Define template to be used with component
    url: 'pane.html',             |  - URL of template HTML
    cssUrl: 'pane.css'            |  - URL of CSS to be used with the component
  })                              |
})                                |
class Pane {                      | Component controller class
  title:string;                   |  - title property 
  open:boolean;
  
  constructor() {
    this.title = '';
    this.open = true;
  }
  
  // Public API
  toggle() => this.open = !this.open;
  open() => this.open = true;
  close() => this.open = false;
}
```

`pane.html`:
```
<div class="outter">
  <h1>{{title}}</h1>
  <div class="inner" [hidden]="!visible">
    <content></content>
  </div>
</div>
```

`pane.css`:
```
.outter, .inner { border: 1px solid blue;}
.h1 {background-color: blue;}
```

Example of usage:
```
<pane #pane title="Example Title">
  Some text to wrap.
</pane>
<button (click)="pane.toggle()">toggle</button>

```



## Instantiator

Instantiator is a directive which can controll instantiation of child views which are then inserted into the DOM. (Examples are `ng-if` and `ng-repeat`.) 

* Instantiators can only be placed on `<template>` elements (or the short hand version which uses `<element template>` attribute.)
* Only one instantiator can be present per DOM template element.
* The instantiator is is created over the `template` element. This is known as the `ViewPort`. 
* Instantiator can insert child views into the `ViewPort`. The child views show up as siblings of the `ViewPort` in the DOM.

>> TODO(misko): Relationship with Injection
>> TODO(misko): Instantiator can not be injected into child Views


```
@Instantiator({
  selector: '[ng-if]',
  bind: {
    'ng-if': 'condition'
  }
})
export class NgIf {
  viewPort: ViewPort;
  view: View;

  constructor(viewPort: ViewPort) {
    this.viewPort = viewPort;
    this.view` = null;
  }

  set condition(value) {
    if (value) {
      if (this.view == null) {
        this.view = this.viewPort.create();
      }
    } else {
      if (this.view != null) {
        this.viewPort.remove(this.view);
        this.view = null;
      }
    }
  }
}
```

## Dependency Injection

Dependency Injection (DI) is a key aspect of directives. DI allows directives to be assembled into different [compositional](http://en.wikipedia.org/wiki/Object_composition) hieranchies. Angular encourages [composition over inheritance](http://en.wikipedia.org/wiki/Composition_over_inheritance) in the application design (but inheritance based approaches are still supported).

When Angular directives are instantiated, the directive can ask for other related directives to be injected into it. By assembing the directives in different order and subtypes the application behavior can be controlled. A good mental model is that DOM structure controlles the directive instantiation graph.

Directive instantiation is triggered by the directive CSS selector matching the DOM structure. The directive in its constructor can ask for other directives or application services. When asking for directives the dependency is locating by fallowing the DOM hieranchy and if not found using the application level injector.

To better understand the kinds of injections which are supported in Angular we have broken them down into use case examples.


### Injecting Services

Service injection is the most straight forward kind of injection which Angular supports. It involves a component configureing the `componentServices` and than letting the directive ask for the configured service. 

This example ilustrates how to inject `MyService` into `House` directive.


```
class MyService {}                   | Assume a service which needs to be injected 
                                     | into a directive.
                                     |
@Component({                         | Assume a top level application component which 
  selector: 'my-app',                | configures the service, template and the 
  componentServices: [MyService],    | directive into which we wish to inject the 
  template: new TemplateConfig({     | service.
    url: 'my_app.html',              |
    directives: [House]              |
  })                                 |
})                                   |
class MyApp {}                       |
                                     |
@Decorator({                         | This is the directive into which we would like 
  selector: '[house]'                | to inject the MyService.
})                                   |
class House {                        |
  constructor(myService:MyService) { | Notice that in the constructor we can simply  
  }                                  | ask for MyService.
}                                    |


```

Assume fallowing DOM structure for `my_app.html`:
```
<div house>     | The house attribute triggers the creation of House directive. 
</div>          | This is equivalent to:
                |   new House(injector.get(MyService));
```


### Injecting other Directives

Injecting other directives into directives fallows similar mechanism as injecting services, but with added constraint of visibility governed by DOM structure.

There are five kinds of visibilities:

* (no annotation): Inject a directives only if it is on the curent element. 
* `@ancestor`: Inject a directive if it is at any element above the current element.
* `@parent`: Inject a directive which is direct parent of the current element.
* `@child`: Inject a list of direct children which match a given type. (Used with `Query`)
* `@descendant`: Inject a list of any children which match a given type. (Used with `Query`)

NOTE: if the injection constraint can not be satisfied by the current visibility constraint, than it is forward to normal injector which may provide a default value for the directive or it may throw an error.

Here is an example of the kinds of injections which can be achieved:


```
@Component({                         |
  selector: 'my-app',                | 
  template: new TemplateConfig({     | 
    url: 'my_app.html',              |
    directives: [Form, FieldSet,     |
      Field, Primary]                |
  })                                 |
})                                   |
class MyApp {}                       |
                                     |
@Decorator({ selector: 'form' })     |
class Form {                         |
  constructor(                       |
    @descendant sets:Query<FieldSet> |
  ) {                                | 
  }                                  | 
}                                    |
                                     |
@Decorator({ selector: 'fieldset' }) |
class FieldSet {                     |
  constructor(                       |
    @child sets:Query<Field>         |
  ) { ... }                          | 
}                                    |
                                     |
@Decorator({ selector: 'field' })    |
class Field {                        |
  constructor(                       |
    @ancestor field:Form,            |
    @parent field:FieldSet,          |
  ) { ... }                          | 
}                                    |
                                     |
@Decorator({ selector: '[primary]'}) |
class Primary {                      |
  constructor(field:Field ) { ... }  | 
}                                    |
```

Assume fallowing DOM structure for `my_app.html`:
```
<form>                         |
  <div>                        |
    <fieldset>                 |
       <field primary></field> |
       <field></field>         |
    </div>                     |
  </fieldset>                  |
</form>                        |  
```


### Shadow DOM effects on Dependency Injection

Shadow DOM provides encapsulation for components, so as a general rule it does not allow directive injections to cross the shadow DOM boundries.



## Further Reading

* [Composition](http://en.wikipedia.org/wiki/Object_composition)
* [Composition over Inheritance](http://en.wikipedia.org/wiki/Composition_over_inheritance)