View
====

View is an important part of MVC and is responsible for presenting data to end users.


Basics
------

Yii uses PHP in view templates by default so in a web application a view typically contains some HTML, `echo`, `foreach`
and such basic constructs. It may also contain widget calls. Using complex code in views is considered a bad practice.
Such code should be moved to controller or widgets.

View is typically called from controller action like the following:

```php
public function actionIndex()
{
	return $this->render('index', ['username' => 'samdark']);
}
```

First argument is the view name. In context of the controller Yii will search for its views in `views/site/` where `site`
is controller ID. For details on how view name is resolved please refer to [yii\base\Controller::render] method.
Second argument is data array that contains key-value pairs. Value is available in the view as a variable named the same
as the corresponding key.

So the view for the action above should be in `views/site/index.php` and can be something like:

```php
<p>Hello, <?= $username ?>!</p>
```

Instead of just scalar values you can pass anything else such as arrays or objects.

Widgets
-------

Widgets are a self-contained building blocks for your views. A widget may contain advanced logic, typically takes some
configuration and data and returns HTML. There is a good number of widgets bundled with Yii such as [active form](form.md),
breadcrumbs, menu or [wrappers around bootstrap component framework](bootstrap-widgets.md). Additionally there are
extensions providing additional widgets such as official one for jQueryUI components.

In order to use widget you need to do the following:

```php
// Note that you have to "echo" the result to display it
echo \yii\widgets\Menu::widget(['items' => $items]);

// Passing an array to initialize the object properties
$form = \yii\widgets\ActiveForm::begin([
	'options' => ['class' => 'form-horizontal'],
	'fieldConfig' => ['inputOptions' => ['class' => 'input-xlarge']],
]);
... form inputs here ...
\yii\widgets\ActiveForm::end();
```

In the code above `widget` method is used for a widget that just outputs content while `begin` and `end` are used for a
widget that wraps content between method calls with its own output. In case of the form this output is the `<form>` tag
with some properties set.

Security
--------

One of the main security principles is to always escape output. If violated it leads to script execution and,
most probably, to cross-site scripting known as XSS leading to leaking of admin passwords, making a user to automatically
perform actions etc.

Yii provides a good toolset in order help you escaping your output. The very basic thing to escape is a text without any
markup. You can deal with it like the following:

```php
<?php
use yii\helpers\Html;
?>

<div class="username">
	<?= Html::encode($user->name) ?>
</div>
```

When you want to render HTML it becomes complex so we're delegating the task to excellent
[HTMLPurifier](http://htmlpurifier.org/) library. In order to use it you need to modify your `composer.json` first by
adding the following to `require`:

```javascript
"ezyang/htmlpurifier": "v4.5.0"
```

After it's done run `php composer.phar install` and wait till package is downloaded. Now everything is prepared to use
Yii's HtmlPurifier helper:

```php
<?php
use yii\helpers\HtmlPurifier;
?>

<div class="post">
	<?= HtmlPurifier::process($post->text) ?>
</div>
```

Note that besides HTMLPurifier does excellent job making output safe it's not very fast so consider
[caching result](caching.md).

Alternative template languages
------------------------------

There are official extensions for [Smarty](http://www.smarty.net/) and [Twig](http://twig.sensiolabs.org/). In order
to learn more refer to [Using template engines](template.md) section of the guide.

Using View object in templates
------------------------------

An instance of `yii\web\View` component is available in view templates as `$this` variable. Using it in templates you
can do many useful things including setting page title and meta, registering scripts and accessing the context.

### Setting page title

A common place to set page title are view templates. Since we can access view object with `$this`, setting a title
becomes as easy as:

```php
$this->title = 'My page title';
```

### Adding meta tags

Adding meta tags such as encoding, description, keywords is easy with view object as well:

```php
$this->registerMetaTag(['encoding' => 'utf-8']);
```

The first argument is an map of `<meta>` tag option names and values. The code above will produce:

```html
<meta encoding="utf-8">
```

Sometimes there's a need to have only a single tag of a type. In this case you need to specify the second argument:

```html
$this->registerMetaTag(['description' => 'This is my cool website made with Yii!'], 'meta-description');
$this->registerMetaTag(['description' => 'This website is about funny raccoons.'], 'meta-description');
```

If there are multiple calls with the same value of the second argument (`meta-description` in this case), the latter will
override the former and only a single tag will be rendered:

```html
<meta description="This website is about funny raccoons.">
```

### Registering link tags

`<link>` tag is useful in many cases such as customizing favicon, pointing to RSS feed or delegating OpenID to another
server. Yii view object has a method to work with these:

```php
$this->registerLinkTag([
	'title' => 'Lives News for Yii Framework',
	'rel' => 'alternate',
	'type' => 'application/rss+xml',
	'href' => 'http://www.yiiframework.com/rss.xml/',
]);
```

The code above will result in

```html
<link title="Lives News for Yii Framework" rel="alternate" type="application/rss+xml" href="http://www.yiiframework.com/rss.xml/" />
```

Same as with meta tags you can specify additional argument to make sure there's only one link of a type registered.

### Registering CSS

You can register CSS using `registerCss` or `registerCssFile`. Former is for outputting code in `<style>` tags directly
to the page which is not recommended in most cases (but still valid). Latter is for registering CSS file. In Yii it's
much better to [use asset manager](assets.md) to deal with these since it provides extra features so `registerCssFile`
is manly useful for external CSS files.

```php
$this->registerCss("body { background: #f00; }");
```

The code above will result in adding the following to the head section of the page:

```html
<style>
body { background: #f00; }
</style>
```

If you want to specify additional properties of the style tag, pass array of name-values to the second argument. If you
need to make sure there's only a single style tag use third argument as was mentioned in meta tags description.

```php
$this->registerCssFile("http://example.com/css/themes/black-and-white.css", ['media' => 'print'], 'css-print-theme');
```

The code above will add a link to CSS file to the head section of the page. The CSS will be used only when printing the
page. We're using third argument so one of the views could override it.

### Registering scripts

With View object you can register scripts. There are two dedicated methods for it: `registerJs` for inline scripts
and `registerJsFile` for external scripts. Inline scripts are useful for configuration and dynamically generated code.
The method for adding these can be used as follows:

```php
$this->registerJs("var options = ".json_encode($options).";", View::POS_END, 'my-options');
```

First argument is the actual code where we're converting a PHP array of options to JavaScript one. Second argument
determines where script should be in the page. Possible values are:

- `View::POS_HEAD` for head section.
- `View::POS_BEGIN` for right after opening `<body>`.
- `View::POS_END` for right before closing `</body>`.
- `View::POS_READY` for executing code on document `ready` event. This one registers jQuery automatically.

The last argument is unique script ID that is used to identify code block and replace existing one with the same ID
instead of adding a new one.

External script can be added like the following:

```php
$this->registerJsFile('http://example.com/js/main.js');
```

Same as with external CSS it's preferred to use asset bundles for external scripts.

### Registering asset bundles

As was mentioned earlier it's preferred to use asset bundles instead of using CSS and JavaScript directly. You can get
details on how to define asset bundles in [asset manager](assets.md) section of the guide. As for using already defined
asset bundle, it's very straightforward:

```php
frontend\config\AppAsset::register($this);
```

### Layout

A layout is a very convenient way to represent the part of the page that is common for all or at least for most pages
generated by your application. Typically it includes `<head>` section, footer, main menu and alike elements.
You can fine a fine example of the layout in a [basic application template](apps-basic.md). Here we'll review the very
basic one without any widgets or extra markup.

```php
<?php
use yii\helpers\Html;
?>
<?php $this->beginPage(); ?>
<!DOCTYPE html>
<html lang="<?= Yii::$app->charset ?>">
<head>
	<meta charset="<?= Yii::$app->charset ?>"/>
	<title><?= Html::encode($this->title) ?></title>
	<?php $this->head(); ?>
</head>
<body>
<?php $this->beginBody(); ?>
	<div class="container">
		<?= $content ?>
	</div>
	<footer class="footer">© 2013 me :)</footer>
<?php $this->endBody(); ?>
</body>
</html>
<?php $this->endPage(); ?>
```

In the markup above there's some code. First of all, `$content` is a variable that will contain result of views rendered
with controller's `$this->render()` method.

TBD

### Partials

Often you need to reuse some HTML markup in many views and often it's too simple to create a full-featured widget for it.
In this case you may use partials.

Partial is a view as well. It resides in one of directories under `views` and by convention is often started with `_`.
For example, we need to render a list of user profiles and, at the same time, display individual profile elsewhere.

First we need to define a partial for user profile in `_profile.php`:

```php
<?php
use yii\helpers\Html;
?>

<div class="profile">
	<h2><?= Html::encode($username) ?></h2>
	<p><?= Html::encode($tagline) ?></p>
</div>
```

Then we're using it in `index.php` view where we display a list of users:

```php
<div class="user-index">
	<?php
	foreach ($users as $user) {
		echo $this->render('_profile', [
			'username' => $user->name,
			'tagline' => $user->tagline,
		]);
	}
	?>
</div>
```

Same way we can reuse it in another view displaying a single user profile:

```php
echo $this->render('_profile', [
	'username' => $user->name,
	'tagline' => $user->tagline,
]);
```

### Accessing context

Views are generally used either by controller or by widget. In both cases the object that called view rendering is
available in the view as `$this->context`. For example if we need to print out the current internal request route in a
view rendered by controller we can use the following:

```php
echo $this->context->getRoute();
```

### Caching blocks

To learn about caching of view fragments please refer to [caching](caching.md) section of the guide.

Customizing View component
--------------------------

Since view is also an application component named `view` you can replace it with your own component that extends
from `yii\base\View` or `yii\web\View`. It can be done via application configuration file such as `config/web.php`:

```php
return [
	// ...
	'components' => [
		'view' => [
			'class' => 'app\components\View',
		],
		// ...
	],
];
```