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Managing assets

An asset in Yii is a file that is included into the page. It could be CSS, JavaScript or any other file. Framework provides many ways to work with assets from basics such as adding <script src=" tag for a file that is handled by View section to advanced usage such as publishing files that are not under webserve document root, resolving JavaScript dependencies or minifying CSS.

Declaring asset bundle

In order to publish some assets you should declare an asset bundle first. The bundle defines a set of asset files or directories to be published and their dependencies on other asset bundles.

Both basic and advanced application templates contain AppAsset asset bundle class that defines assets required application wide. Let's review basic application asset bundle class:

class AppAsset extends AssetBundle
{
	public $basePath = '@webroot';
	public $baseUrl = '@web';
	public $css = [
		'css/site.css',
	];
	public $js = [
	];
	public $depends = [
		'yii\web\YiiAsset',
		'yii\bootstrap\BootstrapAsset',
	];
}

In the above $basePath specifies web-accessible directory assets are served from. It is a base for relative $css and $js paths i.e. @webroot/css/site.css for css/site.css. Here @webroot is an alias that points to application's web directory.

$baseUrl is used to specify base URL for the same relative $css and $js i.e. @web/css/site.css where @web is an alias that corresponds to your website base URL such as http://example.com/.

In case you have asset files under non web accessible directory, that is the case for any extension, you need to additionally specify $sourcePath. Files will be copied or symlinked from source path to base path prior to being registered. In case source path is used baseUrl is generated automatically at the time of publishing asset bundle.

Dependencies on other asset bundles are specified via $depends property. It is an array that contains fully qualified names of bundle classes that should be published in order for this bundle to work properly.

Here yii\web\YiiAsset adds Yii's JavaScript library while yii\bootstrap\BootstrapAsset includes Bootstrap frontend framework.

Asset bundles are regular classes so if you need to define another one, just create alike class with unique name. This class can be placed anywhere but the convention for it is to be under assets directory of the application.

Registering asset bundle

Asset bundle classes are typically registered in views or, if it's main application asset, in layout. Doing it is as simple as:

use app\assets\AppAsset;
AppAsset::register($this);

Since we're in a view context $this refers to View class.

Overriding asset bundles

Sometimes you need to override some asset bundles application wide. A good example is loading jQuery from CDN instead of your own server. In order to do it we need to configure assetManager application component via config file. In case of basic application it is config/web.php:

return [
    // ...
    'components' => [
        'assetManager' => [
            'bundles' => [
                'yii\web\JqueryAsset' => [
                     'sourcePath' => null,
                     'js' => ['//ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js']
                ],
            ],
        ],
    ],
];

In the above we're adding asset bundle definitions to bundles property of asset manager. Keys there are fully qualified class paths to asset bundle classes we want to override while values are key-value arrays of class properties and corresponding values to set.

Setting sourcePath to null tells asset manager not to copy anything while js overrides local files with a link to CDN.

Enabling symlinks

Asset manager is able to use symlinks instead of copying files. It is turned off by default since symlinks are often disabled on shared hosting. If your hosting environment supports symlinks you certainly should enable the feature via application config:

return [
    // ...
    'components' => [
        'assetManager' => [
            'linkAssets' => true,
        ],
    ],
];

There are two main benefits in enabling it. First it is faster since no copying is required and second is that assets will always be up to date with source files.

Compressing and combining assets

To improve application performance you can compress and then combine several CSS or JS files into lesser number of files therefore reducing number of HTTP requests and overall download size needed to load a web page. Yii provides a console command that allows you to do both.

Preparing configuration

In order to use asset command you should prepare a configuration first. A template for it can be generated using

yii asset/template /path/to/myapp/config.php

The template itself looks like the following:

<?php
/**
 * Configuration file for the "yii asset" console command.
 * Note that in the console environment, some path aliases like '@webroot' and '@web' may not exist.
 * Please define these missing path aliases.
 */
return [
	// The list of asset bundles to compress:
	'bundles' => [
		// 'yii\web\YiiAsset',
		// 'yii\web\JqueryAsset',
	],
	// Asset bundle for compression output:
	'targets' => [
		'app\config\AllAsset' => [
			'basePath' => 'path/to/web',
			'baseUrl' => '',
			'js' => 'js/all-{ts}.js',
			'css' => 'css/all-{ts}.css',
		],
	],
	// Asset manager configuration:
	'assetManager' => [
		'basePath' => __DIR__,
		'baseUrl' => '',
	],
];

In the above keys are properties of AssetController. bundles list contains bundles that should be compressed. These are typically what's used by application. targets contains a list of bundles that define how resulting files will be written. In our case we're writing everything to path/to/web that can be accessed like http://example.com/ i.e. it is website root directory.

Note: in the console environment some path aliases like '@webroot' and '@web' may not exist, so corresponding paths inside the configuration should be specified directly.

JavaScript files are combined, compressed and written to js/all-{ts}.js where {ts} is replaced with current UNIX timestamp.

Providing compression tools

The command relies on external compression tools that are not bundled with Yii so you need to provide CSS and JS compressors which are correspondingly specified via cssCompressor and jsCompression properties. If compressor is specified as a string it is treated as a shell command template which should contain two placeholders: {from} that is replaced by source file name and {to} that is replaced by output file name. Another way to specify compressor is to use any valid PHP callback.

By default for JavaScript compression Yii tries to use Google Closure compiler that is expected to be in a file named compiler.jar.

For CSS compression Yii assumes that YUI Compressor is looked up in a file named yuicompressor.jar.

In order to compress resources with these two you need to download both and place where your yii console bootstrap file is using named mentioned above. Since both are Java tools you need JRE installed.

Performing compression

After configuration is adjusted you can run the compress action, using created config:

yii asset /path/to/myapp/config.php /path/to/myapp/config/assets_compressed.php

Now processing takes some time and finally finished. You need to adjust your web application config to use compressed assets file like the following:

'components' => [
	// ...
	'assetManager' => [
		'bundles' => require /path/to/myapp/config/assets_compressed.php,
	],
],

Using asset converter

Instead of using CSS and JavaScript directly often developers are using their improved versions such as LESS or SCSS for CSS or Microsoft TypeScript for JavaScript. Using these with Yii is easy.

First of all, corresponding compression tools should be installed and should be available from where yii console bootstrap file is. The following lists file extensions and their corresponding conversion tool names that Yii converter recognizes:

LESS: less - lessc
SCSS: scss, sass - sass
Stylus: styl - stylus
CoffeeScript: coffee - coffee
TypeScript: ts - tsc

So if the corresponding tool is installed you can specify any of these in asset bundle:

class AppAsset extends AssetBundle
{
	public $basePath = '@webroot';
	public $baseUrl = '@web';
	public $css = [
		'css/site.less',
	];
	public $js = [
		'js/site.ts',
	];
	public $depends = [
		'yii\web\YiiAsset',
		'yii\bootstrap\BootstrapAsset',
	];
}

In order to adjust conversion tool call parameters or add new ones you can use application config:

// ...
'components' => [
	'assetManager' => [
		'converter' => [
			'class' => 'yii\web\AssetConverter',
			'commands' => [
				'less' => ['css', 'lessc {from} {to} --no-color'],
				'ts' => ['js', 'tsc --out {to} {from}'],
			],
		],
	],
],

In the above we've left two types of extra file extensions. First one is less that can be specified in css part of an asset bundle. Conversion is performed via running lessc {from} {to} --no-color where {from} is replaced with LESS file path while {to} is replaced with target CSS file path. Second one is ts that can be specified in js part of an asset bundle. The command that is run during conversion is in the same format that is used for less.

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