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1 、官方文档给出的定义

“Facades 为应用的 服务容器 提供了一个「静态」 接口。Laravel 自带了很多 Facades，可以访问绝大部分功能。Laravel Facades 实际是服务容器中底层类的 「静态代理」 ，相对于传统静态方法，在使用时能够提供更加灵活、更加易于测试、更加优雅的语法。”

如何使用Facades？

2 、Facade 简化调用的例子

我们使用一个 Laravel 中的例子，来说明一下 Facade 是如何简化调用的。我们需要调用设置数据缓存的方法，使用 Facade 的语法如下：

#所有的 Laravel Facades 都定义在 Illuminate\Support\Facades 命名空间下。
#所以，我们可以轻松的使用 Facade ：use Illuminate\Support\Facades\Cache;
Route::get('/cache', function() {// 事先保证执行下面的put方法，将缓存存入// Cache::put('key', 'HelloKang', 10);// 获取缓存项key的内容return Cache::get('key');
}

如果不使用 Facade 来调用，那么调用的语法如下：

Route::get('/cache', function() {   // 获取缓存项key的内容return app('cache')->get('key');
}

语法过程就是先从服务容器中解析出来缓存对象，再利用缓存对象将缓存项提取。

对比这两种使用方式，第一种显而易见，更加直观，简单一些。这就是 Facade 的主要目的。

3 、其它

Facade的更通用的用法是省略前缀Illuminate\Support\Facades，直接use Cache进来，或者直接在代码中以\Cache::get()的方式调用，因为在配置目录config目录下的app.php文件中 aliases 已经为我们导入了类别名。

'aliases' => ['App' => Illuminate\Support\Facades\App::class,'Artisan' => Illuminate\Support\Facades\Artisan::class,'Auth' => Illuminate\Support\Facades\Auth::class,'Blade' => Illuminate\Support\Facades\Blade::class,'Broadcast' => Illuminate\Support\Facades\Broadcast::class,'Bus' => Illuminate\Support\Facades\Bus::class,'Cache' => Illuminate\Support\Facades\Cache::class,'Config' => Illuminate\Support\Facades\Config::class,'Cookie' => Illuminate\Support\Facades\Cookie::class,'Crypt' => Illuminate\Support\Facades\Crypt::class,'DB' => Illuminate\Support\Facades\DB::class,'Eloquent' => Illuminate\Database\Eloquent\Model::class,'Event' => Illuminate\Support\Facades\Event::class,'File' => Illuminate\Support\Facades\File::class,'Gate' => Illuminate\Support\Facades\Gate::class,'Hash' => Illuminate\Support\Facades\Hash::class,'Lang' => Illuminate\Support\Facades\Lang::class,'Log' => Illuminate\Support\Facades\Log::class,'Mail' => Illuminate\Support\Facades\Mail::class,'Notification' => Illuminate\Support\Facades\Notification::class,'Password' => Illuminate\Support\Facades\Password::class,'Queue' => Illuminate\Support\Facades\Queue::class,'Redirect' => Illuminate\Support\Facades\Redirect::class,'Redis' => Illuminate\Support\Facades\Redis::class,'Request' => Illuminate\Support\Facades\Request::class,'Response' => Illuminate\Support\Facades\Response::class,'Route' => Illuminate\Support\Facades\Route::class,'Schema' => Illuminate\Support\Facades\Schema::class,'Session' => Illuminate\Support\Facades\Session::class,'Storage' => Illuminate\Support\Facades\Storage::class,'URL' => Illuminate\Support\Facades\URL::class,'Validator' => Illuminate\Support\Facades\Validator::class,'View' => Illuminate\Support\Facades\View::class,'Excel' => Maatwebsite\Excel\Facades\Excel::class,'Input' => Illuminate\Support\Facades\Input::class,'EasyWeChat' => Overtrue\LaravelWeChat\Facade::class,'Image' => Intervention\Image\Facades\Image::class,'QrCode' => SimpleSoftwareIO\QrCode\Facades\QrCode::class,],

总结：

Facade，就是访问服务方法的快捷语法，不同我们从服务容器中手动解析，直接调用封装好的 Facade 即可完成任务。

看到这里就明白了，缓存Facade，Cache类的实现只是告诉服务容器，我需要的服务名字叫“cache”，你把之前绑定的类实例化后返回给我用。至于“cache”服务是何时绑定到容器中的，这就涉及框架的启动过程以及容器类的内部实现了。