laravel的facade原理以及static介绍

之前就说过new static 和new self的区别( php的newstatic和self )。今天在laravel中又看到了后期绑定的介绍,深入学习了一下static的用法。

今天看了laravel的官方文档,介绍了facade,说实话,以前一直用,但总是知其然不知其所以然。今天看了代码,了解了一下原理,同时也了解了static的用法。

那就从facade说起。

在laravel中有很多的facade可供我们使用,均在lluminate\Support\Facades中,说白了facade就是一个静态代理。就拿Cache来说,有时会这样用:

Cache::get('key')

看一下这个类:

class Cache extends Facade
{
    /**
     * Get the registered name of the component.
     *
     * @return string
     */
    protected static function getFacadeAccessor()
    {
        return 'cache';
    }
}

这个类本身没有get方法,那怎么实现的呢?

这就要归功于Facade抽象类了。先把整个抽象类的代码拿出来:

abstract class Facade
{
    /**
     * The application instance being facaded.
     *
     * @var \Illuminate\Contracts\Foundation\Application
     */
    protected static $app;

    /**
     * The resolved object instances.
     *
     * @var array
     */
    protected static $resolvedInstance;

    /**
     * Convert the facade into a Mockery spy.
     *
     * @return void
     */
    public static function spy()
    {
        if (! static::isMock()) {
            $class = static::getMockableClass();

            static::swap($class ? Mockery::spy($class) : Mockery::spy());
        }
    }

    /**
     * Initiate a mock expectation on the facade.
     *
     * @return \Mockery\Expectation
     */
    public static function shouldReceive()
    {
        $name = static::getFacadeAccessor();

        $mock = static::isMock()
                    ? static::$resolvedInstance[$name]
                    : static::createFreshMockInstance();

        return $mock->shouldReceive(...func_get_args());
    }

    /**
     * Create a fresh mock instance for the given class.
     *
     * @return \Mockery\Expectation
     */
    protected static function createFreshMockInstance()
    {
        return tap(static::createMock(), function ($mock) {
            static::swap($mock);

            $mock->shouldAllowMockingProtectedMethods();
        });
    }

    /**
     * Create a fresh mock instance for the given class.
     *
     * @return \Mockery\MockInterface
     */
    protected static function createMock()
    {
        $class = static::getMockableClass();

        return $class ? Mockery::mock($class) : Mockery::mock();
    }

    /**
     * Determines whether a mock is set as the instance of the facade.
     *
     * @return bool
     */
    protected static function isMock()
    {
        $name = static::getFacadeAccessor();

        return isset(static::$resolvedInstance[$name]) &&
               static::$resolvedInstance[$name] instanceof MockInterface;
    }

    /**
     * Get the mockable class for the bound instance.
     *
     * @return string|null
     */
    protected static function getMockableClass()
    {
        if ($root = static::getFacadeRoot()) {
            return get_class($root);
        }
    }

    /**
     * Hotswap the underlying instance behind the facade.
     *
     * @param  mixed  $instance
     * @return void
     */
    public static function swap($instance)
    {
        static::$resolvedInstance[static::getFacadeAccessor()] = $instance;

        if (isset(static::$app)) {
            static::$app->instance(static::getFacadeAccessor(), $instance);
        }
    }

    /**
     * Get the root object behind the facade.
     *
     * @return mixed
     */
    public static function getFacadeRoot()
    {
        return static::resolveFacadeInstance(static::getFacadeAccessor());
    }

    /**
     * Get the registered name of the component.
     *
     * @return string
     *
     * @throws \RuntimeException
     */
    protected static function getFacadeAccessor()
    {
        throw new RuntimeException('Facade does not implement getFacadeAccessor method.');
    }

    /**
     * Resolve the facade root instance from the container.
     *
     * @param  string|object  $name
     * @return mixed
     */
    protected static function resolveFacadeInstance($name)
    {
        if (is_object($name)) {
            return $name;
        }

        if (isset(static::$resolvedInstance[$name])) {
            return static::$resolvedInstance[$name];
        }

        return static::$resolvedInstance[$name] = static::$app[$name];
    }

    /**
     * Clear a resolved facade instance.
     *
     * @param  string  $name
     * @return void
     */
    public static function clearResolvedInstance($name)
    {
        unset(static::$resolvedInstance[$name]);
    }

    /**
     * Clear all of the resolved instances.
     *
     * @return void
     */
    public static function clearResolvedInstances()
    {
        static::$resolvedInstance = [];
    }

    /**
     * Get the application instance behind the facade.
     *
     * @return \Illuminate\Contracts\Foundation\Application
     */
    public static function getFacadeApplication()
    {
        return static::$app;
    }

    /**
     * Set the application instance.
     *
     * @param  \Illuminate\Contracts\Foundation\Application  $app
     * @return void
     */
    public static function setFacadeApplication($app)
    {
        static::$app = $app;
    }

    /**
     * Handle dynamic, static calls to the object.
     *
     * @param  string  $method
     * @param  array   $args
     * @return mixed
     *
     * @throws \RuntimeException
     */
    public static function __callStatic($method, $args)
    {
        $instance = static::getFacadeRoot();

        if (! $instance) {
            throw new RuntimeException('A facade root has not been set.');
        }

        return $instance->$method(...$args);
    }
}

入口是__callStatic。当你调用一个不存在的静态方法时,这个方法会被调用。

然后再看内部的:

    $instance = static::getFacadeRoot();

它是为了获取实例,看一下内部;

 public static function getFacadeRoot()
    {
        return static::resolveFacadeInstance(static::getFacadeAccessor());
    }

这里面的static::getFacadeAccessor()很重要。它调用的是当前子类的getFacadeAccessor()方法,即Cache重构的方法。PHP官方文档把这个叫做后期状态绑定。这个和self完全不同。

resolveFacadeInstance方法其实就是根据容器名称从注册的容器中获取对应的类的实例。随后就调用实际实例的方法。

好了,关于Facades说了这么多,那么,我们为什么需要它呢?直接调用对应的类不好吗?

其实,最最前面的Cache的例子已经说明了,这让我们的调用变得非常的方便。

Cache的getFacadeAccessor返回的是一个字符串cache.

其实cache是一个已经注册到Container中的Provider。其原型:


class CacheServiceProvider extends ServiceProvider
{
    /**
     * Indicates if loading of the provider is deferred.
     *
     * @var bool
     */
    protected $defer = true;

    /**
     * Register the service provider.
     *
     * @return void
     */
    public function register()
    {
        $this->app->singleton('cache', function ($app) {
            return new CacheManager($app);
        });

        $this->app->singleton('cache.store', function ($app) {
            return $app['cache']->driver();
        });

        $this->app->singleton('memcached.connector', function () {
            return new MemcachedConnector;
        });
    }

关于laravel的容器,服务注册等介绍非常号的一篇文章:

laravel容器介绍

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