2.7. 外观模式

2.7.1. 目的

The primary goal of a Facade Pattern is not to avoid you having to read the manual of a complex API. It’s only a side-effect. The first goal is to reduce coupling and follow the Law of Demeter.

Facade通过嵌入多个（当然，有时只有一个）接口来解耦访客与子系统，当然也降低复杂度。

  • Facade 不会禁止你访问子系统
  • 你可以（应该）为一个子系统提供多个 Facade

因此一个好的 Facade 里面不会有 new 。如果每个方法里都要构造多个对象，那么它就不是 Facade，而是生成器或者[抽象|静态|简单] 工厂 [方法]。

优秀的 Facade 不会有 new，并且构造函数参数是接口类型的。如果你需要创建一个新实例，则在参数中传入一个工厂对象。

2.7.2. UML 图

Alt Facade UML Diagram

2.7.3. 代码

你可以在 GitHub 上找到这些代码

Facade.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\Facade;

class Facade
{
    /**
     * @var OperatingSystem
     */
    private $os;

    /**
     * @var Bios
     */
    private $bios;

    /**
     * @param Bios $bios
     * @param OperatingSystem   $os
     */
    public function __construct(Bios $bios, OperatingSystem $os)
    {
        $this->bios = $bios;
        $this->os = $os;
    }

    public function turnOn()
    {
        $this->bios->execute();
        $this->bios->waitForKeyPress();
        $this->bios->launch($this->os);
    }

    public function turnOff()
    {
        $this->os->halt();
        $this->bios->powerDown();
    }
}

OperatingSystem.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\Facade;

interface OperatingSystem
{
    public function halt();

    public function getName(): string;
}

Bios.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\Facade;

interface Bios
{
    public function execute();

    public function waitForKeyPress();

    public function launch(OperatingSystem $os);

    public function powerDown();
}

2.7.4. 测试

Tests/FacadeTest.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\Facade\Tests;

use DesignPatterns\Structural\Facade\Facade;
use DesignPatterns\Structural\Facade\OperatingSystem;
use PHPUnit\Framework\TestCase;

class FacadeTest extends TestCase
{
    public function testComputerOn()
    {
        /** @var OperatingSystem|\PHPUnit_Framework_MockObject_MockObject $os */
        $os = $this->createMock('DesignPatterns\Structural\Facade\OperatingSystem');

        $os->method('getName')
            ->will($this->returnValue('Linux'));

        $bios = $this->getMockBuilder('DesignPatterns\Structural\Facade\Bios')
            ->setMethods(['launch', 'execute', 'waitForKeyPress'])
            ->disableAutoload()
            ->getMock();

        $bios->expects($this->once())
            ->method('launch')
            ->with($os);

        $facade = new Facade($bios, $os);

        // the facade interface is simple
        $facade->turnOn();

        // but you can also access the underlying components
        $this->assertSame('Linux', $os->getName());
    }
}