结构型设计模式-外观 Facade

简介

外观模式特点是屏蔽后端复杂的实现逻辑，将其整合成简单的一个或几个接口提供出去，client 只需与 Facade 层提供的简单接口交互即可。

例如银行办业务，后面涉及到很多财务细节，但用户面对的就是一个窗口业务员。

角色

• Facade 外观层
• Subsystem 负责具体事务的子系统们

类图

代码

class Facade
{
    protected $subsystem1;

    protected $subsystem2;

    public function __construct(
        Subsystem1 $subsystem1 = null,
        Subsystem2 $subsystem2 = null
    ) {
        $this->subsystem1 = $subsystem1 ?: new Subsystem1();
        $this->subsystem2 = $subsystem2 ?: new Subsystem2();
    }

    public function operation(): string
    {
        $result = "Facade initializes subsystems:\n";
        $result .= $this->subsystem1->operation1();
        $result .= $this->subsystem2->operation1();
        $result .= "Facade orders subsystems to perform the action:\n";
        $result .= $this->subsystem1->operationN();
        $result .= $this->subsystem2->operationZ();

        return $result;
    }
}

class Subsystem1
{
    public function operation1(): string
    {
        return "Subsystem1: Ready!\n";
    }

    public function operationN(): string
    {
        return "Subsystem1: Go!\n";
    }
}

class Subsystem2
{
    public function operation1(): string
    {
        return "Subsystem2: Get ready!\n";
    }

    public function operationZ(): string
    {
        return "Subsystem2: Fire!\n";
    }
}

function clientCode(Facade $facade)
{
    echo $facade->operation();
}

$subsystem1 = new Subsystem1();
$subsystem2 = new Subsystem2();
$facade = new Facade($subsystem1, $subsystem2);
clientCode($facade);

output:

Facade initializes subsystems:
Subsystem1: Ready!
Subsystem2: Get ready!
Facade orders subsystems to perform the action:
Subsystem1: Go!
Subsystem2: Fire!