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TypeScript 外观模式讲解和代码示例

外观是一种结构型设计模式， 能为复杂系统、 程序库或框架提供一个简单 （但有限） 的接口。

尽管外观模式降低了程序的整体复杂度， 但它同时也有助于将不需要的依赖移动到同一个位置。

** 进一步了解外观模式 **

复杂度：******

流行度：******

使用示例： 使用 TypeScript 开发的程序中会经常使用外观模式。 它在与复杂程序库和 API 协作时特别有用。

识别方法： 外观可以通过使用简单接口， 但将绝大部分工作委派给其他类的类来识别。 通常情况下， 外观管理其所使用的对象的完整生命周期。

概念示例

本例说明了外观设计模式的结构并重点回答了下面的问题：

• 它由哪些类组成？
• 这些类扮演了哪些角色？
• 模式中的各个元素会以何种方式相互关联？

** index.ts:  概念示例

/**
 * The Facade class provides a simple interface to the complex logic of one or
 * several subsystems. The Facade delegates the client requests to the
 * appropriate objects within the subsystem. The Facade is also responsible for
 * managing their lifecycle. All of this shields the client from the undesired
 * complexity of the subsystem.
 */
class Facade {
    protected subsystem1: Subsystem1;

    protected subsystem2: Subsystem2;

    /**
     * Depending on your application's needs, you can provide the Facade with
     * existing subsystem objects or force the Facade to create them on its own.
     */
    constructor(subsystem1?: Subsystem1, subsystem2?: Subsystem2) {
        this.subsystem1 = subsystem1 || new Subsystem1();
        this.subsystem2 = subsystem2 || new Subsystem2();
    }

    /**
     * The Facade's methods are convenient shortcuts to the sophisticated
     * functionality of the subsystems. However, clients get only to a fraction
     * of a subsystem's capabilities.
     */
    public operation(): string {
        let 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;
    }
}

/**
 * The Subsystem can accept requests either from the facade or client directly.
 * In any case, to the Subsystem, the Facade is yet another client, and it's not
 * a part of the Subsystem.
 */
class Subsystem1 {
    public operation1(): string {
        return 'Subsystem1: Ready!\n';
    }

    // ...

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

/**
 * Some facades can work with multiple subsystems at the same time.
 */
class Subsystem2 {
    public operation1(): string {
        return 'Subsystem2: Get ready!\n';
    }

    // ...

    public operationZ(): string {
        return 'Subsystem2: Fire!';
    }
}

/**
 * The client code works with complex subsystems through a simple interface
 * provided by the Facade. When a facade manages the lifecycle of the subsystem,
 * the client might not even know about the existence of the subsystem. This
 * approach lets you keep the complexity under control.
 */
function clientCode(facade: Facade) {
    // ...

    console.log(facade.operation());

    // ...
}

/**
 * The client code may have some of the subsystem's objects already created. In
 * this case, it might be worthwhile to initialize the Facade with these objects
 * instead of letting the Facade create new instances.
 */
const subsystem1 = new Subsystem1();
const subsystem2 = new Subsystem2();
const facade = new Facade(subsystem1, subsystem2);
clientCode(facade);

** Output.txt:  执行结果

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