责任链模式

• 定义：使多个对象都有机会处理请求，从而避免请求的发送者和接收者之间的耦合关系。将这个对象连成一条链，并沿着这条链传递该请求，直到有一个对象处理它为止。

interface Handler {
    setNext(handler: Handler): Handler;

    handle(request: string): string;
}

abstract class AbstractHandler implements Handler
{
    private nextHandler: Handler;

    public setNext(handler: Handler): Handler {
        this.nextHandler = handler;
        // Returning a handler from here will let us link handlers in a
        // convenient way like this:
        // monkey.setNext(squirrel).setNext(dog);
        return handler;
    }

    public handle(request: string): string {
        if (this.nextHandler) {
            return this.nextHandler.handle(request);
        }

        return null;
    }
}

class MonkeyHandler extends AbstractHandler {
    public handle(request: string): string {
        if (request === 'Banana') {
            return `Monkey: I'll eat the ${request}.`;
        }
        return super.handle(request);

    }
}

class SquirrelHandler extends AbstractHandler {
    public handle(request: string): string {
        if (request === 'Nut') {
            return `Squirrel: I'll eat the ${request}.`;
        }
        return super.handle(request);
    }
}

class DogHandler extends AbstractHandler {
    public handle(request: string): string {
        if (request === 'MeatBall') {
            return `Dog: I'll eat the ${request}.`;
        }
        return super.handle(request);
    }
}

// 使用
const monkey = new MonkeyHandler();
const squirrel = new SquirrelHandler();
const dog = new DogHandler();
// 设置责任链
monkey.setNext(squirrel).setNext(dog);

clientCode(monkey);
function clientCode(handler: Handler) {
    const foods = ['Nut', 'Banana', 'Cup of coffee'];

    for (const food of foods) {
        console.log(`Client: Who wants a ${food}?`);

        const result = handler.handle(food);
        if (result) {
            console.log(`  ${result}`);
        } else {
            console.log(`  ${food} was left untouched.`);
        }
    }
}

优缺点

• 优点：
  • 可以控制处理的顺序
  • 单一职责
  • 开闭原则
• 缺点：部分请求可能未被响应