javascript 访客行为设计模式

Posted 2021-05-03

## Real world example
Consider someone visiting Dubai. They just need a way (i.e. visa) to enter Dubai. After arrival, they can come and visit any place in Dubai on their own without having to ask for permission or to do some leg work in order to visit any place here just let them know of a place and they can visit it. Visitor pattern let's you do just that, it helps you add places to visit so that they can visit as much as they can without having to do any legwork.

## In plain words
Visitor pattern let's you add further operations to objects without having to modify them.

## Wikipedia says
In object-oriented programming and software engineering, the visitor design pattern is a way of separating an algorithm from an object structure on which it operates. A practical result of this separation is the ability to add new operations to existing object structures without modifying those structures. It is one way to follow the open/closed principle.

class Monkey {
    shout() {
        console.log('Ooh oo aa aa!')
    }

    accept(operation) {
        operation.visitMonkey(this)
    }
}

class Lion {
    roar() {
        console.log('Roaaar!')
    }
    
    accept(operation) {
        operation.visitLion(this)
    }
}

class Dolphin {
    speak() {
        console.log('Tuut tuttu tuutt!')
    }
    
    accept(operation) {
        operation.visitDolphin(this)
    }
}


const speak = {
    visitMonkey(monkey){
        monkey.shout()
    },
    visitLion(lion){
        lion.roar()
    },
    visitDolphin(dolphin){
        dolphin.speak()
    }
}


const jump = {
    visitMonkey(monkey) {
        console.log('Jumped 20 feet high! on to the tree!')
    },
    visitLion(lion) {
        console.log('Jumped 7 feet! Back on the ground!')
    },
    visitDolphin(dolphin) {
        console.log('Walked on water a little and disappeared')
    }
}


// IMPLEMENTATION
const monkey = new Monkey()
const lion = new Lion()
const dolphin = new Dolphin()

monkey.accept(speak)   // Ooh oo aa aa!
monkey.accept(jump)    // Jumped 20 feet high! on to the tree!

lion.accept(speak)     // Roaaar!
lion.accept(jump)      // Jumped 7 feet! Back on the ground! 

dolphin.accept(speak)  // Tuut tutt tuutt! 
dolphin.accept(jump)   // Walked on water a little and disappeared