let a = {
    name: 'a',
    sendLetter: function(target) {
        target.receiveLetter(this)
    }
}
 
let b = {
    name: 'b',
    receiveLetter: function(from) {
        console.log(`${this.name} have received the letter from ${from.name}`)
    }
}
 
//无代理情况
a.sendLetter(b)
    // b have received the letter from a
 
//有代理情况
 
let proxy = { // 代理对象
    name: 'proxy',
    sendLetter: function() {
        b.receiveLetter(this)
    },
    receiveLetter: function(from) {
        console.log(`proxy have received the letter from ${from.name}`)
        this.sendLetter()
    }
}
 
a.sendLetter(proxy)
    //proxy have received the letter from a
    //b have received the letter from proxy

let proxy = { // 代理对象
    name: 'proxy',
    sendLetter: function() {
        b.receiveLetter(this)
    },
    receiveLetter: function(from, letter) {
        console.log(`proxy have received the letter from ${from.name}`)
        if (letter.length > 200) { // 对收信方的"保护"
            console.log(`the letter from ${from.name} is too long`)
            console.log(`proxy have thrown the letter from ${from.name}`)
            return
        }
        this.sendLetter()
    }
}
 
// 使用带length属性的对象来模仿letter
a.sendLetter(proxy, { length: 100 })
    // proxy have received the letter from a
    // b have received the letter from proxy
 
a.sendLetter(proxy, { length: 300 })
    // proxy have received the letter from a
    // the letter from a is too long
    // proxy have thrown the letter from a

let b = {
    name: 'b',
    currentMood: function() { // 模拟b的情绪变化
        if (Math.random() > 0.5)
            return 'mad'
        return 'happy'
    },
    receiveLetter: function(from) {
        console.log(`${this.name} have received the letter from ${from.name}`)
    }
}
 
let proxy = { // 代理对象
    name: 'proxy',
    sendLetter: function() {
        b.receiveLetter(this)
    },
    receiveLetter: function(from) {
        console.log(`proxy have received the letter from ${from.name}`)
        this.listenMood()
    },
    listenMood: function() { // 监听b的心情
        if (b.currentMood() === 'mad') {
            console.log('b is at mad now')
            console.log('proxy will send letter after one second')
            setTimeout(() => {
                this.listenMood()
            }, 1000)
            return;
        }
        this.sendLetter()
    }
}
 
a.sendLetter(proxy)

let proxy = {
    cache: {}, //  存储请求的文件信息
    ajaxMes: function(url) {
        // ... 模拟文件获取，这里将文件获取的操作省略
        return message;
    },
    getMes: function(url) {
        if (!this.cache.url) {
            this.cache.url = this.ajaxMes(url) // 将获取的文件信息放入缓存中
        }
        return this.cache.url // 返回请求的信息
    }
}
 
// 假设有id位btn1,btn2的两个按钮，按下就会发起请求
let btn1 = documnet.getElementById('btn1')
let btn2 = document.getElementById('btn2')
 
 
// 这里的url1,url2为模拟url
btn1.onclick = function() {
    proxy.getMes(url1)
}
 
btn2.onclick = function() {
    proxy.getMes(url2)
}