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　Web端即时通讯技术：即时通讯技术简单的说就是实现这样一种功能：服务器端可以即时地将数据的更新或变化反应到客户端，例如消息即时推送等功能都是通过这种技术实现的。但是在Web中，由于浏览器的限制，实现即时通讯需要借助一些方法。这种限制出现的主要原因是，一般的Web通信都是浏览器先发送请求到服务器，服务器再进行响应完成数据的现实更新。

　　实现Web端即时通讯的方法：实现即时通讯主要有四种方式，它们分别是轮询、长轮询(comet)、长连接(SSE)、WebSocket。它们大体可以分为两类，一种是在HTTP基础上实现的，包括短轮询、comet和SSE；另一种不是在HTTP基础上实现是，即WebSocket。下面分别介绍一下这四种轮询方式，以及它们各自的优缺点。

当我们要实现一个实时投票系统，或者是实时通讯系统，我们的页面数据总需要更新

我们不能让用户一直去刷新页面。所以就有了轮询，长轮询，以及websock的出现

轮询

既然我想要实时获取后端的数据，那我就每隔2秒给后端发一次请求

这种我们就叫轮询~那它会有一些缺点就是存在延时~就算每秒发一次~也会存在一定的延迟

下面我们看下轮询的代码：

from flask import Flask, render_template, request, jsonifyapp = Flask(__name__)USERS = {
    1: {"name": "悟空", "count": 0},
    2: {"name": "悟能", "count": 0},
    3: {"name": "悟净", "count": 0},}@app.route("/")
def index():
    return render_template("index.html", users=USERS)@app.route("/vote", methods=["POST"])
def vote():
    uid = request.json.get("uid")
    USERS[uid]["count"] += 1
    return "投票成功"@app.route("/get_vote")
def get_vote():
    return jsonify(USERS)if __name__ == '__main__':
    app.run()

app.py

<!DOCTYPE html>
<html lang="en"><head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>Title</title>
    <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
    <script src="https://cdn.bootcss.com/axios/0.19.0-beta.1/axios.js"></script></head>
<body>
<h1>选丑大赛</h1><ul>
    {% for key, value in users.items()%}
        <li id="{{key}}" onclick="vote({{key}})">{{value.name}} ({{value.count}})</li>
    {% endfor %}
</ul><script>
    function vote(uid) {
        axios.request({
            url: "/vote",
            method: "POST",
            data: {
                "uid": uid
            }
        }).then(function (res) {
            console.log(res.data)
        })
    }    function get_vote() {
        axios.request({
            url: "/get_vote",
            method: "GET"
        }).then(function (res) {
             console.log(res)
            for(let key in res.data){
                 let liEle = document.getElementById(key);
                 let username = res.data[key]["name"]
                 let count = res.data[key]["count"]
                 liEle.innerText = `${username} (${count})`
            }
        })
    }    window.onload = function () {
        setInterval(get_vote, 2000)
    }</script></body>
</html>

index.html

长轮询

轮询缺点就是延迟，那么如果前端发送过来请求，如果没有数据的更新

后端的请求就阻塞了，直到有数据返回或者超时再返回，这样延迟就可以得到很好的解决

python中有个queue对象，当我们从这个队列里拿不到值的时候，可以阻塞住请求的

import queue
from flask import Flaskapp = Flask(__name__)
q = queue.Queue()@app.route("/get")
def index():
    try:
        val = q.get(timeout=20)
    except queue.Empty:
        val = "超时"
    return val@app.route("/vote")
def vote():
    q.put("")
    return "投票成功"if __name__ == '__main__':
    app.run()

queueDemo.py

如果我为每个请求都建立一个q对象，然后阻塞住他们的请求，有数据更新的时候，给他们的q对象返回值就可以了。

from flask import Flask, render_template, request, jsonify, session
import queue
import uuidapp = Flask(__name__)
app.secret_key = "lajdgia"USERS = {
    1: {"name": "悟空", "count": 0},
    2: {"name": "悟能", "count": 0},
    3: {"name": "悟净", "count": 0},}
# 为每个用户建立一个q对象
# 以用户的uuid为key 值为q对象
Q_DICT = {}@app.route("/")
def index():
    user_uuid = str(uuid.uuid4())
    session["user_uuid"] = user_uuid
    Q_DICT[user_uuid] = queue.Queue()
    return render_template("index2.html", users=USERS)@app.route("/vote", methods=["POST"])
def vote():
    # 投票 循环q对象的dict 给每个q对象返回值
    uid = request.json.get("uid")
    USERS[uid]["count"] += 1
    for q in Q_DICT.values():
        q.put(USERS)
    return "投票成功"@app.route("/get_vote", methods=["POST", "GET"])
def get_vote():
    # 获取投票结果 去自己的q对象里取值 没有夯住 知道有或者超时返回
    user_uuid = session.get("user_uuid")
    q = Q_DICT[user_uuid]
    try:
        users = q.get(timeout=30)
    except queue.Empty:
        users = ""
    return jsonify(users)if __name__ == '__main__':
    app.run()

app2.py

<!DOCTYPE html>
<html lang="en"><head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>Title</title>
    <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
    <script src="https://cdn.bootcss.com/axios/0.19.0-beta.1/axios.js"></script></head>
<body>
<h1>选丑大赛</h1><ul>
    {% for key, value in users.items()%}
        <li id="{{key}}" onclick="vote({{key}})">{{value.name}} ({{value.count}})</li>
    {% endfor %}
</ul><script>
    function vote(uid) {
        axios.request({
            url: "/vote",
            method: "POST",
            data: {
                "uid": uid
            }
        }).then(function (res) {
            console.log(res.data)
        })
    }    function get_votes() {
        axios.request({
            url: "/get_vote",
            method: "POST"
        }).then(function (res) {
             console.log(res);
            if(res.data != ""){
                 for(let key in res.data){
                 let liEle = document.getElementById(key);
                 let username = res.data[key]["name"]
                 let count = res.data[key]["count"]
                 liEle.innerText = `${username} (${count})`
                }
            }
            get_votes()
        })
    }    window.onload = function () {
        get_votes()
    }</script></body>
</html>

index2.html

websocket

websocket是一个协议，协议规定

连接的时候需要握手，发送的数据需要加密~~连接之后不断开

Flask不带websocket，我们需要下载

下载：pip install gevent-websocket

from flask import Flask, request, render_template
from geventwebsocket.handler import WebSocketHandler
from gevent.pywsgi import WSGIServer
import jsonapp = Flask(__name__)USERS = {
    1: {"name": "悟空", "count": 0},
    2: {"name": "悟能", "count": 0},
    3: {"name": "悟净", "count": 0},}@app.route("/")
def index():
    return render_template("index3.html", users=USERS)WEBSOCKET_LIST = []
@app.route("/vote")
def vote():
    ws = request.environ.get("wsgi.websocket")
    if not ws:
        return "HTTP请求"
    WEBSOCKET_LIST.append(ws)
    while True:
        uid = ws.receive()
        if not uid:
            WEBSOCKET_LIST.remove(ws)
            ws.close()
            break
        uid = int(uid)
        USERS[uid]["count"] += 1
        name = USERS[uid]["name"]
        new_count = USERS[uid]["count"]
        for client in WEBSOCKET_LIST:
            client.send(json.dumps({"uid": uid, "name": name, "count": new_count}))if __name__ == '__main__':
    http_server = WSGIServer(('127.0.0.1', 5000), app, handler_class=WebSocketHandler)
    http_server.serve_forever()

app3.py

<!DOCTYPE html>
<html lang="en"><head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>Title</title>
    <script src="https://cdn.bootcss.com/axios/0.19.0-beta.1/axios.js"></script></head>
<body>
<h1>选丑大赛</h1><ul>
        {% for (key, value) in users.items() %}
            <li onclick="vote({{key}})" id="{{key}}">{{value.name}} ({{value.count}})</li>
        {% endfor%}</ul><script>
    let ws = new WebSocket('ws://127.0.0.1:5000/vote')    function vote(uid) {
        ws.send(uid)
    }
    ws.onmessage = function (event) {
        let data = JSON.parse(event.data);
        let liEle = document.getElementById(data.uid);
        liEle.innerText = `${data.name} (${data.count})`
    }</script></body>
</html>

index3.html

轮询、长轮询、websock

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轮询

长轮询

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