【websocket】小白快速上手flask-socketio
大家好，我是一个初级的Python开发工程师。本文是结合官方教程和代码案例，简单说下我对flask-socketio的使用理解。
一、websocket简介
websocket 说白一点就是，建立客户端和服务端双向通讯通道， 服务器可以主动向客户端发消息。
二、flask-socketio理解与使用
1. 环境准备：Python3.7
pip install eventlet==0.33.3 pip install flask-socketio==5.8.0 pip install flask==1.1.4
2. 代码来自官方教程
下面的代码亲测可用，请放心食用。
（1）项目结构
（2）app.py代码
from threading import Lock from flask import Flask, render_template, session, request, copy_current_request_context from flask_socketio import SocketIO, emit, join_room, leave_room, close_room, rooms, disconnect # Set this variable to "threading", "eventlet" or "gevent" to test the # different async modes, or leave it set to None for the application to choose # the best option based on installed packages. async_mode = None app = Flask(__name__) app.config['SECRET_KEY'] = 'secret!' socketio = SocketIO(app, async_mode=async_mode) thread = None thread_lock = Lock() def background_thread(): """Example of how to send server generated events to clients.""" count = 0 while True: socketio.sleep(10) count += 1 socketio.emit('my_response', {'data': 'Server generated event', 'count': count}) @app.route('/') def index(): return render_template('index.html', async_mode=socketio.async_mode) @socketio.event def my_event(message): session['receive_count'] = session.get('receive_count', 0) + 1 emit('my_response', {'data': message['data'], 'count': session['receive_count']}) @socketio.event def my_broadcast_event(message): session['receive_count'] = session.get('receive_count', 0) + 1 emit('my_response', {'data': message['data'], 'count': session['receive_count']}, broadcast=True) @socketio.event def join(message): join_room(message['room']) session['receive_count'] = session.get('receive_count', 0) + 1 emit('my_response', {'data': 'In rooms: ' + ', '.join(rooms()), 'count': session['receive_count']}) @socketio.event def leave(message): leave_room(message['room']) session['receive_count'] = session.get('receive_count', 0) + 1 emit('my_response', {'data': 'In rooms: ' + ', '.join(rooms()), 'count': session['receive_count']}) @socketio.on('close_room') def on_close_room(message): session['receive_count'] = session.get('receive_count', 0) + 1 emit('my_response', {'data': 'Room ' + message['room'] + ' is closing.', 'count': session['receive_count']}, to=message['room']) close_room(message['room']) @socketio.event def my_room_event(message): session['receive_count'] = session.get('receive_count', 0) + 1 emit('my_response', {'data': message['data'], 'count': session['receive_count']}, to=message['room']) @socketio.event def disconnect_request(): @copy_current_request_context def can_disconnect(): disconnect() session['receive_count'] = session.get('receive_count', 0) + 1 # for this emit we use a callback function # when the callback function is invoked we know that the message has been # received and it is safe to disconnect emit('my_response', {'data': 'Disconnected!', 'count': session['receive_count']}, callback=can_disconnect) @socketio.event def my_ping(): emit('my_pong') @socketio.event def connect(): global thread with thread_lock: if thread is None: thread = socketio.start_background_task(background_thread) emit('my_response', {'data': 'Connected', 'count': 0}) @socketio.on('disconnect') def test_disconnect(): print('Client disconnected', request.sid) if __name__ == '__main__': socketio.run(app, host='0.0.0.0', debug=True)
（3）index.html代码
<!DOCTYPE HTML> <html> <head> <title>Flask-SocketIO Test</title> <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.min.js" integrity="sha512-bLT0Qm9VnAYZDflyKcBaQ2gg0hSYNQrJ8RilYldYQ1FxQYoCLtUjuuRuZo+fjqhx/qtq/1itJ0C2ejDxltZVFg==" crossorigin="anonymous"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/socket.io/3.0.4/socket.io.js" integrity="sha512-aMGMvNYu8Ue4G+fHa359jcPb1u+ytAF+P2SCb+PxrjCdO3n3ZTxJ30zuH39rimUggmTwmh2u7wvQsDTHESnmfQ==" crossorigin="anonymous"></script> <script type="text/javascript" charset="utf-8"> $(document).ready(function() { // Connect to the Socket.IO server. // The connection URL has the following format, relative to the current page: // http[s]://<domain>:<port>[/<namespace>] var socket = io.connect('http://' + document.domain + ':' + location.port); // Event handler for new connections. // The callback function is invoked when a connection with the // server is established. socket.on('connect', function() { socket.emit('my_event', {data: 'I\'m connected!'}); }); // Event handler for server sent data. // The callback function is invoked whenever the server emits data // to the client. The data is then displayed in the "Received" // section of the page. socket.on('my_response', function(msg, cb) { $('#log').append('<br>' + $('<div/>').text('Received #' + msg.count + ': ' + msg.data).html()); if (cb) cb(); }); // Interval function that tests message latency by sending a "ping" // message. The server then responds with a "pong" message and the // round trip time is measured. var ping_pong_times = []; var start_time; window.setInterval(function() { start_time = (new Date).getTime(); $('#transport').text(socket.io.engine.transport.name); socket.emit('my_ping'); }, 1000); // Handler for the "pong" message. When the pong is received, the // time from the ping is stored, and the average of the last 30 // samples is average and displayed. socket.on('my_pong', function() { var latency = (new Date).getTime() - start_time; ping_pong_times.push(latency); ping_pong_times = ping_pong_times.slice(-30); // keep last 30 samples var sum = 0; for (var i = 0; i < ping_pong_times.length; i++) sum += ping_pong_times[i]; $('#ping-pong').text(Math.round(10 * sum / ping_pong_times.length) / 10); }); // Handlers for the different forms in the page. // These accept data from the user and send it to the server in a // variety of ways $('form#emit').submit(function(event) { socket.emit('my_event', {data: $('#emit_data').val()}); return false; }); $('form#broadcast').submit(function(event) { socket.emit('my_broadcast_event', {data: $('#broadcast_data').val()}); return false; }); $('form#join').submit(function(event) { socket.emit('join', {room: $('#join_room').val()}); return false; }); $('form#leave').submit(function(event) { socket.emit('leave', {room: $('#leave_room').val()}); return false; }); $('form#send_room').submit(function(event) { socket.emit('my_room_event', {room: $('#room_name').val(), data: $('#room_data').val()}); return false; }); $('form#close').submit(function(event) { socket.emit('close_room', {room: $('#close_room').val()}); return false; }); $('form#disconnect').submit(function(event) { socket.emit('disconnect_request'); return false; }); }); </script> </head> <body> <h1>Flask-SocketIO Test</h1> <p> Async mode is: <b>{{ async_mode }}</b><br> Current transport is: <b><span id="transport"></span></b><br> Average ping/pong latency: <b><span id="ping-pong"></span>ms</b> </p> <h2>Send:</h2> <form id="emit" method="POST" action='#'> <input type="text" name="emit_data" id="emit_data" placeholder="Message"> <input type="submit" value="Echo"> </form> <form id="broadcast" method="POST" action='#'> <input type="text" name="broadcast_data" id="broadcast_data" placeholder="Message"> <input type="submit" value="Broadcast"> </form> <form id="join" method="POST" action='#'> <input type="text" name="join_room" id="join_room" placeholder="Room Name"> <input type="submit" value="Join Room"> </form> <form id="leave" method="POST" action='#'> <input type="text" name="leave_room" id="leave_room" placeholder="Room Name"> <input type="submit" value="Leave Room"> </form> <form id="send_room" method="POST" action='#'> <input type="text" name="room_name" id="room_name" placeholder="Room Name"> <input type="text" name="room_data" id="room_data" placeholder="Message"> <input type="submit" value="Send to Room"> </form> <form id="close" method="POST" action="#"> <input type="text" name="close_room" id="close_room" placeholder="Room Name"> <input type="submit" value="Close Room"> </form> <form id="disconnect" method="POST" action="#"> <input type="submit" value="Disconnect"> </form> <h2>Receive:</h2> <div id="log"></div> </body> </html>
（4）运行app.py代码，浏览器访问5000端口，如下：
（5）代码理解（最重要的部分！！！）
flask-socketio包的常用方法理解：
1. socketio.on和socketio.event是等价的，都是用来定义事件处理器（event handlers）的。区别是.on的第一个参数是事件名称（event name），.event没有这个参数，而是使用被装饰的函数名作为事件名称。其他参数是一样的。事件名称 connect / disconnect / message / json 都是SocketIO生成的特殊事件名，任何其他的事件名都被视为自定义事件。其他参数还有namespace（命名空间）。
2. send和emit都被服务器用来向客户端发送消息。send直接发送消息，emit需要指定事件和消息。一般情况下，都是使用emit指定事件名发送消息。emit的其他参数有：
A. namespace（命名空间），和事件名配合使用。默认为"/"。
B. broadcast（广播模式True/False），是否向所有客户端Client发送消息。
C. to，通常为room_id，发送给指定房间的所有用户。
D. callback（回调函数），指定回调函数，发送到另一端执行。
启动后的运行流程理解：
1. 启动时的初始运行流程。客户端访问http://host:5000后，触发index.html里面的js代码，客户端执行了后，
var socket = io.connect('http://' + document.domain + ':' + location.port);
客户端和后台服务器建立了连接，注意，此时先触发服务器端的代码：
@socketio.event def connect(): global thread with thread_lock: if thread is None: thread = socketio.start_background_task(background_thread) emit('my_response', {'data': 'Connected', 'count': 0})
然后紧接着触发了客户端的代码：
socket.on('connect', function() {
socket.emit('my_event', {data: 'I\'m connected!'});
});
所以，浏览器请求的web页面 Receive部分：先是 Received #0: Connected，再是 Received #1: I'm connected!
2. 接下来，看看 浏览器请求的web页面 Send部分：
（1）echo：输入123，浏览器会向服务器端的my_event事件处理器发送数据{"data": 123}
socket.emit('my_event', {data: $('#emit_data').val()});
服务器端的my_event事件处理器为：
@socketio.event def my_event(message): session['receive_count'] = session.get('receive_count', 0) + 1 emit('my_response', {'data': message['data'], 'count': session['receive_count']})
可以看到，服务器端在接收到数据后，又向客户端的my_response事件处理器发送数据。在看看index.html里的my_response事件处理器是如何定义的：
socket.on('my_response', function(msg, cb) {
$('#log').append('<br>' + $('<div/>').text('Received #' + msg.count + ': ' + msg.data).html());
if (cb)
cb();
});
最终，浏览器的web页面显示为 Received #2: 123。通过这个例子，也充分展示了websocket的功能，服务端和客户端都主动可以向另一端发送数据。这是有别于http的。http协议只能客户端发起请求，服务端响应请求。服务端无法主动向客户端发送数据。
（2）broadcast暂时不说了。
（3）Join Room：这个和Leave Room是成对使用的。就像一个聊天室一样，加入指定聊天室后，当执行Send to Room，就可以接收这个房间内的所有消息。
（4）Close Room：关闭房间
（5）Disconnect：客户端主动断开连接，客户端触发服务端的disconnect_request事件处理器，
# 客户端 socket.emit('disconnect_request'); # 服务端 @socketio.event def disconnect_request(): @copy_current_request_context def can_disconnect(): disconnect() session['receive_count'] = session.get('receive_count', 0) + 1 # for this emit we use a callback function # when the callback function is invoked we know that the message has been # received and it is safe to disconnect emit('my_response', {'data': 'Disconnected!', 'count': session['receive_count']}, callback=can_disconnect)
服务端收到请求后，会向客户端的my_response事件处理器发送数据，同时发送一个callback回调函数can_disconnect，让客户端执行该函数。
最终浏览器的页面显示：Received #2: Disconnected!
三、写在最后
至此，你应该已经对使用flask-socketio库有了基本的认识了。如果还有不了解的，可以留言交流。
在生产环境中，还需要添加异常处理，比如socketio.on_error()和socketio.on_error_default()。
本文只是入门使用教程，感兴趣的话请大家自行查文档深入理解。
附上官方教程链接：
1. https://blog.miguelgrinberg.com/post/easy-websockets-with-flask-and-gevent
2. https://flask-socketio.readthedocs.io/en/latest/index.html
