我正在使用Node.js和socket.io创建一个私人消息传递系统。我需要在2位用户之间发送最新消息。 get_messages_between_users
功能和get_message_user
功能分别可以正常工作。但是,如果我按如下方式使用它们,则messages
数组在套接字发出数据时保持为空。实际上由于Javascript的异步结构,这是正常的,但是我该如何解决这个问题呢?这是代码。
function get_message_user(message, callback) {
var sql = "SELECT * from users WHERE id = " + message.from_user
var query = dbh.query(sql, function(err, result) {
callback(message, result[0]);
});
};
function get_messages_between_users(user1, user2, callback) {
var sql = "SELECT id, from_user, to_user, content, msg_date, status FROM messages WHERE ((to_user = "+user1+" AND from_user = "+user2+") OR (to_user = "+user2+" AND from_user = "+user1+")) ORDER BY id ASC LIMIT 20;"
var query = dbh.query(sql, function(err, result) {
callback(result);
});
};
io.on('connection',function (socket) {
//Other stuff for handling clients
socket.on("page messages", function (data) {
var messages = [];
get_messages_between_users(socket.session.user_id, data.to_user, function(result){
for(var i = 0; i < result.length; i++){
get_message_user(result[i], function(message, user){
var obj = {id: message.id, user_id: user.id, username: user.username, avatar:user.avatar_url, content: message.content, msg_date:message.msg_date};
messages.push(obj);
});
}
});
//The problem is messages array is empty here.
socket.emit('message history', JSON.stringify(messages));
});
});
我正在message
中使用get_message_user
参数,只是为了绕过变量。
您需要做几件事,以便重构代码来处理所有相互依赖的异步操作:
1]将get_message_user
和get_messages_between_users
函数转换为基于Promise的函数。
([注:如下面的注释中所建议,最佳解决方案是使用当前数据库库支持的基于promise的功能,或切换到使用Promises进行这些操作的功能。可以轻松地将回调函数转换为基于Promise的函数,其中一种是Promisify
2)将socket.on("page message")
回调转换为异步函数
3)在套接字回调中,调用get_messages_between_users
函数,并为异步结果调用await
4)返回结果列表后,在其上循环并使用get_message_user
函数为所有这些异步请求的结果创建对await
异步函数和Promise.all()
的调用数组。
5)返回结果后,在其上映射(或循环),并将数据添加到message数组,并通过socket.emit
函数发出数据
function get_message_user(message) {
// Return a Promise that will be handled via a then() method
return new Promise((resolve,reject)=>{
let sql = "SELECT * from users WHERE id = " + message.from_user;
dbh.query(sql, function(err, result) {
if ( err ){ reject(err); } // Handle Error via a catch() method
resolve([message,result[0]]);
});
})
};
function get_messages_between_users(user1, user2) {
return new Promise((resolve,reject)=>{
let sql = "SELECT id, from_user, to_user, content, msg_date, status FROM messages WHERE ((to_user = "+user1+" AND from_user = "+user2+") OR (to_user = "+user2+" AND from_user = "+user1+")) ORDER BY id ASC LIMIT 20;"
dbh.query(sql, function(err, result) {
if ( err ){ reject(err); }
resolve(result);
});
})
};
io.on('connection',function (socket) {
// We need an async function here, in order to use await on async operations
socket.on("page messages", async function (data) {
let messages = [];
// Wait for the asynchronous operation to give back a result using await
const result = await get_messages_between_users(socket.session.user_id, data.to_user);
const requests = []; // This will hold a list of async requests for each result
for ( let i = 0; i < result.length; i++){
requests.push( get_message_user(result[i]) );
}
const results = await Promise.all(requests);
results.map((result)=>{
let message = result[0];
let user = result[1];
let obj = {
id: message.id,
user_id: user.id,
username: user.username,
avatar:user.avatar_url,
content: message.content,
msg_date:message.msg_date
};
messages.push(obj);
});
socket.emit('message history', JSON.stringify(messages));
});
});
参考:
注意:无需支付Udemy课程费用。您可以在Udacity中查看有关异步编程的这些优秀课程:
警告:正如上面的注释所正确指出的那样,处理问题中发布的SQL查询的代码需要进行大量的重构,以确保代码的安全。提供此答案中的代码仅是为了演示如何处理多个异步请求。其余代码保留不安全和必须重构。