Question

我试图了解这个非阻塞业务。

你能告诉我这里我做错了什么吗？我试图添加一个 retrieveContactName 函数，该函数在写入socket之前从db获取更多信息。我试图让它成为一个回调但我在底部得到错误。

case 'getConversations':
    var sip  = parts[1];
    var pass = parts[2].replace(/[\n\r]/g, ''); //strip that carriage return
    var sql = "SELECT DISTINCT(session) FROM im WHERE sip = "+connection.escape(sip)+" AND password = "+connection.escape(pass)+" ORDER BY timestamp DESC";
    connection.query(sql, function(err, results) {
        if (err) winston.warn(err);  
        for (var i=0; i < results.length; i++) { 
            retrieveContactName(results[i].session, sip, pass, function(value) {
                sock.write('Conversations '+results[i].session+' '+value+'\n');
            });
        }
    });
break;

别处

function retrieveContactName(session, user, pass, callback) {
    var sql = "SELECT `im_from` FROM `im` WHERE `session` = "+connection.escape(session)+" AND `im_to` != "+connection.escape(session)+" AND `sip` = "+connection.escape(user)+"  AND `password` = "+connection.escape(pass)+" LIMIT 1";
    connection.query(sql, function(err, results) {
        if (err) winston.warn(err);  
        if (results.length > 0 ) { 
            callback(results[0].im_from);
        } else {
            callback(session.replace("contact:",""));
        }
    });
}

和我的错误

TypeError: Cannot read property 'session' of undefined
    at /home/ubuntu/socket/server.js:44:47
    at Query._callback (/home/ubuntu/socket/server.js:79:4)
    at Query.end (/home/ubuntu/socket/node_modules/mysql/lib/protocol/sequences/Sequence.js:75:24)
    at Query._handleFinalResultPacket (/home/ubuntu/socket/node_modules/mysql/lib/protocol/sequences/Query.js:143:8)
    at Query.EofPacket (/home/ubuntu/socket/node_modules/mysql/lib/protocol/sequences/Query.js:127:8)
    at Protocol._parsePacket (/home/ubuntu/socket/node_modules/mysql/lib/protocol/Protocol.js:172:24)
    at Parser.write (/home/ubuntu/socket/node_modules/mysql/lib/protocol/Parser.js:62:12)
    at Protocol.write (/home/ubuntu/socket/node_modules/mysql/lib/protocol/Protocol.js:37:16)
    at Socket.ondata (stream.js:38:26)
    at Socket.emit (events.js:88:20)

Answer 1

你的问题归结为没有完全理解范围，基本上问题出现在这里：

for (var i=0; i < results.length; i++) { 
    retrieveContactName(results[i].session, sip, pass, function(value) {
        sock.write('Conversations '+results[i].session+' '+value+'\n');
    });
}

上述内容无法正常运行，因为当您的回调触发i时，您的预期值不会超出预期值...它很可能会等于results.length您是undefined中的results位置。这是因为for很可能会在回调发生之前继续并完成它的执行。这是非阻塞的原则，代码不等待，它继续，并且你的回调必须为此做好准备。

为了使用i的值，或任何可能在回调范围之外更改其值的变量，您需要捕获该值并将其与回调一起存储。有几种方法可以做到这一点，但最好的方法是将你的回调所需的数据作为参数传递 - 所以你需要将results[i].session（传递给retrieveContactName）传递给你的回调。

function retrieveContactName(session, user, pass, callback) {
    var sql = "SELECT `im_from` FROM `im` WHERE `session` = "+connection.escape(session)+" AND `im_to` != "+connection.escape(session)+" AND `sip` = "+connection.escape(user)+"  AND `password` = "+connection.escape(pass)+" LIMIT 1";
    connection.query(sql, function(err, results) {
        if (err) winston.warn(err);  
        if (results.length > 0 ) { 
            callback(results[0].im_from, session);
        } else {
            callback(session.replace("contact:",""), session);
        }
    });
}

然后：

for (var i=0; i < results.length; i++) { 
    retrieveContactName(results[i].session, sip, pass, function(value, session) {
        sock.write('Conversations '+session+' '+value+'\n');
    });
}

显然这只是一个简单的例子，最好传入实际的结果行 - 取决于你想要/需要的东西。

Answer 2

结果[i]可能超出回调范围

case 'getConversations':
var sip  = parts[1];
var pass = parts[2].replace(/[\n\r]/g, ''); //strip that carriage return
var sql = "SELECT DISTINCT(session) FROM im WHERE sip = "+connection.escape(sip)+" AND password = "+connection.escape(pass)+" ORDER BY timestamp DESC";
connection.query(sql, function(err, results) {
    if (err) winston.warn(err);  
    for (var i=0; i < results.length; i++) { 
        retrieveContactName(results[i].session, sip, pass, function(value) {
            sock.write('Conversations '+results[i].session+' '+value+'\n');
        });
    }
});

中断;

你可以在上面放置一个console.log（结果）：

 sock.write('Conversations '+results[i].session+' '+value+'\n');

node.js在mysql结果循环中回调

2 个答案: