一次响应多次回调-Node JS

Question

我收到来自客户端的请求，该请求需要 select 来自两个 table 的数据，来自一个 table 我需要 select 1000 条记录table 两条我只需要 select 两条记录，通过回调处理它。我想要两个回调函数执行后需要发送给我的客户端的响应，如何在node js中实现？

Answer 1

欢迎来到回调地狱！您只需要构建金字塔形式的回调！

示例：

function getDataFromDB (query, successCb, errorCb) {
    executeFirstQuery(query, function(firstQueryData) {
        console.log('First query ok! Now execute second.');

        executeSecondQuery(query, function(secondQueryData) {
            console.log('Second query ok! Now execute successCb().');

            successCb([firstQueryData, secondQueryData]);
        }, function(error) {
            errorCb("Error in second query");
        })
    }, function(error) {
        errorCb("Error in first query");
    });
}

function executeFirstQuery (query, successCb, errorCb) {
    console.log("Execute first query!");
    successCb("Some data from first query");
    // errorCb("Some Error from first query");
}

function executeSecondQuery (query, successCb, errorCb) {
    console.log("Execute second query!");
    successCb("Some from second query");
    // errorCb("Some Error from second query");
}

getDataFromDB("Some sql", function(data) {
    console.log(data);
}, function(error) {
    console.error(error);
});

或者使用 Promise：http://documentup.com/kriskowal/q/

Answer 2

您有以下选择：

遵循节点风格的回调模式。另请阅读，Callback-hell
关注 promises.

回调模式

var f  = function(somedata, cb){
    // pick user from db
    pickUserFromTable1(somedata, function(err, users){
        // callled on completion of pickUserFromDB
        if(err){
            //handle error.
            cb(err, null);
            return;
        }

        pickUserFromTable2(users, function(err, finallySelectedUsers){
            // called on completion of pickUserFromTable2
            if(err){
                // handle error.
                cb(err, null);
                // returning is important. or use else part of if to separate the case
                return;
            }

            // call initiator's callback function on completion of both pickUserFromTable1 and pickUserFromTable2
            cb(null, finallySelectedUsers);
        });
    })
}

注意：这种方法适用于短项目，但如果依赖链很长，则会导致一些维护问题（回调地狱）。

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