在async.series中调用async.series会产生不可预测的输出

Question

使用caolan的node.js异步库,我一直在尝试调用一个async.series在另一个使用async.series的函数内部使用的函数,但我仍然无法使函数以正确的顺序运行,详情如下:

终端输出显示第一个函数在第一个函数之前被调用,原因不明:

The "sys" module is now called "util". It should have a similar interface.
Starting the second step in the series
Value of b: undefined
Invoking the function firstStep
the value of toObtain is: [object Object]

这是相应的源代码:

var im = require('imagemagick');
var async = require('async');

var toObtain;


var b;
async.series([

function (callback) {
    //It appears that this function is being invoked after the second function.
    //Why is this happening?
    firstStep();
    callback();
},

function (callback) {
    //Why is the output of this function being displayed BEFORE the output of the function above? It's the opposite of the order in which I'm calling the functions.
    console.log("Starting the second step in the series");
    console.log("Value of b: " + b);
}]);


function firstStep(){
    async.series([

    function (next) { // step one - call the function that sets toObtain
        im.identify('kittens.png', function (err, features) {
            if (err) throw err;
            console.log("Invoking the function firstStep");
            toObtain = features;
            //console.log(toObtain);
            b = toObtain.height;
            next(); // invoke the callback provided by async
        });
    },

    function (next) { // step two - display it
        console.log('the value of toObtain is: %s',toObtain.toString());
    }]);
}

Answer 1

经过大约一个小时的实验,我让它正常工作.我只是修改了firstStep函数,以便它将回调函数作为参数,并在firstStep函数的末尾调用回调函数.

var im = require('imagemagick');
var async = require('async');

var toObtain = false;


var b;
async.series([

function (callback) {
    firstStep(callback); //the firstStep function takes a callback parameter and calls the callback when it finishes running. Now everything seems to be working as intended.
},

function (callback) {
    console.log("Starting the second step in the series");
    console.log("Value of b: " + b);
}]);


function firstStep(theCallback){
    async.series([

    function (next) { // step one - call the function that sets toObtain
        im.identify('kittens.png', function (err, features) {
            if (err) throw err;
            console.log("Invoking the function firstStep");
            toObtain = features;
            //console.log(toObtain);
            b = toObtain.height;
            next(); // invoke the callback provided by async
        });
    },

    function (next) { // step two - display it
        console.log('the value of toObtain is: %s',toObtain.toString());
        theCallback();
    }]);
}