为什么then（）链接的方法不能按顺序运行？

Question

We are trying to execute a number of AJAX calls in a particular order. The following code has methodA, methodB and methodC (each returns an AJAX promise object running async=true ).

They are chained using the then() function in jQuery .

 self.methodA().then( self.methodB() ).then( self.methodC() )

I have made one of the methods slow ( methodB ) (I use a slow URL).

I would expect A ... 10 second wait...then B then C .

Instead I get A, C ....10 second wait and B .

Why is it doing that? Does it have something to do with me using the alert() in an always() function?

Here is my code in a fiddle : http://jsfiddle.net/h8tfrvy4/13/

Code:

function Tester() {
    var self = this;
    this.url = 'https://public.opencpu.org/ocpu/library/';
    this.slowurl = 'http://fake-response.appspot.com/?sleep=5';


    this.save = function() {
        self.methodA().then( self.methodB() ).then( self.methodC() )
    }

    this.methodA = function () {
        var self = this;

        return $.ajax({
            url: self.url,
            async: true
        }).always(function (processedDataOrXHRWrapper, textStatus, xhrWrapperOrErrorThrown) {

            //check for errors... and if OK
            alert('A OK');


        })
    }
    this.methodB = function () {
        var self = this;

        return $.ajax({
            url: self.slowurl,
            async: true
        }).always(function (processedDataOrXHRWrapper, textStatus, xhrWrapperOrErrorThrown) {

            //check for errors... and if OK
            alert('B OK');


        })
    }
    this.methodC = function () {
        var self = this;

        return $.ajax({
            url: self.url,
            async: true
        }).always(function (processedDataOrXHRWrapper, textStatus, xhrWrapperOrErrorThrown) {
            //OK
            alert('C OK');

        })
    }
}
new Tester().save();

Answer 1

This is wrong:

self.methodA().then( self.methodB() ).then( self.methodC() )

You're invoking each method immediately , and passing the promises into then .

If you want each function to wait until the previous finishes, you need to give each then a callback to execute when the previous promise resolves:

self.methodA().then(function () { return self.methodB() }).then(function() { return self.methodC() });

Answer 2

Short and simple:

this.save = function() {
    self.methodA().then( self.methodB ).then( self.methodC )
}

It has bothered me all **&^*$& day that @meagar was right and I was wrong on this one but I was sure that I was right. His answer seemed too complicated and but I was fuzzy headed in the morning and my answer wasn't right either. This is the right answer and it works perfectly when you plug it into the JSFiddle.