jQuery延迟承诺无序执行？

Question

编辑：重要说明这是使用 jQuery 1.7.2，不，不能从此版本更改

我对 promises 不熟悉，并试图围绕它们进行思考。我正在尝试按顺序执行一系列函数，等待它们完成，然后再创建一些子视图（这是在 Backbone.js 中）。这是我的代码：

initialize: function () {
    console.log('AppView::initialized!');
    var _this = this;

    $.when( _this.processCookies() )
        .then( _this.loadAdScripts() )
        .then( _this.createChildViews() );
},

processCookies: function () {
    var def = $.Deferred();
    console.log('(1) PROCESS COOKIES');
    return def.resolve();
},


/**
 * Instantiates new instances of the child views.
 */
createChildViews: function () {
    var _this = this;
    console.log('(4) CREATING CHILD VIEWS');
},

loadAdScripts: function () {

    var _this = this,
        def = $.Deferred();

    $.when(
        _this.insertScript({
            name: 'example1',
            async: false,
            src: '//www.example.com/script1.js',
        }),
        _this.insertScript({
            is_mobile: is_mobile,
            name: 'example2',
            async: true,
            src: '//example.com/script2.js'
        })
    )
    .done(function () {
        console.log('(3) ALL SCRIPTS LOADED');
        def.resolve();
    });
},

insertScript: function (script) {
    var def = $.Deferred(),
        protocol = (document.location.protocol === 'https:' ? 'https:' : 'http:');

    // dont script 2 on mobile.
    if (script.name === 'example2' && script.is_mobile) {
        console.log('skipping script');
        return def.resolve();
    }

    var promise = $.ajax({
        dataType: 'script',
        cache: false,
        async: script.async,
        url: protocol + script.src,
    });

    promise.done( function () {
        console.log('(2) SINGLE SCRIPT LOADED');
        return def.resolve();
    });


},

所以，这里所需的流程是：

1. processCookies()函数完成后，
2. 执行loadAdScripts函数 2a。 insertScript() 触发，脚本 1 加载 2b。 insertScript() 触发，脚本 2 加载
3. 当两个脚本都完成后，执行createChildViews函数。

所以，观察代码中的console.log() 占位符，我期望在我的控制台中看到：

'(1) PROCESS COOKIES'
'(2) SINGLE SCRIPT LOADED'
'(2) SINGLE SCRIPT LOADED'
'(3) ALL SCRIPTS LOADED'
'(4) CREATING CHILD VIEWS'

然而我实际上看到的是：

'(1) PROCESS COOKIES'
'(3) ALL SCRIPTS LAODED'
'(4) CREATING CHILD VIEWS'
'(2) SINGLE SCRIPT LOADED'
'(2) SINGLE SCRIPT LOADED'

我的承诺有什么问题，为什么它们没有按预期的顺序执行？

Answer 1

$.when( _this.processCookies() )
        .then( _this.loadAdScripts() )
        .then( _this.createChildViews() );

似乎是立即调用loadScripts()、createChildViews()，而是尝试在.then(_this.loadAdScripts)回调中引用函数名称。

return def.resolve() 返回 jQuery.Deferred() 对象，而不是 jQuery 承诺对象；尝试在.resolve() 之后添加.promise() 以返回jQuery 承诺对象。

jQuery.ajax() 返回 jQuery 承诺对象；不需要创建新的 jQuery $.Deferred() 对象，可以返回 $.ajax()

initialize: function () {
    console.log('AppView::initialized!');
    var _this = this;

    $.when( _this.processCookies() )
        // reference function name, not invoked
        .then( _this.loadAdScripts )
        .then( _this.createChildViews );
},

processCookies: function () {
    // no asynchronous operations appear here, 
    // no need to include `$.Deferred()` or `.promise()` object
    // var def = $.Deferred();
    console.log('(1) PROCESS COOKIES');
    // return jQuery promise object, not deferred object
    // return def.resolve().promise();
},


/**
 * Instantiates new instances of the child views.
 */
createChildViews: function () {
    var _this = this;
    console.log('(4) CREATING CHILD VIEWS');
},

loadAdScripts: function () {

    //var _this = this,
    //    def = $.Deferred();

    return $.when(
        _this.insertScript({
            name: 'example1',
            async: false,
            src: '//www.example.com/script1.js',
        }),
        _this.insertScript({
            is_mobile: is_mobile,
            name: 'example2',
            async: true,
            src: '//example.com/script2.js'
        })
    )
    .done(function () {
        console.log('(3) ALL SCRIPTS LOADED');
        // def.resolve();
    });
},

insertScript: function (script) {
    // var def = $.Deferred(),
        protocol = (document.location.protocol === 'https:' ? 'https:' : 'http:');

    // dont script 2 on mobile.
    // if (script.name === 'example2' && script.is_mobile) {
    //    console.log('skipping script');
    //    return def.resolve().promise();
    // }

    var promise = script.name === 'example2' && script.is_mobile 
                  ? $.when() 
                  : $.ajax({
                      dataType: 'script',
                      cache: false,
                      async: script.async,
                      url: protocol + script.src,
                    });

    promise.done( function () {
        console.log('(2) SINGLE SCRIPT LOADED');
        // def.resolve();
    });    
},

---

编辑：重要说明这是使用 jQuery 1.7.2，不，它不能 从这个版本改变

编辑、更新

如果不修改源代码，使用 jQuery 版本 1.7.2 可能无法实现预期的顺序。

在deferred.then 更新后，使用 jQuery 版本 1.8+ 时出现返回预期顺序；见http://blog.jquery.com/2012/08/09/jquery-1-8-released/，http://bugs.jquery.com/ticket/11010

1.8.0

var dfd = {
    initialize: function () {
        console.log('AppView::initialized!');
        _this = dfd;

        $.when(_this.processCookies())
        // reference function name, not invoked
        .then(_this.loadAdScripts)
        .then(_this.createChildViews);
    },

    processCookies: function () {
        // no asynchronous operations appear here, 
        // no need to include `$.Deferred()` or `.promise()` object
        var def = $.Deferred(function (d) {
            setTimeout(function () {
                d.resolve('(1) PROCESS COOKIES')
            }, Math.floor(Math.random() * 1000));
        }).promise();
        def.then(function (msg) {
            console.log(msg);
        });
        return def.promise()
    },


    /**
     * Instantiates new instances of the child views.
     */
    createChildViews: function () {
        _this = dfd;
        console.log('(4) CREATING CHILD VIEWS');
    },

    loadAdScripts: function () {
        _this = dfd;
        return $.when.apply(_this, [_this.insertScript({
            name: 'example1',
            async: false,
            src: '//www.example.com/script1.js',
        }),
        _this.insertScript({
            is_mobile: true,
            name: 'example2',
            async: true,
            src: '//example.com/script2.js'
        })]).then(function () {
            console.log('(3) ALL SCRIPTS LOADED');
        })
    },

    insertScript: function (script) {
        // var def = $.Deferred(),
        protocol = (document.location.protocol === 'https:' ? 'https:' : 'http:');

        // dont script 2 on mobile.
        // if (script.name === 'example2' && script.is_mobile) {
        //    console.log('skipping script');
        //    return def.resolve();
        // }

        var promise = $.when( script.name === 'example2' && script.is_mobile ? $.Deferred(function (d) {
            setTimeout(function () {
                d.resolve('(2) skipping script', protocol + script.src)
            }, Math.floor(Math.random() * 1000))
        }).promise() : $.Deferred(function (d) {
            setTimeout(function () {
                d.resolve('(2) SINGLE SCRIPT LOADED', protocol + script.src)
            }, Math.floor(Math.random() * 1000))
        }).promise())
        
        return promise.then(function(msg) {
          console.log(msg)
        });
    }
};

dfd.initialize();

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.0/jquery.min.js"></script>

1.72

var dfd = {
    initialize: function () {
        console.log('AppView::initialized!');
        _this = dfd;

        $.when(_this.processCookies())
        // reference function name, not invoked
        .then(_this.loadAdScripts)
        .then(_this.createChildViews);
    },

    processCookies: function () {
        // no asynchronous operations appear here, 
        // no need to include `$.Deferred()` or `.promise()` object
        var def = $.Deferred(function (d) {
            setTimeout(function () {
                d.resolve('(1) PROCESS COOKIES')
            }, Math.floor(Math.random() * 1000));
        }).promise();
        def.then(function (msg) {
            console.log(msg);
        });
        return def.promise()
    },


    /**
     * Instantiates new instances of the child views.
     */
    createChildViews: function () {
        _this = dfd;
        console.log('(4) CREATING CHILD VIEWS');
    },

    loadAdScripts: function () {
        _this = dfd;
        return $.when.apply(_this, [_this.insertScript({
            name: 'example1',
            async: false,
            src: '//www.example.com/script1.js',
        }),
        _this.insertScript({
            is_mobile: true,
            name: 'example2',
            async: true,
            src: '//example.com/script2.js'
        })]).then(function () {
            console.log('(3) ALL SCRIPTS LOADED');
        })
    },

    insertScript: function (script) {
        // var def = $.Deferred(),
        protocol = (document.location.protocol === 'https:' ? 'https:' : 'http:');

        // dont script 2 on mobile.
        // if (script.name === 'example2' && script.is_mobile) {
        //    console.log('skipping script');
        //    return def.resolve();
        // }

        var promise = $.when( script.name === 'example2' && script.is_mobile ? $.Deferred(function (d) {
            setTimeout(function () {
                d.resolve('(2) skipping script', protocol + script.src)
            }, Math.floor(Math.random() * 1000))
        }).promise() : $.Deferred(function (d) {
            setTimeout(function () {
                d.resolve('(2) SINGLE SCRIPT LOADED', protocol + script.src)
            }, Math.floor(Math.random() * 1000))
        }).promise())
        
        return promise.then(function(msg) {
          console.log(msg)
        });
    }
};

dfd.initialize();

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>