Windows 8 Javascript应用程序XML对象

我目前正在尝试制作一个HTML/JavaScriptWindows8现代应用程序，我想在其中访问安装目录中的本地XML文件。在阅读了网上的许多想法和代码片段后，我想出了一种访问文件的复杂异步方法，这种方法很有效。但是，这是访问本地XML文件这样简单的操作的最佳/正确方法吗？此外，我希望能够让一个函数加载xml文件，并将XMLDocument对象保存为"全局"变量，以便在按下按钮和其他触发器时，可以访问和解析XMLDocument对象。这就是所有问题的开始，因为一个方法是异步的，然后变量是未定义的，等等…

(function () {
"use strict";
WinJS.UI.Pages.define("/pages/reader/reader.html", {
    // This function is called whenever a user navigates to this page. It
    // populates the page elements with the app's data.
    ready: function (element, options) {
        // TODO: Initialize the page here.
        var button = document.getElementById("changeText");
        button.addEventListener("click", this.buttonClickHandler, false);
        var dropdown = document.getElementById("volumeDropdown");
        dropdown.addEventListener("change", this.volumeChangeHandler, false);
        var loadSettings = new Windows.Data.Xml.Dom.XmlLoadSettings;
        loadSettings.prohibitDtd = false;
        loadSettings.resolveExternals = false;
        //previous attempt, also didn't work:
        //this.xmlDoc = null;
        //this.loadXMLdoc(this, this.testXML);
        //also not working:
        this.getXmlAsync().then(function (doc) {
            var xmlDoc = doc;
        });
        //this never works also, xmlDoc always undefined, or an error:
        //console.log(xmlDoc);            
    },
    buttonClickHandler: function (eventInfo) {
        // doesn't work, xmlDoc undefined or error:
        console.log(xmlDoc);
    },
    volumeChangeHandler: function (eventInfo) {
        var e = document.getElementById("volumeDropdown");
        // of course doesn't work, since I can't save the XMLDocument object into a variable (works otherwise):
        var nodelist2 = xmlDoc.selectNodes('//volume[@name="volumeName"]/chapter/@n'.replace('volumeName', list[0]));
        var volumeLength = nodelist2.length;
        for (var index = 0; index < volumeLength; index++) {
            var option = document.createElement("option");
            option.text = index + 1;
            option.value = index + 1;
            var volumeDropdown = document.getElementById("chapterDropdown");
            volumeDropdown.appendChild(option);
        }
    },
    getXmlAsync: function () {
        return Windows.ApplicationModel.Package.current.installedLocation.getFolderAsync("books").then(function (externalDtdFolder) {
            externalDtdFolder.getFileAsync("book.xml").done(function (file) {
                return Windows.Data.Xml.Dom.XmlDocument.loadFromFileAsync(file);
            })
        })
    },
    loadXMLdoc: function (obj, callback) {
        var loadSettings = new Windows.Data.Xml.Dom.XmlLoadSettings;
        loadSettings.prohibitDtd = false;
        loadSettings.resolveExternals = false;
        Windows.ApplicationModel.Package.current.installedLocation.getFolderAsync("books").then(function (externalDtdFolder) {
            externalDtdFolder.getFileAsync("book.xml").done(function (file) {
                Windows.Data.Xml.Dom.XmlDocument.loadFromFileAsync(file, loadSettings).then(function (doc) {
                    var nodelist = doc.selectNodes("//volume/@name");
                    var list = [];
                    for (var index = 0; index < nodelist.length; index++) {
                        list.push(nodelist[index].innerText);
                    };
                    for (var index = 0; index < list.length; index++) {
                        var option = document.createElement("option");
                        option.text = list[index] + "new!";
                        option.value = list[index];
                        var volumeDropdown = document.getElementById("volumeDropdown");
                        volumeDropdown.appendChild(option);
                    };
                    var nodelist2 = doc.selectNodes('//volume[@name="volumeName"]/chapter/@n'.replace('volumeName', list[0]));
                    var volumeLength = nodelist2.length;
                    for (var index = 0; index < volumeLength; index++) {
                        var option = document.createElement("option");
                        option.text = index + 1;
                        option.value = index + 1;
                        var volumeDropdown = document.getElementById("chapterDropdown");
                        volumeDropdown.appendChild(option);
                    };

                    obj.xmlDoc = doc;
                    callback(obj);
                })
            })
        });
    },
    initializeXML: function (doc, obj) {
        console.log("WE ARE IN INITIALIZEXML NOW")
        obj.xmlDoc = doc;
    },
    testXML: function (obj) {
        console.log(obj.xmlDoc);
    },

});

})（）；

总之，在所有这些复杂的方法都失败的情况下，我应该如何做一些简单的事情，比如加载一个XML文件，然后将其作为一个可以被其他函数使用的对象，等等。？

谢谢你的帮助！

PS：我对JavaScript和Windows8现代应用程序/WinAPI非常陌生。以前在Python和Java方面的经验（在那里做这件事很琐碎！）。