mvvm windows app DataBinding

前台html 绑定(view):
<div id="aa" data-win-bind="innerText:UserData.word" style="height: 40px; width: 100px; background: white; color: red"></div>

//js 实现 viewmodel
(function () {
    "use strict";

    WinJS.Binding.optimizeBindingReferences = true;

    var app = WinJS.Application;
    var activation = Windows.ApplicationModel.Activation;

    app.onactivated = function (args) {
        if (args.detail.kind === activation.ActivationKind.launch) {
            if (args.detail.previousExecutionState !== activation.ApplicationExecutionState.terminated) {
                WinJS.Namespace.define("ViewModel", WinJS.Binding.as({
                    UserData: {
                        word: 'aaaaaa',
                        wordLength: null
                    }
                }));

                document.querySelector("#btn").onclick = function () {

                    var text = document.querySelector('#text').value;
                    ViewModel.UserData.word = text; //text 就可以用model 去获取，这里用 输入框代替了
                }
            }
            args.setPromise(WinJS.UI.processAll().then(function () {

                return WinJS.Binding.processAll(document.querySelector('#aa'), ViewModel);

            }));
        }
    };
    app.start();
})();