WinJS Listview shows undefined when navigating quickly - windows-8

Question

I have a WinJS application with listviews in which if quickly navigate between pages before the listview is fully loaded, the next page shows the listview with all elements in it bound as "undefined".
So say I have a hub page with a "to do" that is filtered to only show 6 items, and there is a header that navigates to the full "to do" page, when the hub page is displayed but before it is fully loaded I click on the header link to the "to do" page, the app then goes to the "to do" page, but the items show up with all the properties in the tile as "undefined".
I am using IndexedDB as my data store.
My home page code looks like this:
WinJS.UI.Pages.define("/pages/home/home.html", {
ready: function (element, options) {
WinJS.Utilities.query("a").listen("click", function (e) {
e.preventDefault();
WinJS.Navigation.navigate(e.currentTarget.href);
}, false);
viewModel = new HomeViewModel(element);
viewModel.load(); //loads from indexed db
},
//etc...
To Do Page:
WinJS.UI.Pages.define("/pages/ToDo/ToDo.html", {
ready: function (element, options) {
viewModel = new ToDoViewModel(element);
viewModel.load();
},
etc//
I know there isn't much to go off, but any ideas would be appreciated.
Also tips on how to debug something like this would be great.
Update
I narrowed it down to this one line from the Hub Page:
myLib.GetData(todaysDate, function (result) {
that.trendsModel.today = result;
WinJS.Binding.processAll(that.el.querySelector("#dataPanel"), that.trendsModel); //<--Right Here
});
If I remove that, then when I load the second page the data doesn't show as undefined. What is interesting is the data initially shows correctly on the second page and then it changes to "undefined".
Solution
My fix:
myLib.GetData(todaysDate, function (result) {
var element = that.el.querySelector("#dataPanel");
that.trendsModel.today = result;
if(element) {
WinJS.Binding.processAll(element, that.trendsModel);
}
});
At the point when when the callback returns, I am already on the second page. So the selector was not found returning null. If you pass null to processAll it tries to bind the whole page which is why I was able to see the correct data for a second then it changes to undefined...Wow, what a doozy. I guess it makes sense but what a pain to find.
Hope it helps someone in the future :)

Answer 1

Your ToDoViewModel, and HomeViewModel need to be observable. This means they need to mix in from WinJS.Binding.mixin, and for the properties that you pull in asynchronously, they need to call this.notify("propertyName", newVal, oldVal) from the property setter.
Note that you need to have getter/setter properties. e.g.
var bindingBase = WinJS.Class.mix(function() {}, WinJS.Binding.mixin);
WinJS.Namespace.define("YourNamespace", {
ToDoViewModel: WinJS.Class.derive(bindingBase, function constructor() {
}, {
_titleStorage: "",
title: {
get: function() { return this._titleStorage; },
set: function(newValue) {
if(newValue === this._titleStorage) {
return;
}
var old = this._titleStorage;
this._titleStorage = newValue;
this.notify("title", newValue, old);
}
}
}),
});

Answer 2

myLib.GetData(todaysDate, function (result) {
var element = that.el.querySelector("#dataPanel");
that.trendsModel.today = result;
if(element) {
WinJS.Binding.processAll(element, that.trendsModel);
}
});
At the point when when the callback returns, I am already on the second page. So the selector was not found returning null. If you pass null to processAll it tries to bind the whole page which is why I was able to see the correct data for a second then it change to undefined...Wow, what doozy. I guess it makes sense but what a pain to find.