Windows Store apps, save and load the state of Bing Maps Pushpins on navigation between pages - windows-8

Question

I'm making a Windows Store App that asks for user input then produces a bunch of pushpins based on that input. When a pushpin is tapped the app navigates to a page with more detail.
Now the problem i'm having is this:
My pages all inherit from the automatically generated LayoutAwarePage so I could potentially make use of SaveState and LoadState to save the pushpins so they don't get wiped on navigation. The thing is that i can't get the pins to save into the Dictionary object supplied by SaveState.
The error I get is "Value cannot be null" and it's referring to the _pageKey variable in LayoutAwarePage.OnNavigatedFrom() and i don't know why it's happening.
I've tried serialising them into a JSON string so i can deserialise it in LoadState, but i get the same result using a string or a List of UIelement.
I think this is all due to my lack of understanding of how SaveState, LayoutAwarePAge and SuspensionManager work. I thought what i was doing would work as the Dictionary is only asking for a string and an object.
I'm not using any other methods from LayoutAwarePage so if there is a better way than using SaveState and LoadState, I'm all ears.
These are the two versions of SaveState i've tried:
Using JSON
protected override void SaveState(Dictionary<String, Object> pageState)
{
List<string> pindata = new List<string>();
List<string> serialisedpins = new List<string>();
foreach (Pushpin ele in map.Children)
{
pindata = ele.Tag as List<string>;
serialisedpins.Add(JsonConvert.SerializeObject(pindata));
}
string jasoned = JsonConvert.SerializeObject(serialisedpins);
pageState["pins"] = jasoned;
}
using a List of UIElement
protected override void SaveState(Dictionary<String, Object> pageState)
{
List<UIElement> pins = new List<UIElement>(map.Children);
pageState["pins"] = pins;
}

Answer 1

The error you're getting (_pagekey value cannot be null) is not really related to what you're saving into the Dictionary. The exception is most likely being thrown in OnNavigateFrom() method of LayoutAwarePage:
protected override void OnNavigatedFrom(NavigationEventArgs e)
{
var frameState = SuspensionManager.SessionStateForFrame(this.Frame);
var pageState = new Dictionary<String, Object>();
this.SaveState(pageState);
frameState[_pageKey] = pageState; // <-- throws exception because _pageKey is null
}
If you take a look at the rest of the code of LayoutAwarePage you'll find out the value of _pageKey is being set in OnNavigatedTo method of LayoutAwarePage:
protected override void OnNavigatedTo(NavigationEventArgs e)
{
// Returning to a cached page through navigation shouldn't trigger state loading
if (this._pageKey != null) return;
var frameState = SuspensionManager.SessionStateForFrame(this.Frame);
this._pageKey = "Page-" + this.Frame.BackStackDepth; <-- this line sets the _pageKey value
if (e.NavigationMode == NavigationMode.New)
{
// Clear existing state for forward navigation when adding a new page to the
// navigation stack
var nextPageKey = this._pageKey;
int nextPageIndex = this.Frame.BackStackDepth;
while (frameState.Remove(nextPageKey))
{
nextPageIndex++;
nextPageKey = "Page-" + nextPageIndex;
}
// Pass the navigation parameter to the new page
this.LoadState(e.Parameter, null);
}
else
{
// Pass the navigation parameter and preserved page state to the page, using
// the same strategy for loading suspended state and recreating pages discarded
// from cache
this.LoadState(e.Parameter, (Dictionary<String, Object>)frameState[this._pageKey]);
}
}
Usually the reason for that is that you're overriding OnNavigatedTo in your own page without calling base.OnNavigatedTo(e) inside it. The basic pattern of overriding it should always be:
protected override void OnNavigatedTo(NavigationEventArgs e)
{
base.OnNavigatedTo(e);
// the rest of your own code
}
This will make sure the base implementation will execute and set the _pageKey value as well as call LoadState() to load the previously saved state if there's any.