Supervision problems in WP7, I can’t determine if I need to restore or create instances / request new data

Question

I just ran into a thunder issue / issue in WP7. let's say I have 3 pages, FirstPage.xaml, SecondPage.xaml and ThirdPage.xaml. natural flow will be:

FirstPage.xaml → SecondPage.xaml → ThirdPage.xaml

In other words,

main page → page with a list of objects → page with a detailed display of one object from the previous page

when I switch from FirstPage.xaml to SecondPage.xaml, I need to do a database query to get the List in SecondPage.xaml. Then I need to go to ThirdPage.xaml from SecondPage.xaml (after I select one MyObject from the list). at the moment, the tomb is becoming very confused for me.

I know when going FirstPage.xaml -> SecondPage.xaml, the constructor of SecondPage.xaml.cs is called. I know when navigating ThirdPage.xaml → SecondPage.xaml (returning back by pressing the "Back" button or NavigationService.GoBack ()), the constructor of SecondPage.xaml.cs is NOT called. when I switch from SecondPage.xaml to ThirdPage.xaml, I store view-model objects (VM) in PhoneApplicationService.Current.State (SecondPage.xaml.cs.OnPageNavigatedFrom ()).

my (incorrect) strategy was, well, if the constructor of SecondPage.xaml.cs is called in one instance (FirstPage.xaml → SecondPage.xaml), but not in another instance (ThirdPage.xaml → SecondPage.xaml), then I can install in constructor a logical flag whether to make a new database query or restore the state of the page (from PhoneApplication.Current.State). the boolean flag is first set to false, but only true in the SecondPage.xaml.cs constructor.

I thought this worked fine, but then when I clicked the start button to leave the application, and then click the back button to return to the application, the SecondPage.xaml.cs constructor was called. therefore, I am making another new DB query instead of restoring a state that is NOT the intended behavior.

My question is how to find out when to make a new database query and restore when the user clicks on start and then back to go to the application? I thought about how to solve this myself, but most of what I came up with were kludges; it seemed indecent, and as if I was messing around to make it work. for example, I thought I could go to querystring from FirstPage.xaml to SecondPage.xaml (i.e. /SecondPage.xaml?freshDbQuery=1), but when I switch from ThirdPage.xaml back to SecondPage.xaml, this is the key querystring pair , freshDbQuery = 1, always like that! (since you can tell, I don't know wp7 very well).

any help is appreciated.

Answer 1

All your processing for the tomb should be done in the OnNavigatingFrom ** and OnNavigatedTo .

You can create all the target handlers for your situation with the following:

 protected override void OnNavigatingFrom(NavigatingCancelEventArgs e) { if (e.NavigationMode != System.Windows.Navigation.NavigationMode.Back) { this.State.Clear(); this.State.Add("db_data", ***Serialized version of the DB returned data***); } base.OnNavigatingFrom(e); } protected override void OnNavigatedTo(NavigationEventArgs e) { if (this.State.ContainsKey("db_data")) { this.SomethingOnPage = DeserializeToAppropriateType(this.State["db_data"]); } base.OnNavigatedTo(e); } 

** Use it preferably OnNavigatedFrom where possible.