导航后，依赖项属性恢复为默认值

Question

I'm missing something really obvious here but just cant work it out :(

I have a 2 page windows store application with a Dependency Property called Seconds. If you click the start button a timer will start and the value of Seconds will decrease as expected BUT if I navigate to Page2 and back again to MainPage the DP is reset to Default Value in UI despite the actual value being correct.

I can see this by putting a breakpoint on the _timer.tick event the Seconds DP isnt at its default value but decreasing as expected as the timer is still running. I would like this reflected in the UI Any help appreciated?

MainPage.xaml

<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="1*"/>
        <ColumnDefinition Width="1*"/>
    </Grid.ColumnDefinitions>
    <StackPanel>
        <Button x:Name="btnNav" Content="Page 2" Width="150" Height="60" Click="btnNav_Click"></Button>
        <Button x:Name="btnStart" Content="Start" Width="150" Height="60" Click="btnStart_Click"></Button>
    </StackPanel>
    <StackPanel Grid.Column="1">            
        <TextBlock x:Name="txtSeconds" FontSize="25" Text="{Binding Path=Seconds}" />
    </StackPanel>
</Grid>

MainPage.cs

 public sealed partial class MainPage : Page
 {
    private static DispatcherTimer _timer;

    public MainPage()
    {
        this.InitializeComponent();

        Loaded += (s, args) =>
            {


                if (_timer == null)
                {
                    _timer = new DispatcherTimer();
                    _timer.Interval = TimeSpan.FromMilliseconds(1000);
                    _timer.Tick += (sender, e) =>
                        {                                
                            Seconds--;
                        };
                }

                this.DataContext = this;
            };
    }


    public int Seconds
    {
        get { return (int)GetValue(SecondsProperty); }
        set { SetValue(SecondsProperty, value); }
    }

    // Using a DependencyProperty as the backing store for Seconds.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty SecondsProperty =
        DependencyProperty.Register("Seconds", typeof(int), typeof(MainPage), new PropertyMetadata(100));


    private void btnNav_Click(object sender, RoutedEventArgs e)
    {
        this.Frame.Navigate(typeof(Page2));
    }

    private void btnStart_Click(object sender, RoutedEventArgs e)
    {
        _timer.Start();
    }
}

Page2.xaml

<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
    <AppBarButton Icon="Back" Click="AppBarButton_Click"></AppBarButton>
</Grid>

Page2.cs

public sealed partial class Page2 : Page
{
    public Page2()
    {
        this.InitializeComponent();
    }

    private void AppBarButton_Click(object sender, RoutedEventArgs e)
    {
        this.Frame.Navigate(typeof(MainPage));
    }
}

Answer 1

As stated in comment :

Problem is not in DP. When you navigate back to same page, page is recreated. You can verify that by putting breakpoint on MainPage constructor. Since after navigating back to main page, you haven't started timer again so no update in GUI.

For your statement

_timer is static.

that change nothing at all. Since timer is static and won't be null in case of second load so Tick event never gets hooked up for second instance of MainPage.

Tick event is getting hit for first instance of MainPage. First instance never gets destroyed because of memory leak you have induced by making timer as static. So, it is still decrementing Seconds value for first instance and not current instance.

Move Tick event hooking outside of null check and you will see GUI will update but starting from 100 onwards because new instance will have default value 100.

 if (_timer == null)
 {
     _timer = new DispatcherTimer();
     _timer.Interval = TimeSpan.FromMilliseconds(1000);
 }
 _timer.Tick += (sender, e) =>
            {                                
               Seconds--;
            };

Answer 2

As mentioned in comments, you navigate forward to new instance of MainPage, so it has own (default) value of DP.

I guess you're looking for GoBack method of Frame