Page Navigation and Passing, Sharing and Retaining Data in Windows Phone 7

Charles Petzold
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This chapter is taken from book "Programming Windows Phone 7" by Charles Petzold published by Microsoft press. http://www.charlespetzold.com/phone/index.html

Page navigation would seem to be an advanced Silverlight programming topic, and a topic that applies only to Silverlight programming rather than XNA programming. However, there are issues involved with navigation that are related to the very important topic of tombstoning, which is what happens to your Windows Phone 7 application when the user navigates to another application through the phone's Start screen. Tombstoning is very much an issue that also affects XNA programmers.

Basic Navigation

The SilverlightSimpleNavigation project begins as usual with a MainPage class, and as usual I set the two TextBlock elements for the titles and the content area of MainPage.xaml contains only a TextBlock that sets a handler for its ManipulationStarted event:

<StackPanel x:Name="TitlePanel" Grid.Row="0" Margin="12,17,0,28">
    <TextBlock x:Name="ApplicationTitle" Text="SIMPLE NAVIGATION" Style="{StaticResource PhoneTextNormalStyle}"/>
    <TextBlock x:Name="PageTitle" Text="main page" Margin="9,-7,0,0" Style="{StaticResource PhoneTextTitle1Style}"/>
</StackPanel>


<Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">
    <TextBlock Text="Navigate to 2nd Page"
               HorizontalAlignment="Center"
               VerticalAlignment="Center"
               Padding="0 34"
               ManipulationStarted="OnTextBlockManipulationStarted" />
</Grid>

Notice the Text property on the TextBlock: "Navigate to 2nd page." The code-behind file contains the handler for ManipulationStarted but also overrides the OnManipulationStarted method for the whole page:

namespace SilverlightSimpleNavigation
{
    public partial class MainPage : PhoneApplicationPage
    {
        Random rand = new Random();
        public MainPage()
        {
            InitializeComponent();
        }
        void OnTextBlockManipulationStarted(object sender, ManipulationStartedEventArgs args)
        {
            this.NavigationService.Navigate(new Uri("/SecondPage.xaml", UriKind.Relative));
            args.Complete();
            args.Handled = true;
        }
        protected override void OnManipulationStarted(ManipulationStartedEventArgs args)
        {
            ContentPanel.Background = new SolidColorBrush(
                Color.FromArgb(255, (byte)rand.Next(255),
                                    (byte)rand.Next(255),
                                    (byte)rand.Next(255)));
            base.OnManipulationStarted(args);
        }
    }
}

If you touch anywhere on the page outside of the TextBlock, the background of the ContentPanel is set to a random color. Touch the TextBlock, and the handler accesses the NavigationService property of the page.

I created a second page in the SilverlightSimpleNavigation project by right-clicking the project name in the Visual Studio solution explorer, and selecting Add and New Item. From the Add New Item dialog box, I picked Windows Phone Portrait Page and gave it a name of SecondPage.xaml.

I gave the titles In SecondPage.xaml the same application name as FirstPage.xaml but a page title of "second page" and the content area of SecondPage.xaml is very much like MainPage.xaml but the TextBlock reads "Go Back to 1st Page":

<StackPanel x:Name="TitlePanel" Grid.Row="0" Margin="12,17,0,28">
    <TextBlock x:Name="ApplicationTitle" Text="SIMPLE NAVIGATION" Style="{StaticResource PhoneTextNormalStyle}"/>
    <TextBlock x:Name="PageTitle" Text="second page" Margin="9,-7,0,0" Style="{StaticResource PhoneTextTitle1Style}"/>
</StackPanel>


<Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">
    <TextBlock Text="Go Back to 1st Page"
               HorizontalAlignment="Center"
               VerticalAlignment="Center"
               Padding="0 34"
               ManipulationStarted="OnTextBlockManipulationStarted" />
</Grid>

The code-behind file of the SecondPage class is also very much like the FirstPage class:

namespace SilverlightSimpleNavigation
{
    public partial class SecondPage : PhoneApplicationPage
    {
        Random rand = new Random();
        public SecondPage()
        {
            InitializeComponent();
        }
        void OnTextBlockManipulationStarted(object sender, ManipulationStartedEventArgs args)
        {
            this.NavigationService.GoBack();
            args.Complete();
            args.Handled = true;
        }
        protected override void OnManipulationStarted(ManipulationStartedEventArgs args)
        {
            ContentPanel.Background = new SolidColorBrush(
                Color.FromArgb(255, (byte)rand.Next(255),
                                    (byte)rand.Next(255),
                                    (byte)rand.Next(255)));
            base.OnManipulationStarted(args);
        }
    }
}

Let's run the program. The program begins with the main page, and you can touch the screen to change the color:

Now touch the TextBlock that says "Navigate to 2nd Page" and the second page comes into view:

You can touch that screen to change to a different color:

Now touch the TextBlock that says "Go Back to 1st Page". (Alternatively, you can press the phone's hardware Back button.) You'll be whisked back to the main page with the color just as you left it:

Passing Data to Pages

The possible use of pages as dialog boxes provokes two questions:

How do I pass data from a source page to a destination page?
How do I return data when going back to the original page?

The following project is called SilverlightPassData. It is very much like the first project in this chapter except that when MainPage navigates to SecondPage, it provides SecondPage with its current background color, and SecondPage initializes itself with that color. Here's the content area of MainPage.xaml, the same as in the previous program:

<Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">
    <TextBlock Text="Navigate to 2nd Page"
               HorizontalAlignment="Center"
               VerticalAlignment="Center"
               padding="0 34"
               ManipulationStarted="OnTextBlockManipulationStarted" />
</Grid>

I won't show you the OnManipulationStarted override because it's the same as in the previous program, but the ManipulationStarted event handler for the TextBlock is a bit enhanced:

void OnTextBlockManipulationStarted(object sender, ManipulationStartedEventArgs args)
{
    string destination = "/SecondPage.xaml";
    if (ContentPanel.Background is SolidColorBrush)
    {
        Color clr = (ContentPanel.Background as SolidColorBrush).Color;
        destination += String.Format("?Red={0}&Green={1}&Blue={2}",
        clr.R, clr.G, clr.B);
    }
    this.NavigationService.Navigate(new Uri(destination, UriKind.Relative));
    args.Complete();
    args.Handled = true;
}

The SilverlightPassData project also contains a SecondPage class that is the same as the one in the first project except that the code-behind file contains an override of the OnNavigatedTo method:

protected override void OnNavigatedTo(NavigationEventArgs args)
{
    IDictionary<string, string> parameters = this.NavigationContext.QueryString;
    if (parameters.ContainsKey("Red"))
    {
        byte R = Byte.Parse(parameters["Red"]);
        byte G = Byte.Parse(parameters["Green"]);
        byte B = Byte.Parse(parameters["Blue"]);
        ContentPanel.Background =
        new SolidColorBrush(Color.FromArgb(255, R, G, B));
    }
    base.OnNavigatedTo(args);
}

Now as you navigate from MainPage to SecondPage, the background color remains the same. As you go back, however, that's not the case. There is no built-in facility like the query string to return data from one page to another.

Sharing Data Among Pages

Keep in mind that all the pages in your program have convenient access to the App class that derives from Application. The static Application.Current property returns the Application object associated with the program, and you can simply cast that to App. This means that you can use the App class for storing data you want to share among multiple pages of the application.

In the SilverlightShareData project, I defined a simple public property in the App class, I defined this property of type nullable Color rather than just Color for those cases where a SolidColorBrush has not been set on the Background property of ContentPanel.

public partial class App : Application
{
    // public property for sharing data among pages
    public Color? SharedColor { set; get; }
    ....
}

Much of the program remains the same, except that when you touch the TextBlock in MainPage, the handler first attempts to save a color in the new App class property before navigating to SecondPage:

void OnTextBlockManipulationStarted(object sender, ManipulationStartedEventArgs args)
{
    if (ContentPanel.Background is SolidColorBrush)
        (Application.Current as App).SharedColor =
            (ContentPanel.Background as SolidColorBrush).Color;

    this.NavigationService.Navigate(new Uri("/SecondPage.xaml", UriKind.Relative));
    args.Complete();
    args.Handled = true;
}

The OnNavigatedTo override in SecondPage than accesses that property:

protected override void OnNavigatedTo(NavigationEventArgs args)
{
    Color? sharedColor = (Application.Current as App).SharedColor;
    if (sharedColor != null)
        ContentPanel.Background = new SolidColorBrush(sharedColor.Value);
    base.OnNavigatedTo(args);
}

Similarly, when you press the TextBlock on SecondPage, the handler saves whatever color the background now happens to be back in the App class before calling GoBack:

void OnTextBlockManipulationStarted(object sender, ManipulationStartedEventArgs args)
{
    if (ContentPanel.Background is SolidColorBrush)
        (Application.Current as App).SharedColor =
            (ContentPanel.Background as SolidColorBrush).Color;

    this.NavigationService.GoBack();
    args.Complete();
    args.Handled = true;
}

Retaining Data across Instances

If we want SecondPage to "remember" the last color it was set to, something outside of SecondPage must be responsible for saving that data. That could be MainPage. Or, SecondPage could save its state in isolated storage. Isolated storage is much like regular disk storage. To access it, you use classes in the System.IO.IsolatedStorage namespace. Every Windows Phone 7 application has access to isolated storage but only to files that the application itself has created. Isolated storage allows an application to save data between multiple executions, and is ideal for saving application settings.

Let's modify the previous program so that SecondPage uses this State property. In the SilverlightRetainData project, everything is the same except for a using directive for the Microsoft.Phone.Shell namespace and two overrides in SecondPage. Here they are:

protected override void OnNavigatedFrom(NavigationEventArgs args)
{
    if (ContentPanel.Background is SolidColorBrush)
    {
        Color clr = (ContentPanel.Background as SolidColorBrush).Color;
        if (args.Content is MainPage)
            (args.Content as MainPage).ReturnedColor = clr;
            // Save color
            PhoneApplicationService.Current.State["Color"] = clr;
        }
        base.OnNavigatedFrom(args);
    }

protected override void OnNavigatedTo(NavigationEventArgs args)
{
    // Retrieve color
    if (PhoneApplicationService.Current.State.ContainsKey("Color"))
    {
        Color clr = (Color)PhoneApplicationService.Current.State["Color"];
            ContentPanel.Background = new SolidColorBrush(clr);
    }
    base.OnNavigatedTo(args);
}

Now go back to MainPage. The color you set in SecondPage is displayed. From MainPage, press the phone's hardware Start button, leaving the program. Navigate around a bit if you want but eventually start pressing the Back button to come back to SilverlightRetainData and MainPage.

Summary

I hope this article helps you to learn Page Navigation and Passing, Sharing and Retaining Data in Windows Phone 7.