How to save ListView and ScrollViewer family controls scroll position

archiecoder Windows Phone

One of the suggested guidelines when creating a Windows Phone application is that when the user quits an application, the states of the pages are saved, so that when the user re-opens the application, it feels like he never quit.

With the Mango update, it is now possible to save the states of application pages, providing there is enough memory. However, there will be some cases when the application will tombstone. The purpose of my utility static class StateManager will help to save a ListView/ScrollViewer scroll position or other controls that have an internal ScrollViewer.

Before explaining my utility class works, let’s see what happen when the scroll position is not saved:

1- Create a “Windows Phone Databound Application”.
2- Run the app then scroll the list to the last item.
3- Press the Start button.
4- Press the Back button.

The state is preserved. Great, but…

Now, stop debugging the application and go to the properties of the project. In the Debug section, check the option “Tombstone upon deactivation while debugging”. This is the only way to force the tombstoning process while debugging.

Execute the last 3 steps and you’ll see that the list scroll position is not saved.

To fix this issue, insert the StateManager class (provided at the end of the article) in your project and call StateManager.SaveScrollViewerOffset and StateManager.RestoreScrollViewer on the list that you want to save/restore scroll position. The best places to insert those calls are in the method OnNavigatedTo and OnNavigatedFrom.

protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
    base.OnNavigatedTo(e);

    StateManager.RestoreScrollViewerOffset(MainListBox);
    
}

protected override void OnNavigatedFrom(System.Windows.Navigation.NavigationEventArgs e)
{
    base.OnNavigatedFrom(e);

    StateManager.SaveScrollViewerOffset(MainListBox);
}

Download StateManager.cs
Download Sample project

How to get the Internet connection type without blocking the UI

archiecoder Windows Phone

All Windows Phone applications that use Internet data should always check if there is a valid Internet connection. If there is no connection, a proper message should be displayed to the user. If your application relies on a specific server, don’t assume that Internet is not available if a call to the server returns an error, because the server might be down while the Internet is still available.

The proper way to check if you have an Internet connection (WIFI, Ethernet, or none) is calling the property:

NetworkInterface.NetworkInterfaceType

Unfortunately, even if it’s not obvious, calling this property on the UI thread can block the UI for many seconds. To avoid this problem, I created a NetworkInformationUtility class (the class and the sample project are available at the end of the post):

using System.Threading;
using Microsoft.Phone.Net.NetworkInformation;

namespace DotNetApp.Utilities
{
    public class NetworkTypeEventArgs {
        #region Constructor

        public NetworkTypeEventArgs(NetworkInterfaceType type, bool hasTimeout = false)
        {
            Type = type;
            HasTimeout = hasTimeout;
        }

        #endregion #region Properties

        public bool HasTimeout { get; private set; }

        public bool HasInternet
        {
            get { return Type != NetworkInterfaceType.None; }
        }

        public NetworkInterfaceType Type { get; private set; }

        #endregion }

    /// <summary> /// Static class to get the NetworkInterfaceType without blocking the UI thread. /// </summary> public static class NetworkInformationUtility {
        #region Fields

        private static bool _isGettingNetworkType;
        private static readonly object _synchronizationObject = new object();
        private static Timer _timer;

        #endregion #region Methods

        /// <summary> /// Get the NetworkInterfaceType asynchronously. /// </summary> /// <param name="timeoutInMs">Specifies the timeout in milliseconds.</param> public static void GetNetworkTypeAsync(int timeoutInMs)
        {
            lock (_synchronizationObject)
            {
                if (!_isGettingNetworkType)
                {
                    _isGettingNetworkType = true;

                    if (System.Net.NetworkInformation.NetworkInterface.GetIsNetworkAvailable())
                    {
                        Thread thread = new Thread(GetNetworkType) {IsBackground = true};
                        thread.Start(timeoutInMs);
                    }
                    else {
                        FireGetNetworkTypeCompleted(NetworkInterfaceType.None);
                    }
                }
            }
        }

        #endregion #region Delegates

        public delegate void NetworkTypeEventHandler(object sender, NetworkTypeEventArgs networkTypeEventArgs);

        #endregion #region Events

        public static event NetworkTypeEventHandler GetNetworkTypeCompleted;

        #endregion #region Event Handlers

        private static void OnTimerElapsed(object state)
        {
            FireGetNetworkTypeCompleted(NetworkInterfaceType.None, true);
        }

        #endregion #region Private Methods

        private static void GetNetworkType(object state)
        {
            _timer = new Timer(OnTimerElapsed, null, (int)state, 0);

            // This is a blocking call, this is why a thread is used to let the UI to be fluid NetworkInterfaceType type = NetworkInterface.NetworkInterfaceType;

            _timer.Dispose();
            _timer = null;

            FireGetNetworkTypeCompleted(type);
        }

        private static void FireGetNetworkTypeCompleted(NetworkInterfaceType type, bool hasTimeout = false)
        {
            lock (_synchronizationObject)
            {
                if (_isGettingNetworkType)
                {
                    _isGettingNetworkType = false;

                    NetworkTypeEventHandler networkTypeEventHandler = GetNetworkTypeCompleted;

                    if (networkTypeEventHandler != null)
                    {
                        networkTypeEventHandler(null, new NetworkTypeEventArgs(type, hasTimeout));
                    }
                }
            }
        }

        #endregion }
}

Here are the steps to use this class in your code:

1. Add the NetworkInformationUtility.cs to your project.
2. Attach method to the event NetworkInformationUtility.GetNetworkTypeCompleted.
3. Call NetworkInformationUtility.GetNetworkTypeAsync(3000 /*timeout in ms*/);
4. Retrieve the result on the GetNetworkTypeCompleted method that you attached to the event.

Code sample:

using System.Windows;
using DotNetApp.Utilities;

namespace NetworkInformationApp
{
    public partial class MainPage {
        public MainPage()
        {
            InitializeComponent();
        }

        protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
        {
            base.OnNavigatedTo(e);

            NetworkInformationUtility.GetNetworkTypeCompleted += GetNetworkTypeCompleted;

            NetworkInformationUtility.GetNetworkTypeAsync(3000); // Timeout of 3 seconds }

        protected override void OnNavigatedFrom(System.Windows.Navigation.NavigationEventArgs e)
        {
            base.OnNavigatedFrom(e);

            NetworkInformationUtility.GetNetworkTypeCompleted -= GetNetworkTypeCompleted;
        }

        private void GetNetworkTypeCompleted(object sender, NetworkTypeEventArgs networkTypeEventArgs)
        {
            string message;

            if (networkTypeEventArgs.HasTimeout)
            {
                message = "The timeout occurred";
            }
            else if (networkTypeEventArgs.HasInternet)
            {
                message = "The Internet connection type is: " + networkTypeEventArgs.Type.ToString();
            }
            else {
                message = "There is no Internet connection";
            }

            // Always dispatch on the UI thread Dispatcher.BeginInvoke(() => MessageBox.Show(message));
        }
    }
}

Download NetworkInformationUtility.cs
Download Sample project