Question

I’m currently having an issue with WP7’s TimePicker, specifically with binding it to a ViewModel. The TimePicker in question sets the time of day for an alarm. When the page is first loaded, the TimePicker correctly displays the value of the Alarm object (the default value of 12:00am in this case). However, when the user chooses a new value this is not reflected in the model – it’s overridden with the previous value of 12:00am.

I’m using MVVM to create this form and hold the data bindings. Is there anything in particular that I’m doing wrong?

(View) AlarmEditorControl.xaml

<TextBlock Height="30" HorizontalAlignment="Left" Margin="1,6,0,0" Name="lblAlarmTime" Text="Alarm Time:" VerticalAlignment="Top" Grid.Column="2" FontSize="26" />
<!-- Data binding isn't working for updates! -->
<toolkit:TimePicker HorizontalAlignment="Left" Margin="140,34,0,0" Name="tpAlarmTime" VerticalAlignment="Top" Width="161" Grid.Column="1" Grid.ColumnSpan="2" Value="{Binding Path=Time, Mode=TwoWay}" />

(ViewModel) AlarmEditorModel.cs

[DataContractAttribute]
public class AlarmEditorModel
{
    private int _index;

    [DataMemberAttribute]
    public Alarm Alarm { get; set; }

    [DataMemberAttribute]
    public int Index
    {
        get
        {
            return _index;
        }

        set
        {
            _index = value;
        }
    }

    public AlarmEditorModel(int index)
    {
        _index = index;
        Alarm = new Alarm();

        // Get the list of alarms
        AlarmSerializer serializer = new AlarmSerializer();

        // Check the index is in range
        List<Alarm> alarms = serializer.AlarmList;
        if (_index > -1 && index < alarms.Count)
        {
            Alarm = alarms[_index];
        }
    }

    public void Commit()
    {
        // Get the current list of alarms
        AlarmSerializer serializer = new AlarmSerializer();
        List<Alarm> alarms = serializer.AlarmList;

        // Replace our new value
        alarms[_index] = Alarm;
        serializer.AlarmList = alarms;
    }
}

(Model) Alarm.cs

[DataContract]
public class Alarm : INotifyPropertyChanged
{
    private bool _active;
    private DateTime _time;

    [DataMember]
    public string Name { get; set; }

    [DataMember]
    public DateTime Time
    {
        get
        {
            return _time;
        }
        set
        {
            if (_time != value)
            {
                _time = value;
                RaisePropertyChanged("Time");
            }
        }
    }

    [DataMember]
    public AlarmFrequency Frequency { get; set; }

    [DataMember]
    public AlarmTone Tone { get; set; }

    [DataMember]
    public bool Active { 
        get {
            return _active;
        }
        set {
            _active = value;
        } 
    }

    public string AlarmTimeString { 
        get {
            return Time.ToShortTimeString();
        }
    }



    /**
     * Default Constructor
     */
    public Alarm()
    {
        Debug.WriteLine("Alarm: Using default constructor");
        this.Name = "New Alarm";
        this.Time = DateTime.Today;
        this.Frequency = new AlarmFrequency();
        this.Tone = new AlarmTone();
        this.Active = true;

        Debug.WriteLine("Alarm hours is " + this.Time.Hour);
    }

    /**
     * Parameterised constructor
     */
    public Alarm(string Name, DateTime Time, AlarmFrequency Frequency,
                    AlarmTone Tone, bool Active)
    {
        Debug.WriteLine("Alarm: Using parameterised constructor");
        this.Name = Name;
        this.Time = Time;
        this.Frequency = Frequency;
        this.Tone = Tone;
        this.Active = Active;
       }
}

(Calling Page) NewAlarm.xaml.cs

 private List<Channel> feeds;
    private AlarmEditorModel _aem;
    private int _index;

    public NewAlarm()
    {
        InitializeComponent();

        feeds = new List<Channel>();
        feeds.Add(new Channel(null, null, "Feed 1", DateTime.Now));
        feeds.Add(new Channel(null, null, "Feed 2", DateTime.Now));
    }

    /**
     * Setup functions when the page is loaded
     */
    protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
    {
        // Function vars + debug

        Debug.WriteLine("Navigating to");

        // Check if we're recovering from tombstone
        if (!StateUtilities.IsLaunching && this.State.ContainsKey("AlarmState"))
        {
            // Recover the saved model
            _aem = (AlarmEditorModel)this.State["AlarmState"];
        }
        else 
        {
            try
            {
                // Editing an alarm.
                _index = Convert.ToInt32(this.NavigationContext.QueryString["index"]);
                Debug.WriteLine("Editing an alarm");
            }
            catch (KeyNotFoundException knfe)
            {
                Debug.WriteLine(knfe.Message);

                // No index provided, new alarm
                _index = -1;
            }

            // Set the model from the index
            _aem = new AlarmEditorModel(_index);
        }

        AlarmEditor.DataContext = _aem.Alarm;
        Debug.WriteLine(_aem.Alarm.Time.Hour);
    }

    /**
     * Preserve alarm details when tombstoning
     */
    protected override void OnNavigatedFrom(System.Windows.Navigation.NavigationEventArgs e)
    {
        base.OnNavigatedFrom(e);

        if (this.State.ContainsKey("AlarmState"))
        {
            this.State["AlarmState"] = _aem;
        }
        else
        {
            this.State.Add("AlarmState", _aem);
        }

        StateUtilities.IsLaunching = false;
    }

---

EDIT 1

It would appear the setter for Alarm.Time is being called twice. By adding the following debug lines to the Time property:

[DataMember]
    public DateTime Time
    {
        get
        {
            return _time;
        }
        set
        {
            Debug.WriteLine("Current time is " + _time.ToShortTimeString());
            Debug.WriteLine("New time is " + value.ToShortTimeString());

            if (_time != value)
            {
                Debug.WriteLine("Changing time value");
                _time = value;
                RaisePropertyChanged("Time");
            }
        }
    }

The following output is produced in the log when setting the time to 9:10am:

Current time is 4:00 AM
New time is 9:10 AM 
Changing time value
Current time is 12:00 AM 
New time is 4:00 AM 
Changing time value

Answer 1

Problem solved I think. I needed to make an additional check in OnNavigatedTo when recovering from a Tombstone in order to get the value of the TimePicker before it was overwritten by the ViewModel:

protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
        // Check if we're recovering from tombstone
        if (!StateUtilities.IsLaunching && this.State.ContainsKey("AlarmState"))
        {
            // Recover the saved model
            _aem = (AlarmEditorModel)this.State["AlarmState"];

            // Use the value from the TimePicker
            _aem.Alarm.Time = (DateTime)AlarmEditor.tpAlarmTime.Value;
        }
        else 
        ...

Need to do a few more tests on this solution, but it seems to be doing the job so far.