Value Update of Bound ViewModel Property Fails - wpf

Question

I am trying to bind a custom control property to a property of its view model and its failing.
I have defined a Dependency property for settings StartDate and updated the PropertyChangeCallback method
public static readonly DependencyProperty StartDateProperty =
DependencyProperty.Register(StartDatePropertyName,
typeof(DateTime),
typeof(CustomDateTimeControl),
new PropertyMetadata(DateTime.Now.AddYears(-7),
OnStartDatePropertyChanged));
private static void OnStartDatePropertyChanged(DependencyObject d,
DependencyPropertyChangedEventArgs e)
{
DateTime dtNewValue = (DateTime)e.NewValue;
if (dtNewValue != DateTime.MinValue)
{
DateTimeControl dtCtrl = d as DateTimeControl;
dtCtrl.StartDate = (DateTime)e.NewValue;
dtCtrl.CoerceValue(StartDateProperty);
}
}
The StartDate property gets bound to its ViewModel's Start Date, since VM needs to perform some operation which would then be used to define the next available view for the custom control.
<Setter Property="StartDate"
Value="{Binding StartDate,
Mode=OneWayToSource,
UpdateSourceTrigger=PropertyChanged}" />
Also the DependencyProperty defined within is set from the mainWindow view
<CustomDateTimeLib:CustomDateTimeControl StartDate="01/01/2000 00:00:00" />
The binding updates the property in the view model only with the default value of the dependency property but not with the value being set with in the MainView as above even though the dependency property is getting updated with the value from MainView.
ViewModelLocator class
public class ViewModelLocator
{
static ViewModelLocator()
{
ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default);
SimpleIoc.Default.Register<CalendarViewModel>();
}
/// <summary>
/// Gets the Main property.
/// </summary>
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Performance",
"CA1822:MarkMembersAsStatic",
Justification = "This non-static member is needed for data binding purposes.")]
public CustomDateTimeLib:CustomDateTimeControl CalendarVM
{
get
{
return ServiceLocator.Current.GetInstance<CustomDateTimeControl>();
}
}
}
App.Xaml
<Application x:Class="MvvmCustomTestApp.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:vm="clr-namespace:CustomDateTimeLib.ViewModel"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:ignore="http://www.galasoft.ch/ignore"
StartupUri="MainWindow.xaml"
mc:Ignorable="d ignore">
<Application.Resources>
<!--Global View Model Locator-->
<vm:ViewModelLocator x:Key="Locator" d:IsDataSource="True" />
</Application.Resources>
ApplyTemplate override method in CustomDateTimeControl class
public override void OnApplyTemplate()
{
base.OnApplyTemplate();
CalendarViewModel vm = (CalendarViewModel)this.DataContext;
vm.StartDate = this.StartDate;
}
Also defined a property change callback method for StartDate

Answer 1

If I understand you correctly, you have a custom DP that you want to bind to a ViewModel property, however you also want to set the Default Value for this property from the View.
That is not an ideal setup for MVVM or for when you're using bindings. MVVM is supposed to have all the logic, including things like "default value for X" in the ViewModel layer, and the View layer is only used to provide the user with a visual way to interact with the ViewModel (data) layer.
So your solutions are either :
Set the default value in your ViewModel
Provide handling in your Dependency Property that if value == DateTime.Min, use a different default value
Use a Converter (for DateTime) or FallbackValue (for DateTime?) if you really want to have the View supply the default value
Use a second DP to define the default value that should be used
Add a Loaded event handler to the control to read the DataContext and set the Default Value
Option 1 is the best solution if you are using MVVM, since things like a custom DefaultValue should be set in the ViewModel, not the View.
Option 2 is best if this default value is specific to this UserControl, and will be the same anytime this control is used.
Options 3, 4, and 5 are for if you really do insist on setting the default value from the View layer for whatever reason. Which one to use depends on your situation.
Assuming you use #1, I would expect your final XAML to look like this :
<!-- assumes DataContext is of type DateTimeCtrlVM via inheritance or direct binding -->
<CustomDateTimeLib:CustomDateTimeControl StartDate="{Binding StartDate}" />
That's it.
That <Setter> in your XAML code above is actually causing the following to happen :
Control is created with default value of 1/1/2000
Property is bound OneWayToSource to VM property, which means data will only flow from Target (Control) to Source (ViewModel), so the value of 1/1/2000 is getting persisted to your VM
So get rid of that Setter which binds the property as OneWayToSource, use the XAML binding shown above, and set the default value in the ViewModel and it should work.