PropertyChangedEventHandler都为空，但是绑定只能工作一次，为什么？

Question

I made a little Project to learn a bit about MVVM. It is a calculator that calculates when you are allowed to go home from work.

I made a UserControl with two Textboxes and a single label as a simple "TimePicker". This Usercontrol has a ViewModel (the Mainwindow even has one) which manages the time of one Timepicker. It has three Properties: an int called TimeValue which is just the value of hours and minutes and two ints called Hours and Minutes . My two Textboxes are bound to them and display them. Setting one value via Textbox also resets Time, setting time (via Property) resets Hours and Minutes, both textboxes are updated after setting this value.

This works quit fine. Now I wanted to add a second Property called ReadOnly . ReadOnly is needed for the TimePicker which displayes the time to go. It makes no sense to manually set this time so I want to have a possibility to set both Textboxes IsReadOnly Property.

ReadOnly is now a second Property of the UserControl. Because I am lazy I wanted to directly bind the Property and both Textboxes via the UserControl and bind only the IsReadOnly -Property to the UserControl.

This was the Code of my Idea (Usercontrol):

public partial class TimeBox : UserControl, INotifyPropertyChanged
{
    private SingleTimeViewModel viewModel;

    //... other Properties

    public static DependencyProperty ReadOnlyProperty = DependencyProperty.Register("ReadOnly", typeof(Boolean), typeof(TimeBox), new PropertyMetadata(false));

    // Schnittstellen-Ereignis  
    public event PropertyChangedEventHandler PropertyChanged;
    protected internal void OnPropertyChanged(string propertyname)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propertyname));
    }

    public TimeBox()
    {
        InitializeComponent();
        viewModel = new SingleTimeViewModel(SingleTime.CreateSingleTime());
        this.DataContext = viewModel;
    }

    //... Code of other Properties

    private bool _ReadOnly;
    public bool ReadOnly
    {
        get
        {
            return _ReadOnly;
        }
        set
        {
            if (_ReadOnly == value)
                return;
            _ReadOnly = value;
            OnPropertyChanged("ReadOnly");
        }
    }

    //... Other Methods
}

This was bound to both Textboxes via XAML (Bindings for Text lead to ViewModel, IsReadOnly should bind to TimeBox):

<UserControl x:Name="TimeBoxControl" x:Class="TimeCalculator.TimeBox"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         ... >
    <WrapPanel Grid.Column="7" HorizontalAlignment="Stretch" Margin="0,0,0,0" Grid.Row="1" VerticalAlignment="Center" >
        <TextBox x:Name="txtBxHours" ... Text="{Binding Hours}" ... IsReadOnly="{Binding ReadOnly, ElementName=TimeBoxControl}" />
        <Label x:Name="lblSeparator" ... />
        <TextBox x:Name="txtBxMinutes" ... Text="{Binding Minutes}" ...  IsReadOnly="{Binding ReadOnly, ElementName=TimeBoxControl}" />
   </WrapPanel>
</UserControl>

I made the values readonly at the Constructor of the Mainwindow of my Project after InitializeComponent . Therefore I used the following lines:

this.TmBxMayGo.ReadOnly = true;
this.TmBxMustGo.ReadOnly = true;
this.TmBxTimeUntilMayGo.ReadOnly = true;
this.TmBxTimeUntilMustGo.ReadOnly = true;
this.TmBxCurrentOvertime.ReadOnly = true;

This did not work, after some debugging I found out it did not because PropertyChangedEventHandler PropertyChanged was always null . I searched a lot to find a solution for this problem, but I made none of the common mistakes (eg forgot : INotifyPropertyChanged , wrong names in Strings or else).

I finally gave up and made it via ViewModel. But then I realised that PropertyChangedEventHandler PropertyChanged was also null when I set it via ViewModel, but the textboxes were ReadOnly after calling.

Now the two questions I have:

1. Does it make sense to make an own ViewModel for an single Usercontrol?
2. Why is that so? How can it be that PropertyChanged is null twice but only works once?

Answer 1

1. Yes, it makes sense to have a single ViewModel for the single standlone logically seperated UI. It divides the responsibility from the mainviewmodel. So put all your properties in your ViewModel.

2. WPF only attaches the handler to the PropertyChanged event of the INotifyPropertyChanged object when that object is set as the DataContext of the view that is why your PropertyChanged is null before you set the DataContext of your usercontrol. And your textboxes are still disabled because while initializing the binding, getter of these properties are called and UI is updated with the default values you gave.

Answer 2

a cleaner approach for doing the mentioned would be something like this

    public bool ReadOnly
    {
        get { return (bool)GetValue(ReadOnlyProperty); }
        set { SetValue(ReadOnlyProperty, value); }
    }

    // Using a DependencyProperty as the backing store for ReadOnly.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty ReadOnlyProperty =
        DependencyProperty.Register("ReadOnly", typeof(bool), typeof(TimeBox), new PropertyMetadata(false, null, CoerceReadOnly));

    private static object CoerceReadOnly(DependencyObject d, object baseValue)
    {
        if ((d as TimeBox)._Enabled == baseValue)
            return DependencyProperty.UnsetValue;
        return baseValue;
    }

here I am using Coerce Value callback to check the condition, so by returning DependencyProperty.UnsetValue will cancel the property change otherwise it will proceed

more info on Coerce here http://msdn.microsoft.com/en-us/library/ms745795(v=vs.110).aspx#Advanced

for other question, I would say you may probably not require to create a usercontrol unless you want to deploy a control library. Do try to leverage data templates for the same.

and secondly since you are binding to a usercontrol which work on dependency framework the regular notifications may not work as expected.