How to use to use UserControl DP and MVVM - wpf

Question

I haven`t found answer for this question in internet.
Is there some way to use in UserControl DP and MVVM together?
For example, I created UserControl, and i need to display some text on it. My UserControl has DP which takes some data (f.e. string) from consumer.
I have some code behind:
public partial class MyUserControl : UserControl
{
public MyUserControl()
{
InitializeComponent();
}
***
public string Text
{
get { return (PointCollection)GetValue(TextProperty); }
set { SetValue(TextProperty, value); }
}
public static readonly DependencyProperty TextProperty =
DependencyProperty.Register(nameof(Text),
typeof(string),
typeof(MyUserControl),
new FrameworkPropertyMetadata(null, new PropertyChangedCallback(TextChanged))
);
private static void TextChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
((MyUserControl)d).DataContext.Txt = (string)e.NewValue;
}
***
}
Here is code of ViewModel class:
class MyUserControlViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string propertyName)
{
PropertyChangedEventHandler handler = PropertyChanged;
handler?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
private string _txt;
public MyUserControlViewModel(){}
***
public string Txt
{
get{return _txt;}
set
{
if(_txt!= value)
{
_txt= value;
OnPropertyChanged("Txt");
}
}
}
***
}
And here some XAML code of UserControl in which i want to bind text of TextBlock to property Txt of ViewModel, so:
<UserControl x:Class="SimpleWPFChartExample.ChartSurface"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:SimpleWPFChartExample"
mc:Ignorable="d">
<UserControl.DataContext>
<local:SimpleWPFChartExample />
</UserControl.DataContext>
<Grid>
<TextBlock Text="{Binding Path=Txt}"/>
</Grid>
</UserControl>
So, if I do this way I don`t see any changes of DP Text and any changes of Txt, when i put some data in DP Text in consumer application.
What should I do?

Answer 1

You didn't mention why you want to use both dependency properties and a viewmodel, but it's easy to get one working. You just need to use the viewmodel you want to use instead of a different one. This is the wrong viewmodel:
<UserControl.DataContext>
<local:SimpleWPFChartExample />
</UserControl.DataContext>
This is the right one:
<UserControl.DataContext>
<local:MyUserControlViewModel />
</UserControl.DataContext>
At least, that would appear to be the case based on your question. But you have a number of copy and paste errors in there: For example, your Text string property getter casts the return value from GetValue() to PointCollection; that won't even compile. Neither will this: ((MyUserControl)d).DataContext.Txt = (string)e.NewValue;
This will compile:
private static void TextChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
var ctl = ((MyUserControl)d);
var vm = (MyUserControlViewModel)ctl.DataContext;
vm.Txt = (String)e.NewValue;
}
So for all I know you really are using the correct viewmodel in your actual code, in which case the problem is in code you're not sharing here. It may be that the problem is in whatever you're doing to set Text.
It would help if you could provide a complete, minimal, single-property example that compiles, but and then fails at runtime to meet your expectations.