MVVM Command Binding

Question

I am developing a WPF application using the MVVM pattern. I have a window with two controls: a user control and a button. The button's Command property is bound to a RelayCommand (from MVVM Light) on the user control's VM. Here is the xaml:

<local:TraceChartView x:Name="chartView"/>
<Button Content="Copy To Clipboard" 
        Command="{Binding ElementName=chartView, 
                          Path=DataContext.CopyToClipboardCommand}"/>

So clicking the "Copy To Clipboard" button executes the CopyToClipboardCommand RelayCommand in the TraceChartView's VM. This works fine, but it requires me to use a complicated mechanism to get the command from the VM to the TraceChartView's code behind where it can actually implement the action. It seems like I should be able to implement something like a dependency property in the TraceChartView's code behind and bind the button action directly to it, i.e.:

<local:TraceChartView x:Name="chartView"/>
<Button Content="Copy To Clipboard" 
        Command="{Binding ElementName=chartView, 
                          Path=CopyToClipboardCommand}"/>

I tried this and a bunch of variations, but no luck. Is there any way to do this?

Edit:

I tried doing this with a Dependency Property. Here is the TraceChartView code behind:

private static readonly DependencyProperty CopyToClipboardCommandProperty =   
    DependencyProperty.Register("CopyToClipboardCommand",
                                typeof(ICommand),
                                typeof(TraceChartView),
                                new FrameworkPropertyMetadata(null, OnSomePropertyChanged));

private static void OnSomePropertyChanged(DependencyObject d, 
                                          DependencyPropertyChangedEventArgs e)
{
    // never called
}

public ICommand CopyToClipboardCommand
{
    set
    {
        this.SetValue(CopyToClipboardCommandProperty, value);
    }
    get
    {
        return (ICommand)this.GetValue(CopyToClipboardCommandProperty);
    }
}

I tried binding like this:

<local:TraceChartView x:Name="chartView"/>
<Button Content="Copy To Clipboard" 
        Command="{Binding ElementName=chartView, 
                          Path=CopyToClipboardCommand}"/>

This doesn't produce any console errors, but my callback is never called.

if you own TraceChartView it doesn't need to be an attached dependency property, it can be a DependencyProperty on the TraceChartView class itself. — John Gardner
– John Gardner, Commented May 3, 2014 at 0:24
I've you've tried a bunch of variations, show some, and any binding errors you got from the console. because you're right, you should be able to do what you want to do. — John Gardner
– John Gardner, Commented May 3, 2014 at 0:27

user2302873 · Accepted Answer · 2014-05-04 02:23:41Z

1

The problem with my code is that I created a dependency property, but I never set the property to anything. When I did this in the constructor, it worked:

public TraceChartView()
{
    InitializeComponent();

    CopyToClipboardCommand = new RelayCommand(DoCopyToClipboard);
}

For my implementation, I don't really need a dependency property, and the whole thing can be implemented with a single line in the code behind:

public RelayCommand CopyToClipboardCommand { get { return new RelayCommand(DoCopyToClipboard); } }

with this xaml:

<local:TraceChartView x:Name="chartView"/>
<Button Content="Copy To Clipboard" 
    Command="{Binding ElementName=chartView, 
                      Path=CopyToClipboardCommand}"/>

I hope that this will help some other WPF beginner.

answered May 4, 2014 at 2:23

user2302873

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