How can the ViewModel request an update in the View in WPF/MVVM?

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I have a dependency property on a control in my View that is bound to a field on my ViewModel. When the user clicks a menu item I want the control to update the value of that property so the ViewModel can save it in an XML file. What is the correct mechanism to have the ViewModel request that the View update that property?

Edit: move the code from comment below

The XAML:
	
   "open bracket" controls:MyControl 
   MyProperty="{Binding MyProperty, Mode=TwoWay}" "close bracket"
			
   MyControl class:
	
   public static readonly DependencyProperty MyPropertyProperty =
      DependencyProperty.Register("MyProperty",
                  typeof(byte[]),
                  typeof(MyControl),
                  new PropertyMetadata(null)
                  );
				  
   ViewModel property:
	byte [] MyProperty {get;set;}
			

// Code on ViewModel that is run when the MenuItem is clicked:
	
   public void MyMenuItemFunction()
   {
      // Get the up-to-date value of MyProperty from MyControl here
   }

Posted 26-Mar-12 2:33am

bscaer

Updated 26-Mar-12 13:26pm

Nelek

v2

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SteveAdey · Answer 1 · 2012-03-26T10:05:00

Solution 1

You should use command binding. The ViewModel would expose an ICommand property. You would derive a class from ICommand that will handle the Execute. When you instantiate the command object from your ViewModel, you can pass anything else you need into there.

The menu command can then use binding via its Command property. That will then call your Execute method when pressed.

Posted 26-Mar-12 10:05am

SteveAdey

Comments

bscaer 26-Mar-12 16:08pm

Thanks. I do have a menu command bound to the ViewModel but when that command is executed how does the ViewModel tell the control to update the value of its property?

SteveAdey 26-Mar-12 16:10pm

Which property? Is it one on your ViewModel?

bscaer 26-Mar-12 16:14pm

I have a dependency property on a control in my View that is bound to a field on my ViewModel. I want the ViewModel to tell the View to update the value for the property.

SteveAdey 26-Mar-12 16:15pm

Maybe I'm missing something, but why can't you just modify the property on your ViewModel directly, rather than get the control to do it?

bscaer 26-Mar-12 16:54pm

When the menu item is clicked, I need to tell the control to calculate a value and pass it back to the ViewModel. How to tell the control to calculate the value at that point in time?

SteveAdey 26-Mar-12 16:59pm

Ok, this is how I see it.. You have a dependency property on your control that is bound to your ViewModel. You have a Command on your ViewModel which is bound to your Menu Item. When you click the menu item, you want to use that value, do a calculation and then update your view model.

If this is correct, then you need to add another dependency property to your control, bind that to your ViewModel, but in the binding, set its Mode=OneWayToSource. Then when you update the dependency property in your control, it will push the value back to your ViewModel property that you have bound to.

bscaer 26-Mar-12 17:01pm

Thanks. That is all correct. My question is how to do I tell the control to update its property?

SteveAdey 26-Mar-12 17:09pm

Which control is it? Is it your own control, or a standard one? If it's a standard one, then you are best writing a Behavior for it and exposing some dependency properties on it. Then you can bind these to where you need them on your view model. The click will then update the property.

This should give you an idea of where to start with the Behavior:

http://tap-source.com/making-wpf-images-icommand-aware/

That's demonstrating a Behavior for a ComboBox

bscaer 26-Mar-12 18:13pm

Thanks. The problem is that the click is not on the control. The click is on a menu item.

SteveAdey 26-Mar-12 18:31pm

If you'd like to post some code, I can take a look.

bscaer 26-Mar-12 18:43pm

"<pre> The XAML:

"open bracket" controls:MyControl
MyProperty="{Binding MyProperty, Mode=TwoWay}" "close bracket"

MyControl class:

public static readonly DependencyProperty MyPropertyProperty =
DependencyProperty.Register("MyProperty",
typeof(byte[]),
typeof(MyControl),
new PropertyMetadata(null)
);

ViewModel property:

byte [] MyProperty {get;set;}

Code on ViewModel that is run when the MenuItem is clicked:

public void MyMenuItemFunction()
{
// Get the up-to-date value of MyProperty from MyControl here
}</pre>

SteveAdey 27-Mar-12 1:24am

Ok, the quickest way I can think of at the moment (and it is 6 o'clock in the morning here), is to add another dependency property to your control. Something like DoCalc which is a bool and listens for a property change in PropertyMetadata. Put the a property on your ViewModel called the same. Bind your control's DoCalc to your ViewModel's DoCalc. When the menu item function is run set DoCalc=true. This will trigger the property changed event in your control. Do your stuff, set the MyProperty and then reset DoCalc=false. This should now do what you need :-)

bscaer 27-Mar-12 10:08am

Thanks very much. That is exactly what I had already implemented. Sorry that I didn't mention it. I was hoping for a clean, MVVM method that didn't involve having to touch two properties to get one value.