Comments on: Very simple MVVM demo application

By: Jeremy

Jeremy — Tue, 05 Apr 2011 14:26:39 +0000

praveen,

This is not a design issue nor a bug. In WPF the CanExecute methods of your commands are called automatically as soon as the UI changes (for example you click on a button or the keyboard focus change). This is handled by the CommandManager class. When you run the application in debug with a breakpoint you can see the behavior you describe. I think this is because the UI is being shown and hidden. If you add a trace (like Console.WriteLine) I think you’ll see much less call to the method.

In Silverlight this is different because the CommandManager class does not exist. But that’s another story

By: praveen

praveen — Tue, 05 Apr 2011 10:22:42 +0000

Hi Jeremy,

I am new to MVVM and tried your code.I have placed break point in side the below method to monitor the control flow .But when I debug it I saw the following method is hitting like in an infinite loop.

public bool CanExecute(object parameter)
{
return this.canExecute == null ? true : this.canExecute();
}

Is that a design issue..? Or I am missing something. I have tried it in Visual Studio 2010 Professional edition in windows 7 machine.

By: Debashish Gupta

Debashish Gupta — Mon, 02 Aug 2010 04:05:05 +0000

Hi Jermy , i saw MVVMlight Framework.But i have already developed a lot in Silverlight 4 . So i cannot move now to another Framework.When i try to use Silverlight 4 , I get the help of Commands . Now i am only left with CollectionViewSource.GetDefaultView(). If i get something equivalent in SL . As when i selecet any thing in my listbox , i cannot save the employee details in the details plain as a lost focus did not happen in the textbox of the details Tab.

By: Jeremy

Jeremy — Tue, 20 Jul 2010 08:56:50 +0000

This sample application is designed to work with WPF, not Silverlight. That’s the reason why I’m suggesting you to take a look at an MVVM framework which supports Silverlight.

By: Debashish Gupta

Debashish Gupta — Tue, 20 Jul 2010 08:54:50 +0000

Hi Jeremy ,
Not sure , why are u telling me to use MVVM Framework.
As i see ur downloadable application works fine.
I cannot add dlls like PresentationFramework.dll etc.
Regards
Debashish.

By: Jeremy

Jeremy — Tue, 20 Jul 2010 07:35:56 +0000

Debashish,

You don’t have the exact same set of functionalities in Silverlight and WPF. That’s why you have error with CommandManager and TypeDescription.

Please take a look at a MVVM frameworks which supports Silverlight in order to see a working implementation of the ICommand interface. For example, you can look at the MVVM Light Toolkit (http://mvvmlight.codeplex.com) and its RelayCommand class (http://mvvmlight.codeplex.com/SourceControl/changeset/view/47157#750822).

Hope this helps

By: Debashish Gupta

Debashish Gupta — Mon, 19 Jul 2010 14:35:13 +0000

Hi , I was trying to go through it.
But i could not make
CommandManager.RequerySuggested += value;
work as it was not able to find the CommandManager
—————–
and
if (TypeDescriptor.GetProperties(this)[propertyName]
in ViewModelBase
work as its not able to find the TypeDescriptor in System.ComponentModel.

I was trying to make a Silverlight Project and trying to make it run.

Regards
Debashish.

By: Damian

Damian — Thu, 15 Jul 2010 12:51:35 +0000

Ok, I see now It changes but after selecting again from list. But in other way. So thank you very much My weekend will be happier thanks your article ! Have a nice weekend, too

By: Jeremy

Jeremy — Thu, 15 Jul 2010 12:11:17 +0000

I just did the test: if I change the second TextBox in the DetailView to display the Age property, and without the correct OnPropertyChanged, any changes in the first TextBox is not reflected in the second TextBox.

However, adding the OnPropertyChanged fix this issue.

By: Damian

Damian — Thu, 15 Jul 2010 11:57:45 +0000

Thank You for help.

Unfortunately when I add second textbox linked with Age property everything works ok. So I convert ObservableCollection to List, and removed inheritance from ViemModelBase from MasterViewModel and it still works. I appreciate that you have time for me. I’m very newbie.