Command binding in multiwindow WPF app

Question

My application can have multiple designer windows. Each window constitutes of several user controls which communicates dynamically with the help of RelayCommands. I created the following class as the backbone of the commanding infrastructure.

public static class Commands
{
    public static readonly RoutedCommand EntityEditRequest = new RoutedCommand();
    public static  RelayCommand EntityEditorChangeRequest;
    public static RelayCommand XMLUpdateRequest;
    public static RelayCommand SaveRequest;   
}

Each viewmodel for the user controls will do something like this in the constructor

 public XMLEditorViewModel()
 {
        Commands.Commands.SaveRequest = new RelayCommand(Save_Executed);
        Commands.Commands.XMLUpdateRequest = new RelayCommand(UpdateXML); 
 }

However, I completely missed the point that the application can have multiple windows. When each window is opened the static Commands are set for that particular window.

Example:

Window A is opened-the constructors for the usercontrols set the RelayCommands and all is well.

Window B opened-the constructors for the usercontrols set the RelayCommands. Window A's command binding is lost!

So when I change the tab to Window A(the windows are tabbed) no commands are working.

I need some idea so that when I change the tab the active window always sets the commands. I can try to put the commanding in tab_selection_changed event, but somehow it is looking bad to me. Is there a proper way to do this? Any help is much appreciated.

Edit:

The question proved a bit confusing among the readers. I am not trying to make multiple subscribers for a command. At any given point only one window is active. This window consists several user controls-some of them loaded dynamically with the help of commands; but each command is handled by a single view model class-so no multi subscribers. My problem is the application can load multiple windows in tabs-only one window is active at any given point-but the user can do to a different tab and make another window active. As the view model constructor assigns the static RelayCommands, when each new window is loaded the static command is set to a new binding.

Window A opened-window A view model constructor sets the static command bind to its object command handler. Window A is active. Commanding is fine.

Window B loaded-window B view model constructor sets the static command bind to its object command handler. Window B is active. Commanding is fine.

Now, User select the Window A tab to set the Window A as active. Commanding wont work. Of course it wont as the Command is bind to Window B command handler.

Theoretically static commands can handle the scenario as at any given point there will be only one active window. But how??

The whole point of commands is better separation of logic from design, using an event to make commands work is the wrong approach. — Danny Varod, Mar 25 '11 at 22:57
@Danny-maybe, but that's completely different and I have a legitimate reason for doing that. The problem here is how to make global command bind to active window when there are many tabbed windows. — Jimmy, Mar 26 '11 at 01:09

Aaron McIver · Accepted Answer · 2011-03-25T22:41:57.783

2

The global command should be a CompositeCommand or similar approach (CompositeCommand is from Prism). This will allow multiple children to register with the command.

  public static CompositeCommand SaveCommand = new CompositeCommand();

The command can then be accessed across the ViewModels or where applicable like so...

  SaveCommand = new DelegateCommand<object>(Save, CanExecuteSave);
  GlobalCommands.SaveCommand.RegisterCommand(SaveCommand);

You can then leverage the IActiveAware interface to define which Window is the active Window and act on the command accordingly.

There is also an MSDN posting on creating globally available commands. Don't forget to unregister the command to avoid a memory leak.

edited Mar 25 '11 at 22:41

answered Mar 25 '11 at 22:36

Aaron McIver

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@Aaron-multiple children binding is not the problem . Please read my edit in the post. – Jimmy Mar 26 '11 at 01:15
@Jimmy This was to display how a global command should be setup; making use of the IActiveAware interface and a composite approach to commanding. Your current design is extremely coupled in nature; you are trying to tie the command binding to the active instance. Use the IActiveAware strategy and thus the command will be activated/deactivated based on the Window in focus. – Aaron McIver Mar 26 '11 at 04:27
@Aaron-interesting. I need to try this on Monday and I will get back to you. – Jimmy Mar 26 '11 at 22:12
Well, I had a look at IActiveaAware and I can see what do you mean. Its not easy in my case as I cannot use any frameworks in the application(like prism)-so I have to do it without a RegionManager as such. I will find some way to activate the IActiveAware object when the window is active. But yes-you gave me the answer. – Jimmy Mar 29 '11 at 13:56

score 1 · Answer 2 · answered Mar 25 '11 at 22:12

1

Any reason why have you decided to put it into static class?

class XMLEditorViewModel
{
    public ICommand SaveRequest { get; private set; }

    public XMLEditorViewModel()
    {
        SaveRequest = new RelayCommand(Save_Executed)?
    }
}

answered Mar 25 '11 at 22:12

Snowbear

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Commands like Save are generally static since you don't want to litter each and every ViewModel with creating a new instance and definition of Save. The static approach to commands provides a global approach to the application; which would allow you to register where relevant...versus creating a one off everywhere you use Save. – Aaron McIver Mar 25 '11 at 22:29
@snowbear-because there are numerous user controls in the window and the command can be fired by any user conrol. Its not that the command will be fired by XMLEditorView – Jimmy Mar 26 '11 at 01:12

score 1 · Answer 3 · answered Mar 25 '11 at 23:03

1

Commands that are not view specific can be defined on static classes.

Commands that are view specific should either be defined on view-model, passed as DataContext to view, enabling separate implementation for different views with different view models, or at least have the views pass a CommandParameter that can be used to either identify them (e.g. reference to the view) or their DataContext.

If the commands are static, register them once only, perhaps on a singleton used by view models.

answered Mar 25 '11 at 23:03

Danny Varod

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@Danny-please read my edited post. My problem is how can I rebind the static commands according to the change of active window in multiple window scenario – Jimmy Mar 26 '11 at 01:14
My point is that you do not have to rebind them if you slightly change your design. – Danny Varod Mar 26 '11 at 18:31

score 0 · Answer 4 · answered Apr 27 '11 at 11:15

create a globally available command, create an instance of the DelegateCommand or the CompositeCommand and expose it through a static class.

public static class GlobalCommands { public static CompositeCommand MyCompositeCommand = new CompositeCommand(); } In your module, associate child commands to the globally available command.

GlobalCommands.MyCompositeCommand.RegisterCommand(command1); GlobalCommands.MyCompositeCommand.RegisterCommand(command2); To increase the testability of your code, you can use a proxy class to access the globally available commands and mock that proxy class in your tests.

The following code example shows how to bind a button to the command in WPF.

Execute My Composite Command

Command binding in multiwindow WPF app

4 Answers4