I've seen quite a few example of passing a parameter through a command using the RelayCommand class in MVVM Light, but there's one slight difference between what i want and what i have seen.
I want to create a few buttons where all have them have a ModuleType associated. And when their action is executed i want to know which ModuleType it is. And i wanted to do this in a code efficient way, so not having to create the RelayCommands manually, but instead do everything in a foreach loop, also because i don't know how many buttons i have to create at start.
So here is the code. In my ViewModel
public ModuleSelectionViewModel(MachineStatusModel model, int order, List<ModuleType> modules) : base(model)
{
........
// create button for each of the modules
foreach (ModuleType mod in modules)
{
_navBarButtons.Add(new NavButton(mod.ToString(), new RelayCommand<ModuleType>(exec => ButtonExecute(mod)), mod));
}
RaisePropertyChanged("NavBarButtons");
}
// Binding to the Model
public ObservableCollection<NavButton> NavBarButtons
{
get { return _navBarButtons; }
}
// Execut Action
public void ButtonExecute(ModuleType mod)
{
WriteToLog("Selected " + mod.ToString());
}
// Support class
public class NavButton
{
public string ButtonContent { get; set; }
public ICommand ButtonCommand { get; set; }
public ModuleType ButtonModuleType;
public NavButton(string content, ICommand relay, ModuleType moduleType)
{
this.ButtonContent = content;
this.ButtonCommand = relay;
this.ButtonModuleType = moduleType;
}
}
I'm still learning about lambda expressions, so i guess i am doing something wrong on the initialization of the RelayCommand.
If you do a foreach
loop and use the loop variable inside a lambda expression you capture it. Unfortunately the variable is scoped incorrectly (at least in older versions of C#, this changes with C# 5 (thanks, Mafii)).
So you need to do something like:
foreach (ModuleType mod in modules)
{
// New variable that is locally scoped.
var type = mod;
_navBarButtons.Add(new NavButton(mod.ToString(),
new RelayCommand<ModuleType>(param => ButtonExecute(type)), type));
}
Regarding the solution HB posted: not sure why it was not working with lambda functions, why the ButtonExecute was not being execute when the button was pressed. And also i don't understand the logic of defining a lambda function like 'foo => ButtonExecute(foo)' when foo is empty and being passed to the function. But anyway...
This is what i did and is working:
Model
<ItemsControl Name="NavButtonsItemsControl" ItemsSource="{Binding NavBarButtons}">
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<StackPanel Orientation="Horizontal" />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.ItemTemplate>
<DataTemplate>
<Button Content="{Binding ButtonContent}" CommandParameter="{Binding ButtonModuleType}" Command="{Binding ButtonCommand}"/>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
ViewModel (check the rest of the code in my initial question)
.........
_navBarButtons.Add(new NavButton(mod.ToString(), new RelayCommand<ModuleType>(ButtonExecute), type));
.........
private void ButtonExecute(ModuleType state)
{
WriteToLog("Command with parameter " + state.ToString());
}
You can make this more generic by using Object instead of ModuleType. And the solution, instead of using a lambda function, is defining a CommandParameter binding in the button and getting that value as the parameter in the execute function. I don't think that binding is explicitly defined, that's why i was having a hard time understanding how the value was reaching 'ButtonExecute', but following the steps in Dominik Schmidt tutorial it worked.
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