Introduction
In this tip, we will learn about WPF Commands. Commands go very well with MVVM pattern (Model- View-ViewModel). We will also see how actually the view knows about its ViewModel and how the view invokes the operations on ViewModel which can be done by using Commands in WPF.
Background
We will follow this in a step by step approach rather than looking at the complete code at once which will help us to understand what each part of code does in a very better way.
Let’s have a look at the MVVM architecture.
We use the standard conventions for naming the classes as follows:
Views are suffixed with View after the name of the View (e.g.: StudentListView)ViewModels are suffixed with ViewModel after the name of the ViewModel. (e.g.: StudentListViewModel)Models are suffixed with Model after the name of the Model (e.g.: StudentModel).
Using the Code
So enough of theory now, let’s dive into the code and see a working example for MVVM and how to use commands in MVVM.
Create a new WPF project in Visual Studio. Rename the MainWindow as MainWindowView to follow up our conventions.
Next, we need to create a new class with name MainWindowViewModel that will act as the ViewModel for the MainWindowView.
What we do here in MVVM is that we tell the View about what its ViewModel will be. This can be done by setting the Data Context for that view. Models are used in the ViewModel and they do not have any connection between some specific views.
Example code for setting the Datacontext goes like this.
Open the MainWindowView.xaml.cs and set the data context as follows.
MainWindowView.xaml.cs
<Window x:Class="WpfExample.MainWindowView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525"
xmlns:local="clr-namespace:WpfExample">
<Window.DataContext>
<local:MainWindowViewModel/>
</Window.DataContext>
<Grid>
</Grid>
</Window>
Here the local is an alias to our namespace WpfExample. It’s required so that the framework knows where MainWindowViewModel is available.
Now we have set the MVVM pattern. Now the MainWindowView knows that’s its ViewModel is MainWindowViewModel.
We will verify this by using a simple binding.
Let's add a button to view and set the button content using a instance from ViewModel.
The View
We will add the button to the view and we will set its content using binding as follows:
MainWindowView.xaml.cs
<Window x:Class=" WpfMvvmExample.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:local="clr-namespace:WpfMvvmExample"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Window.DataContext>
<local:MainWindowViewModel/>
</Window.DataContext>
<Grid>
<Button Width="100"
Height="100" Content="{Binding ButtonContent}"/>
</Grid>
</Window>
The ViewModel
namespace WpfExample
{
class MainWindowViewModel
{
public string ButtonContent
{
get
{
return "Click Me";
}
}
}
}
In the above code, we are telling the View to fetch the content of button from ButtonContent property present in the ViewModel.
<Button Width="100" Height="100" Content="{Binding ButtonContent}"/>
Now if we run the application, we can see that the button content is set to string “Click Me”.
So this interprets that our MVVM is working properly.
Now We Move In To the ICommand Interface
Now, let’s try to add click functionality to the button using Commands in WPF.
Commands provide a mechanism for the view to update the model in the MVVM architecture.
First, we have a look at the ICommand interface.
bool CanExecute(object parameter);
void Execute(object parameter);
event EventHandler CanExecuteChanged;
We will create a sample application that displays a message box with message “HI” when a button is clicked and we will add another button that toggles whether the Hi button can be clicked.
We create a class called RelayCommand which implements ICommand interface. This class acts as Enhancement for the ICommand and extracts the boiler plate code to a separate class.
public class RelayCommand : ICommand
{
private Action<object> execute;
private Predicate<object> canExecute;
private event EventHandler CanExecuteChangedInternal;
public RelayCommand(Action<object> execute)
: this(execute, DefaultCanExecute)
{
}
public RelayCommand(Action<object> execute, Predicate<object> canExecute)
{
if (execute == null)
{
throw new ArgumentNullException("execute");
}
if (canExecute == null)
{
throw new ArgumentNullException("canExecute");
}
this.execute = execute;
this.canExecute = canExecute;
}
public event EventHandler CanExecuteChanged
{
add
{
CommandManager.RequerySuggested += value;
this.CanExecuteChangedInternal += value;
}
remove
{
CommandManager.RequerySuggested -= value;
this.CanExecuteChangedInternal -= value;
}
}
public bool CanExecute(object parameter)
{
return this.canExecute != null && this.canExecute(parameter);
}
public void Execute(object parameter)
{
this.execute(parameter);
}
public void OnCanExecuteChanged()
{
EventHandler handler = this.CanExecuteChangedInternal;
if (handler != null)
{
handler.Invoke(this, EventArgs.Empty);
}
}
public void Destroy()
{
this.canExecute = _ => false;
this.execute = _ => { return; };
}
private static bool DefaultCanExecute(object parameter)
{
return true;
}
}
The CommandManager.RequerySuggested is responsible for enabling and disabling of "Click to Hii" button.
TheView
<Window x:Class="WpfExample.MainWindowView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525"
xmlns:local="clr-namespace:WpfExample">
<Window.DataContext>
<local:MainWindowViewModel/>
</Window.DataContext>
<Grid>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition/>
</Grid.RowDefinitions>
<Button Grid.Row="0" Command="{Binding HiButtonCommand}"
CommandParameter="Hai" Content="{Binding HiButtonContent}"
Width="100"
Height="100" />
<Button Grid.Row="1" Content="Toggle Can Click"
Command="{Binding ToggleExecuteCommand}" Width="100" Height="100"/>
</Grid>
</Window>
TheViewModel
class MainWindowViewModel
{
private ICommand hiButtonCommand;
private ICommand toggleExecuteCommand { get; set; }
private bool canExecute = true;
public string HiButtonContent
{
get
{
return "click to hi";
}
}
public bool CanExecute
{
get
{
return this.canExecute;
}
set
{
if (this.canExecute == value)
{
return;
}
this.canExecute = value;
}
}
public ICommand ToggleExecuteCommand
{
get
{
return toggleExecuteCommand;
}
set
{
toggleExecuteCommand = value;
}
}
public ICommand HiButtonCommand
{
get
{
return hiButtonCommand;
}
set
{
hiButtonCommand = value;
}
}
public MainWindowViewModel()
{
HiButtonCommand = new RelayCommand(ShowMessage, param => this.canExecute);
toggleExecuteCommand = new RelayCommand(ChangeCanExecute);
}
public void ShowMessage(object obj)
{
MessageBox.Show(obj.ToString());
}
public void ChangeCanExecute(object obj)
{
canExecute = !canExecute;
}
}
Final application looks like this:
I have attached the sample project along with this tip. Hope you find this tip helpful.
