Binding to Commands
Controls that carry out an action, such as Button have a Command property which can be bound to an ICommand. When the control is activated (e.g. when a button is clicked) the ICommand.Execute method will be called.
A good implementation of ICommand can be found in ReactiveUI's ReactiveCommand. If you've created your application using the Avalonia MVVM Application template then this will be available by default. See the ReactiveUI documentation for more information.
An example:
namespace Example
{
public class MainWindowViewModel : ViewModelBase
{
public MainWindowViewModel()
{
DoTheThing = ReactiveCommand.Create(RunTheThing);
}
public ReactiveCommand<Unit, Unit> DoTheThing { get; }
void RunTheThing()
{
// Code for executing the command here.
}
}
}
<Window xmlns="https://github.com/avaloniaui">
<Button Command="{Binding DoTheThing}">Do the thing!</Button>
</Window>
CommandParameter
You can also pass a parameter to the command using the CommandParameter property:
namespace Example
{
public class MainWindowViewModel : ViewModelBase
{
public MainWindowViewModel()
{
DoTheThing = ReactiveCommand.Create<string>(RunTheThing);
}
public ReactiveCommand<string, Unit> DoTheThing { get; }
void RunTheThing(string parameter)
{
// Code for executing the command here.
}
}
}
<Window xmlns="https://github.com/avaloniaui">
<Button Command="{Binding DoTheThing}" CommandParameter="Hello World">Do the thing!</Button>
</Window>
Note that no type conversion is carried out on CommandParameter, so if you need your type parameter to be something other than string you must supply an object of that type in XAML. For example to pass an int parameter you could use:
<Window xmlns="https://github.com/avaloniaui"
xmlns:sys="clr-namespace:System;assembly=mscorlib">
<Button Command="{Binding DoTheThing}">
<Button.CommandParameter>
<sys:Int32>42</sys:Int32>
</Button.CommandParameter>
Do the thing!
</Button>
</Window>
Like any other property, CommandParameter can also be bound.
Binding To Methods
ICommand.Execute
Sometimes you just want to call a method when a button is clicked without the ceremony of creating a command. You can do that too!
namespace Example
{
public class MainWindowViewModel : ViewModelBase
{
public void RunTheThing(string parameter)
{
// Code for executing the command here.
}
}
}
<Window xmlns="https://github.com/avaloniaui">
<Button Command="{Binding RunTheThing}" CommandParameter="Hello World">Do the thing!</Button>
</Window>
ICommand.CanExecute
If you need to have execution dependent on CommandParameter or your ViewModel property, you can define a named method formed by the prefix "Can" and the name of your execution method; the method will accept an object parameter which is the CommandParameter and return a Boolean which determines if the method is executable.
namespace Example
{
public class MainWindowViewModel : ViewModelBase
{
public void RunTheThing(string parameter)
{
// Code for executing the command here.
}
bool CanRunTheThing(/* CommandParameter */object parameter)
{
return parameter != null;
}
}
}
Trigger ICommand.CanExecute
if you want trigger CanExecute from your ViewModel, you have to decorate it with one or more DependsOn attributes, where propertyName is the name of the property that will activate the CanExecute method when it changes value.
namespace Example
{
public class MainWindowViewModel : ViewModelBase
{
bool _isTheThingEnabled = true;
bool IsTheThingEnabled
{
get
{
return _isTheThingEnabled;
}
set
{
if(value == _isTheThingEnabled)
return;
_isTheThingEnabled = value;
PropertyChanged?
.Invoke(this, new PropertyChangedEventArgs(nameof(IsTheThingEnabled)));
}
}
public void RunTheThing(string parameter)
{
// Code for executing the command here.
}
[DependsOn(nameof(IsTheThingEnabled))]
bool CanRunTheThing(/* CommandParameter */object parameter)
{
return IsTheThingEnabled && parameter != null;
}
}
}