Actions

You have a button, and you want to click it and execute a method on your ViewModel? Actions cover this use-case.

Actions and Methods

In 'traditional' WPF, you'd create a property on your ViewModel which implements the ICommand interface, and bind your button's Command attribute to it. This works fairly well (the ViewModel knows nothing about the View, and code-behind is not required), but it's a bit messy - you really want to be calling a method on your ViewModel, not executing a method on some property.

Stylet solves this by introducing Actions. Look at this:

C#	VB.NET
class ViewModel : Screen { public void DoSomething() { Debug.WriteLine("DoSomething called"); } }	Class ViewModel Inherits Screen   Public Sub DoSomething()   Console.WriteLine("DoSomething called")   End Sub End Class

<UserControl x:Class="MyNamespace.View"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:s="https://github.com/canton7/Stylet">
   <Button Command="{s:Action DoSomething}">Click me</Button>
</UserControl>

As you might have guessed, clicking the button called the DoSomething method on the ViewModel to be called.

It's that simple.

If your method accepts a single argument, the value of the button's CommandParameter property will be passed. For example:

C#	VB.NET
class ViewModel : Screen { public void DoSomething(string argument) { Debug.WriteLine(String.Format("Argument is {0}", argument)); } }	Class ViewModel Inherits Screen   Public Sub DoSomething(argument As String)   Debug.WriteLine(String.Format("Argument is {0}", argument)   End Sub End Class

<UserControl x:Class="MyNamespace.View"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:s="https://github.com/canton7/Stylet">
   <Button Command="{s:Action DoSomething}" CommandParameter="Hello">Click me</Button>
</UserControl>

Note that Actions also work on any ICommand property, on anything (e.g. a KeyBinding).

Guard Properties

You can also control whether you button is enabled just as easily, using Guard Properties. A guard property for a given method is a boolean property which has the name "Can<method name>", so if your method is called "DoSomething", the corresponding guard property is called "CanDoSomething".

Stylet will check whether a guard property exists, and if so, will disable the button if it returns false, or enable it if it returns true. It will also watch for PropertyChanged notifications for that property, so you can change whether the button is enabled.

For example:

C#	VB.NET
class ViewModel : Screen { private bool _canDoSomething; public bool CanDoSomething { get { return this._canDoSomething; } set { this.SetAndNotify(ref this._canDoSomething, value); } } public void DoSomething() { Debug.WriteLine("DoSomething called"); } }	Class ViewModel Inherits Screen   Public ReadOnly Property CanDoSomething As Boolean Get Return Me.someOtherConditionIsSatisfied() End Get End Property   Public Sub DoSomething() Debug.WriteLine("DoSomething called") End Sub End Class

Events

But what about if you want to call a ViewModel method when an event occurs? Actions have that covered as well. The syntax is exactly the same, although there's no concept of a guard property here.

<UserControl x:Class="MyNamespace.View"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:s="https://github.com/canton7/Stylet">
   <Button Click="{s:Action DoSomething}">Click me</Button>
</UserControl>

The method which is called must have zero, one, or two parameters. The possible signatures are:

C#	VB.NET
public void HasNoArguments() { }   // This can accept EventArgs, or a subclass of EventArgs public void HasOneSingleArgument(EventArgs e) { }   // Again, a subclass of EventArgs is OK public void HasTwoArguments(object sender, EventArgs e) { }	Public Sub HasNoArguments() End Sub   ' This can accept EventArgs, or a subclass of EventArgs Public Sub HasOneSingleArgument(ByVal e As EventArgs) End Sub   'Again, a subclass of EventArgs is OK Public Sub HasTwoArguments(ByVal sender As Object, ByVal e As EventArgs) End Sub

Method Return Types

Actions don't care about the return type of the method, and the returned value is discarded.

The exception to this is if a Task is returned (e.g. if the method being invoked is async). In this case, the Task will be awaited in an async void method. This means that if the method returns a Task which ends up containing an exception, this exception is rethrown and will bubble up to the Dispatcher, where it will terminate your application (unless you handle it, with e.g. BootstrapperBase.OnUnhandledException). The effect is the same as if the method being invoked was async void, but means it's easier to unit-test async ViewModel methods.

The Action Target

So far I've been telling a little white lie. I've been saying that the Action is invoked on the ViewModel, but that isn't strictly true. Let's go into a bit more detail.

Stylet defines an inherited attached property called View.ActionTarget. When a View is bound to its ViewModel, the View.ActionTarget on the root element in the View is bound to the ViewModel, and it's then inherited by each element in the View. When you invoke an action, it's invoked on the View.ActionTarget.

This means that, by default, actions are invoked on the ViewModel regardless of the current DataContext, which is probably what you want.

This is a very important point, and one that's worth stressing. The DataContext will probably change at multiple points throughout the visual tree. However, the View.ActionTarget will stay the same (unless you manually change it). This means the actions will always be handled by your ViewModel, and not by whatever object is being bound to, which is almost always what you want.

You can of course alter the View.ActionTarget for individual elements, for example:

C#	VB.NET
class InnerViewModel : Screen { public void DoSomething() { } } class ViewModel : Screen { public InnerViewModel InnerViewModel { get; private set; } public ViewModel() { this.InnerViewModel = new InnerViewModel(); } }	Class InnerViewModel Inherits Screen   Public Sub DoSomething() End Sub   End Class   Class ViewModel inherits Screen   Public Property InnerVM As InnerViewModel   Public Sub New() Me.InnerVM = New InnerViewModel() End Sub   End Class

<UserControl x:Class="MyNamespace.View"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:s="https://github.com/canton7/Stylet">
   <Button s:View.ActionTarget="{Binding InnerViewModel}" Command="{s:Action DoSomething}">Click me</Button>
</UserControl>

Here, InnerViewModel.DoSomething will be invoked when the button is clicked. Also, because the View.ActionTarget is inherited, any children of the Button will also have their View.ActionTarget set to the InnerViewModel.

You can also override the action's target using its Target property. However, due to WPF limitations you cannot bind to this, you must use StaticResource, x:Static, x:Type, etc.

<Button Command="{s:Action DoSomething, Target={x:Static my:Globals.ButtonTarget}}">Click me</Button>

Static Methods

Actions can also invoke static methods, if the target is a Type object (use {x:Type ...} in XAML for this). You can set this using both View.ActionTarget and Action's Target property.

public static class CommonButtonTarget
{
    public static void DoSomething() { }
}

<Button Command="{s:Action DoSomething, Target={x:Type my:CommonButtonTarget}}">Click me</Button>

Actions and Styles

Actions will not work from style setters. The classes required to do this in WPF are all internal, which means there is no way to fix the issue. You will need to use old-fashioned Commands in this (rare) case, unfortunately.

Gotchas - ContextMenu and Popup

View.ActionTarget is of course an attached property, which is configured to be inherited by the children of whatever element it is set on. Like any attached property, and indeed the DataContext, there are certain boundaries it is not inherited across, such as:

• Using a ContextMenu
• Using a Popup
• Using a Frame

In these cases, Stylet will do the best it can to find a suitable ActionTarget (it may, for example, find the the ActionTarget associated with the root element in the current XAML file), but this may not be exactly what you expect (e.g. it may ignore a s:View.ActionTarget="{Binding ...}" line you have somewhere in the middle of your page), or it may (in rare circumstances) fail to find an ActionTarget at all.

In this case, please set s:View.ActionTarget to a suitable value. You may struggle to get a reference to anything outside of a ContextMenu from inside of one: I suggest the BindingProxy technique.

Additional Behaviour

There are two cases which will stop an action from working properly: if the View.ActionTarget is null, or if the specified method on the View.ActionTarget doesn't exist. The default behaviour in each of these cases is as follows:

	View.ActionTarget == null	No method on View.ActionTarget
Commands	Disable the control	Throw an exception when the control is clicked
Events	Enable the control	Throw an exception when the event is raised

The justification for this is that if the View.ActionTarget is null, you must have set it yourself, so you probably know what you're doing. However, if the specified method doesn't exist on the View.ActionTarget, that's probably a mistake, and you deserve to know.

Of course, this behaviour is configurable.

To control the behaviour when View.ActionTarget is null, set the NullTarget property on the Action markup extension so either Enable, Disable, or Throw. (Note that Disable is invalid when the Action is linked to an event, as we have no power to disable anything).

For example:

<Button Command="{s:Action MyMethod, NullTarget=Enable}"/>
<Button Click="{s:Action MyMethod, NullTarget=Throw}"/>

Similarly, you can set the ActionNotFound property to the same values:

<Button Command="{s:Action MyMethod, ActionNotFound=Disable}"/>
<Button Click="{s:Action MyMethod, ActionNotFound=Enable}"/>