Some time back, I posted about the Silverlight Extensibility Hacks Preview 2.0. I did a quick port of the code base to WPF under the name WEX or WPF Extensibility Hacks, and now you have the goodness in .NET 3.5 + VS2008.
Just want to have a quick word on what is available. Here are a couple of points about Wex. Wex is built on top of the
- Wex allows you to define multiple conditions for invoking triggers (like you can specify a KeyDown event trigger should be fired only if 'A' is pressed)
- You can specify multiple conditions for invoking each action in a trigger
- Wex introduces a few more Triggers and Actions that you'll see soon
As of now, Slex Preview 2 provides the following Triggers:
EventTrigger - Will be fired when an event is raised. Can listen to events of Elements, or events from your view model.
ReactiveTrigger - Can 'import' an Observable that you may 'export' using MEF. Useful to define and use custom events using
And the following Actions:
InvokeMethodAction - Invoke a method directly in the ViewModel or for a target element, supports passing parameters via XAML
InvokeCommandAction - Invoke an
ICommand in the ViewModel
StoryBoardAction - Start, Stop, Pause, Resume, or Reverse story boards
PropertyAction - Sets a property value (very crude as of now)
You can hook up the event trigger against your ViewModel, or against an element. For example, assume you want to invoke a command in your ViewModel based on various conditions - let us say, when the user presses the key 'A', and if and only if a check box is checked. Here we go:
The concept of reactive trigger is based on the System.Reactive (LINQ to Events) framework and the Managed Extensibility Framework. Basically, you can create an event definition and import that as an event for a control.
Your exported event definition should match the signature
Func<object,IObservable<EventResult>>. You can use the
ObservableExport attribute in Wex.Lib to mark your trigger as an exportable part. Also, the name you provide to the
ExportName attribute will be later used in XAML to 'import' this trigger.
Step 2 - In your application startup, call the Compose method in WexPartComposer, and pass your catalogs
In this case, I'm simply passing an assembly catalog with the current assembly, because I've my trigger as part of my host app. And, I've this line in the App.xaml.cs constructor:
Step 3 - Just use the trigger in your XAML
Here we go, you can import the exported trigger, and this will get fired whenever a key is pressed:
You might have guessed this, but you can create very customized event definitions using
System.Reactive. For example, here is how to create an event definition if you want the trigger to fire only when the arrow keys are pressed:
You can read this article on Reactive Extensions, to understand more on this.
Actions in Wex
You might have already noticed in the above examples how to use actions like
StoryBoardAction, etc. What is interesting is, Wex allows you to fire actions based on conditions (see the EventTrigger example where we are specifying the InvokingConditions for the actions).
I specifically want to comment on the
InvokeMethodAction in Slex, that allows you to Invoke a method in the ViewModel directly. Here we go. Assume that you have an
Add method in your ViewModel, like this:
Now, this is how to invoke the
Add method, passing some parameters. Here, we pass the text in
txt2 as parameters.
A Quick Overview
Have a sneak peak at the actual implementation of Wex. If you examine the Trigger hierarchy, you'll find that we've a
WexTrigger abstract base class, from where other triggers are getting inherited.
WexTrigger - The base class for all Wex framework triggers
EventBasedTrigger - A base class for inheriting event based triggers
ObserverTrigger - An abstract class for creating triggers based on an Observable (
WexTrigger directly inherits from
System.Windows.Interactivity.TriggerBase<DependencyObject>. I'm not going to explain the
System.Windows.Interactivity infrastructure and how each trigger is implemented, but you can definitely go through the source code and find the same yourself. Probably, I'll explain the implementation details in future posts.
Now, let us have a quick look at the Actions involved. Have a look at the actions in the framework.
WexTriggerAction is inherited from
System.Windows.Interactivity.TriggerAction<FrameworkElement>, and from there on, you can go and explore as of now, if you'd like to
As I mentioned, Wex is a quick port of Slex, and seriously, there are lot of areas I still need to re-write to leverage what is already available in WPF (especially on dependency property hooking and all). However, I thought it might be good if I bring out a first cut, so that you can also hack around with me and leverage some of these concepts in your own apps.
And, you may hit a few bugs with this Preview; fix it yourself or wait for Preview 2. But please give feedback!!