Nikhil Kothari's Weblog : TechEd South Africa Slides and Code

This is a quick blog post to share slides and demos from my presentations at TechEd in South Africa. Feel free to post questions here...

From the talk on ViewModel and Application Patterns for Silverlight...
The app I built was a mini-Twitter client named TwitterBug. The demo covered the following:

The ViewModel pattern using data-binding and commanding
Behaviors as a mechanism to encapsulate repetitive view functionality
Implementing an IoC container and dependency injection using MEF
EventAggregator pattern to facilitate view model to view model communication
Designer/developer workflow
Unit testing of view models
Using .NET RIA Services and building a DomainService that uses LINQ to Twitter as its DAL

.NET RIA Services…
For this session I built a basic library management front-end. The code demonstrates a simple CRUD app for managing a list of books:

Unified client and server as part of a single application
LINQ as a cross-tier query language and mechanism
A metadata pipeline from the database to the middle-tier to the client
Validation and authorization
Shared code
Using POCO/non-DAL types in conjunction with Linq to SQL DAL types
Imperative use of client framework, as well as declarative use of DomainDataSource
Use of the DataForm, and customizing it for read-only and editable views

Additionally I also demo’d using the DomainService/DomainContext pattern in a broader variety of application contexts:

Using Azure storage as the DAL
Creating jQuery/Ajax and ASP.NET MVC-based front-ends/presentation layers
Creating an ADO.NET Data Service (aka Astoria service) based on a domain service
Implementing a proof-of-concept offline data model for use in Silverlight Out-of-Browser applications

Most of these scenarios were described at greater length in my MIX blog post. I’ll go over more details on various topics with individual blog posts when I am back in Redmond.

Posted on Tuesday, 8/4/2009 @ 3:02 PM | #Silverlight

Comments

Anil

Posted on 8/7/2009 @ 10:56 PM

Hi Nikhil:

Are there any plans to evolve domaindatasource( or create an adapter that encapsulates domaincontext) into something that can be implemented as "SearchViewModel" easily (with common functionalities such as datapaging, CRUD/Query methods on a given entity and commands that respond to bindable xaml elements???

This MVVM pattern seems cumbersome to implement. Essentially recreating all the functionality that is already provided in the code behind.

Any thoughts in your spare time would be much appreciated.

Thanks,
Anil

Nikhil Kothari

Posted on 8/12/2009 @ 8:24 AM

@Anil
MVVM is about refactoring functionality you'd traditionally have put in code-behind, but written in a way that the logic is decoupled from the UI. So its not fundamentally duplicate work. It does require a bit of practice and experience, and once you're familiar with the approach, it will likely feel very natural, and fairly clean, and at the very least far from cumbersome.

You can certainly think of a standard view model against a DomainContext. However that IMO defeats the purpose of a view model - a view model is where you put _your_ custom logic. If a generic view model could be created, you might as well package it as a component, and drag/drop it or instantiate it with one tag in markup - oh, wait, that is called a DataSource! :-)

Anil

Posted on 8/12/2009 @ 12:04 PM

Nikhil,

"If a generic view model could be created, you might as well package it as a component, and drag/drop it or instantiate it with one tag in markup - oh, wait, that is called a DataSource! :-) "

--LoL!! That was a good one.

I am still trying to get a grip around MVVM. So please bear with me. The way I understand it, it does seem like a bit of work to hook custom commands, events, triggers, actions and behaviors to be wired up to the view model code. I wasn't clear with my question. I wasn't talking about the duplicate work in terms of "functionality" that I would write in the code-behind to respond to some interesting UI events. I am talking about duplicating the mechanism that triggers my code-behind. Take any interesting events on my page (or user control) that can be wired up to code-behind methods. It's a piece of cake. In order to implement MVVM, I have to resort to some framework to provide the bridge between my view and viewmodel. [viz., SilverlightFX, SLExtensions, Prism [I am leaning towards SilverlightFX by the way -;) goes beyond command implementation].

Decoupling code-behind logic into a separate view model is truly amazing. I can see all the benefits and understand the true intention behind it. But as it appears for now, I am still thinking...

How much code in the code-behind is complex enough to justify a separate viewmodel? A simple child window that displays some custom message with some colors/glitz has to be decoupled to a viewmodel?

Is it a rule that there has to be 1:1 relationship between viewmodel and the view? What if my view is loaded with half-a-dozen controls that are delegated to perform some real subtasks. Could they all share the same viewmodel? (I see "$model" and "$datacontext" bits come as rescue though!)

Can I still have the code-behind respond to UI events (default implementation, simple buttonclick) but call upon viewmodel for the functionality that it should respond with?

Thanks for your time and reponse.

--Anil

Nikhil Kothari

Posted on 8/12/2009 @ 10:25 PM

@Anil - All good questions.

Yes, there is some cost specifying the glue between view and its model once you decouple things. At the same time keep in mind that a view model is about encapsulating view state and interaction logic, and not view logic. Code-behind is perfectly fine to have to implement view logic (eg. starting/stopping storyboards). It is also perfectly fine to handle events (from UI elements and the view model itself) and have event handlers in code-behind call into the view model... essentially in that case your glue is imperative rather than declarative. It is not the amount of code-behind that signals whether a view model is warranted or not, but how much code exists that could be decoupled, which is of course harder to think of up front. Even then, you have to apply some level of judgment depending on what you might want to test, develop independent of design, or where the code will be as it evolves...

Also, there isn't a rule to get to zero code-behind, but initially the desire to get to zero helps push one to internalize the viewmodel pattern. Once that happens, typically you tend to be able to separate what is truly view logic that is ok to be expressed as code-behind, and what is interaction logic and associated state that can be naturally decoupled from the view. The question I do ask of remaining view logic is how much of it is truly view specific, vs. how much of it can be encapsulated into a reusable component that is declaratively instantiated (eg. a custom control, or attached behavior).

You can definitely have multiple views bound to the same view model. That is one of the potential benefits of more advanced usage of the viewmodel pattern.

Hope that helps.

jacky

Posted on 8/17/2009 @ 6:51 PM

hi, after i published my silverlight project ,which used ria service ,and run it on IIS6.0 ,visit it with ie8.0 ,i found that the client did not display any data . i track database ,sure that it have queried data ,then debug it with vs2008+IIS6.0 ,it really have data at logic layer ,that use DomainService. But when it return to the callback method ,the result is empty .so ,why ,does here something importent i need to config with my ii6.0? --------by the way ,when debug with vs development server,it all done well .

Nikhil Kothari

Posted on 8/22/2009 @ 10:57 AM

@Quinton
The demo does in fact do polling. I was responding to a question from the audience about other options and alluded to duplex notifications as a possibility when we build offline as a real feature beyond the proof of concept stage. That said, you can use WCF directly and the support for duplex communication in Silverlight to send update notifications, and RIA Services to fetch updated data in response. Its just not integrated end-to-end for this release.

@Vitor
You have absolutely the right observation. When you look at a control, the code in it is like a view model, and its template is the view. It definitely works, and the pattern is quite applicable.

Stefan Buys

Posted on 10/12/2009 @ 5:55 AM

Hi Nikhil,

We attend both your talks (Parts 1 and 2) of .NET RIA Services for Silverlight. I am currently in the process of building a framework to be used for future Silverlight LOB applications within our business. I find that working with SQL Server (LinqToSQL) works very well in RIA Services, but a large portion of our business is built on of Oracle. I manage to scavenge an EDM Provider from codeplex.com/LinqToOracle for Oracle that enables me to utlise LinqToEntities against an Oracle based EDM.

However, most of our Oracle data driven apps utilize Oracle stored procedures for all CRUD events modelled on top of ODP.NET. Is it possible to leverage an existing ODP.NET based DAL in RIA Services?

TechEd South Africa Slides and Code - .NET RIA Services and ViewModel

Comments

Raghuraman

Steve Strong

Donald Massyn

Anil

Abinash Bishoyi

shailesh

Nikhil Kothari

Zeeshan Umar

Anil

Nikhil Kothari

Anil

Quinton

Vitor

jacky

Nikhil Kothari

Amit

Nikhil Kothari

Stefan Buys

Nikhil Kothari

Prabha

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