Introduction

In this short series of articles I’ll cover what MVVM is and how to use it in practice, how to solve issues when applying this pattern and how to take advantage from it.

1. Part I: The Basics
2. Part II: Messaging, Unit Testing and moving to a live data source
3. Part III: Validation, Closer integration with the view

In the previous article we discussed what MVVM is and why it matters. In this article we’ll try to benefit from the pattern and talk about the issues you have to battle through when working with MVVM. Our sample application was able to pull some data from a model, and display it on the screen in a master-detail fashion.

What is MVVM and why do you need it?

Do you know the feeling, when you start to develop an application, and in the first couple of days you feel good because you made a really good progress? Do you know the feeling, when after a couple of days, you feel that your design is not that best and when you have to modify a little code, you are afraid of the effect it might have on other parts of you code? The first sign of a bad design is when applying a hack is easier than to implement it the proper way.

1. Introduction

I recently found myself in the following situation. My boss came to me and asked me: “Dude, we have a big Silverlight project, performing a great number of CRUD operations. We need to find a unified way of showing Modal Dialogs in a MVVM manner, without using any code in the View. The logic of showing dialogs must reside in the ViewModel, not in the code-behind. Also do you remember our previous WinForms projects? There are several pretty useful classes, such as BindingSource and DataRowView, implementing BeginEdit, EndEdit and CancelEdit functionality out of the box.

Introduction

If you work with Silverlight or WPF, you have probably met the phrase “MVVM”. Almost everyone who is anyone in the WPF / Silverlight scene has their own MVVM framework, and their own way of explaining and teaching MVVM. Scary terms like IoC, Dependency Injection, Commanding Frameworks, Event Aggregators, Unit Testing, etc just roll off the tongue of the MVVM experts. This is one of the reasons why MVVM is intimidating for a lot of people. Still, you can create perfectly valid MVVM applications without even knowing what those terms mean.

Introduction

Last month I wrote about how we could take some of the new features of Silverlight 4, Webcam control,  and create a simple application that lets you store captured webcam photos to isolated storage and your file system.

In this article we will take this proof of concept and demonstrate how through the use of commanding and binding we can virtually eliminate all code behind and implement to a strong MVVM architectural pattern.

Getting Started

I think few would argue with the value of a strong separation of concerns within the design of an application.

During a recent work I found some trouble working with the TreeView control in a Model-View-ViewModel scenario. As usually happen, the standard controls are designed to work in an event-driven behavior and this non always marries with a correct MVVM implementation. After some work, I found a way to change the TreeView and transform it to be lazy loadable.

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The Model-View-ViewModel pattern has been introduced in Silverlight by few time and this platform lacks a complete support to the pattern by the standard controls.

Whenever a new version of Silverlight is released, I start examining its features from two perspectives: how would my current (and planned) LOB applications benefit from using these new features, and what cool new things can I build to entertain my children.

This article will guide you through the process of creating a doodling application (you know, for kids ;)), while covering some of the most visible (or not) new features, coming with Silverlight 4. But this is not just about new features, it’s about how they are prepared and served.

1. Introduction

Whatever software application you want to create, the most important problem that must be solved is the tight coupling. Mixing one layer with another is a very common mistake and it is the main reason for your application to be tightly coupled. For example: as a practical example in this article I will create a simple data entry application, which purpose is to load, create, edit, delete and save data. The most straightforward way to create the application is to put everything in the user interface (handling the button’s click events and writing the code there). It is the easiest way but it is far from the best. This will produce a low quality code and high complexity. And when the things are tightly coupled, one change can lead you into chasing breaking changes in the whole code. So the most important thing is to keep the layers separate, one layer – one responsibility. Yes, it is true, that creating an application with separate tiers requires additional work, but this may save you a headache in the future.

The most famous solutions and approaches (patterns) for creating a multi layer application are the MVC and MVP patterns. Since Silverlight does not require reinventing the wheel, these patterns and practices can be applied with great success when you create a Silverlight application. In previous articles I showed you how the Model-View-Presenter (MVP) pattern and the Model-View-Controller (MVC) pattern can be used in Silverlight. Today I decided to continue and to present you another pattern – it is the Model-View-ViewModel (MVVM). MVVM is tailor-made for WPF and it is an adaptation of the MVC and MVP.

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