This view model is obviously designed to manage a list of contacts. It also exposes a delete command and a flag to indicate whether delete is allowed thus maintaining state for the view. The view model here makes a concrete reference to the service, you would most likely wire in that reference externally or use a dependency injection framework. The phone numbers, of course, are faked.
These mechanisms help implement the pattern by binding UI behaviors to the underlying models. In Silverlight, the VSM should be the primary choice for coordination of transitions and animations. Learn more about VSM. The viewmodel becomes wholly responsible for the model in this scenario. You might have heard discussion about view first or viewmodel first. In general, I believe most developers agree that a view should have exactly one viewmodel.
There is no need to attach multiple viewmodels to a single view. A view may be composed of other views, each with its own viewmodel. Viewmodels might compose other viewmodels when necessary often, however, I see people composing and aggregating viewmodels when in fact what they really want is messaging between viewmodels.
While a view should only have one viewmodel, a single viewmodel might be used by multiple views imagine a wizard, for example, that has three views but all bind to the same viewmodel that drives the process.
View first simply means the view is what drives the creation or discovery of the view model. In view first scenarios, the view typically binds to the view model as a resource, uses a locator pattern, or has the view model injected via MEF, Unity, or some other means. This is a very common method for managing views and view models. Here are some of my posts on the topic:. The view is created, then the view model attached. In the App object, it looks like this:. ViewModel first is another method to wire the framework together.
In this scenario, the viewmodel is responsible for creating the view and binding itself to the view. The second issue exists in Silverlight 3 because the ICommand interface is provided, but not implemented. These are implementation details that make it easier to use the MVVM pattern. I also implemented a command. Typically you would have a more generic type of command to handle different situations, but again, for the sake of illustration, I simply created a delete command specific to the function it performs.
I am using a message box to confirm the delete. If you require something more elegant like a ChildWindow , read the scenarios I describe below to better understand how to integrate a dialog box as a service within MVVM. This particular command uses a delegate to callback when it is done, but this would only allow for a single subscriber. A multicast delegate or event will be required if multiple consumers or viewmodels for the command exist.
In Silverlight 4, I can simply bind a button to the command using the Command tag. I built the example in Silverlight 3, which does not have native support. To create the binding, I made a simple trigger — again, specific to this project and for the sake of illustration — to invoke the command, so I can easily bind it in the XAML.
You can also delete a contact. I often receive complaints that blog examples are too simple. You can download the source code for this example here. You can also see it in action here click on the names and try delete … I simulated a slight delay for the initial load and the refresh after a delete.
Click here to view the example in a new window. I think the easiest way to learn the pattern is by seeing a full application being built. In that video, I build some simple viewmodels and show a view that is dynamically swapped based on user selection, and use MEF to wire everything up. A more complex scenario is then introduced in Part 2. So what about those more complicated line of business solutions … the ones that actually have more than one button, multiple views, and complex logic? In my experience, the idea of binding both commands and models or properties is straightforward.
How do we solve these common problems? How do you handle a combo box used for selection of a single, or multiple, items with MVVM? In fact, imagine a scenario where you have a combo-box that has a list of contact names, and another view on the same page that shows the contact details when selected.
In this case, we import a service for getting contacts, wire in the list and set the current contact. The drop down binds to the Contacts collection. The binding would look like this:. This ensures whenever something is selected in the list, the current contact is updated.
View All. Vishnujeet Kumar Updated date Feb 03, Model: Data, entity, ORM. View: Presentation Layer, User Interface. Separation UI and Business layer as view and view model.
Code sharing between pages and forms. Easy to Maintain. Enhance simplicity and testability. Enabled by the robust data binding capability of XAML. No need to use a code behind file minimalist code-behind file.
Provides application development ability for multiple environments. Powerful Data Binding, command, validation and much more. The designer and developer can work together. Why MVVM? It makes code more maintainable, and will help it last longer.
Reusability, assuming we have one WPF application with code and now we want the same business logic in a Windows Phone or Silverlight then that could be quite a bit of hard work. Avoids duplicated code to update views. Unit testable. Easy refactoring. Next Recommended Reading. For me, the most confusing bits were the following: Even though the viewmodels are models of a graphical application, they don't directly reference or use visual concepts.
For example, you don't want references to Windows controls in your ViewModels - those things go in the view. ViewModels simply expose data and behaviors to controls or other objects that will bind to them. For example - do you have a view with a ListBox in it? Your viewmodel is almost certainly going to have some kind of collection in it. Does your view have buttons? Your viewmodel is almost certainly going to have some commands in it.
There are a few kinds of objects that could be considered "viewmodels". The simplest kind of viewmodel to understand is one that directly represents a control or a screen in a relationship, as in "screen XYZ has a textbox, a listbox, and three buttons, so the viewmodel needs a string, a collection, and three commands.
A "thin" viewmodel layer is a set of viewmodels that expose your model objects directly to the views, meaning the views end up binding directly to properties on the model objects.
This can work for things like simple, read-only views, but what if you want to have behavior associated with each object? You don't want that in the model, because the model isn't related to the application, it's only related to your domain.
You can put it in an object that wraps your model object and offers up more binding-friendly data and behaviors. This wrapper object is also considered a viewmodel, and having them results in a "thicker" viewmodel layer, where your views never end up directly binding to anything on a model class.
Collections will contain viewmodels that wrap models instead of just containing models themselves. Improve this answer.
Thanks for clearing that up for me :- — Riegardt Steyn. The double bits u comment above have been confusing me for a very long time. Many articles and blogs only tells the "key features" of MVVM, but when u try to get with it,things begin to be very complicated! Such as "How to navigate those Views?
I'm trying it,it works well so far! So,if i create methods like :ExpandTree at my ViewModel. Is it right way? Wow, this is an excellent article, pretty good job nlawalker — Vivek Shukla. Show 2 more comments. View: The view is a visual element, such as a window, page, user control, or data template. The view model may define logical states that the view can represent visually to the user. Model: Model classes are non-visual classes that encapsulate the application's data and business logic.
InteXX 5, 5 5 gold badges 33 33 silver badges 61 61 bronze badges. Dom Dom Reed Copsey Reed Copsey k 72 72 gold badges silver badges bronze badges. Ok, thanks. I thought of the Model as objects, and ViewModel as methods of handling the objects, such that a view is able to understand the Model "objects".
But I was told that this is wrong, that the ViewModel itself are objects, too. I guess this is what is really confusing me. Rosie: I'd recommend reading or at least skimming through my series I quoted. It's really a "step by step" transition ; — Reed Copsey. Realizing this is zombie-threading a little Filipe Miguel Filipe Miguel 1 1 gold badge 6 6 silver badges 13 13 bronze badges. The link is dead — ibrahim mahrir. I'm downloading the video now.
Sprotty Sprotty 5, 2 2 gold badges 29 29 silver badges 43 43 bronze badges. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown.
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