Mvvm what is the model

It fetches the list of "contacts" right away, which is a hard-coded list of me and someone a little more popular. The phone numbers, of course, are faked. Let's get a little more specific and look at how this would be implemented in a sample application. First, the IConfig represents a configuration service in a newsreader, it may contain the account information and feeds that are being fetched , while the IService is "some service" - perhaps the interface to fetch feeds from RSS sources in a news reader application.

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. Fortunately, it's not alone:. 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. If you think about separation of concerns, this makes sense, because if you have a "contact widget" on the screen bound to a "contact viewmodel", and a "company widget" bound to a "company viewmodel", these should be separate views, not a single view with two viewmodels.

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 example I've included with this post is view-first. The view is created, then the view model attached.

In the App object, it looks like this:. In this example, I'm keeping it simple, and not using any frameworks to wire in interfaces and implementations. 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.

In Silverlight 4, commanding is more "out of the box". Commands facilitate binding of events from the view to the viewmodel. 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. For the examples I've provided here, you can view the sample application.

It is very simple and contains exactly one service and one view model with a "mock database". Two views bind to the same viewmodel, and you can click on a contact to see its details.

You can also delete a contact. I often receive complaints that blog examples are too simple. This is sufficiently complex to show a full app without depending on other frameworks, but certainly doesn't show multiple pages and types of views.

The reason you don't see these as often from me is simply because I am a consultant and contractor, so I am constantly building these line of business frameworks and applications for customers, and am not at liberty to share their code. While I can build small examples for posts, I simply don't have the time to build a larger working model. It's something I'd certainly like to do, but just wasn't practical for the timing of this post.

Imagine that you are coding client for ICQ or something like that. How is the ViewModel supposed to know, that somebody send you a message? A ViewModel only simplifies the output from a Model. If the model is very simple, a ViewModel isn't necessary. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Asked 10 years, 9 months ago. Active 8 years, 11 months ago. Viewed 18k times. Thank you very much! Improve this question. Horse Pen Horse Pen 1 1 gold badge 5 5 silver badges 7 7 bronze badges. Thanks, i've already read it. The key is the "if needed" after the model. Of course defining when needed is an interesting problem. I think it rather depends on the interactions i. Show 1 more comment. Active Oldest Votes.

Having the configure code separately for each view may be simpler in that case. MVVM works well if your app requires many model-to-view transformations. However, not every object will neatly fit into the categories of model, view or view model.

Instead, you should use MVVM in combination with other design patterns. Furthermore, MVVM may not be very useful when you first create your application.

MVC may be a better starting point. This app displays nearby coffee shops provided by Yelp. Everything you need has been included for you in the starter project. The only thing you need to remember is to open CoffeeQuest. Open APIKeys. These map pins are kind of boring. Open MapPin. MapPin takes a coordinate , title , and rating , then converts those into something a map view can display… does this sound familiar?

First, you need to give this class a better name. This will rename both the class name and file name in the File hierarchy. Next, select the Models group in the File hierarchy and press Enter to edit its name.

Rename this to ViewModels. Finally, click on the yellow CoffeeQuest group and select Sort by name. Ultimately, your File hierarchy should look like this:. BusinessMapViewModel needs a few more properties in order to show exciting map annotations, instead of the plain-vanilla pins provided by MapKit. Still inside BusinessMapViewModel , add the following properties after the existing ones; ignore the resulting compiler errors for now:.

You accept image via this initializer and set ratingDescription from the rating. This tells the map to use ratingDescription as the subtitle shown on annotation callout when one is selected. One point of confusion with Silverlight is that it forces service calls to be asynchronous. This can seem strange when building a viewmodel: when do you trigger the call, and how do you know it is complete?

Typically, this is managed by registered to an event when the process is complete, and binding the results. I prefer to hide the implementation details of the events behind a simple Action function. Sometimes you may have a more complex workflow that requires multiple asynchronous calls to execute and complete prior to continuing.

If that is the case, you might want to look into a mechanism for making it easy to code and read the sequential workflow. This post will help you understand one solution using coroutines, and this post describes how to use an existing robust framework to manage those calls with thread safety, error handling, and more.

I would argue it is certain implementations that have this issue, not the pattern itself. The solution is often to page the data, but I find many people approach the problem incorrectly. For some reason, developers want to insist paging is a function of the database and should be isolated to the data access layer. In the most primitive form, you can create a collection that grows as the user pages. If your data is small, you might pull a very large collection and keep it in the Silverlight client, but use a virtualized panel to display the information the problem with some panels is that they create a control for every bound data element, which can crush performance — virtualized panels only create enough controls to fill the visible window on the screen.

These queries contain extension methods that allow you to grab only the first few items in a list, rather than fetching the full list at once. The framework also provides helper classes like the PagedCollectionView to help filter, sort, and page data. It does stress separation of concerns, which is nice. I would suggest that if you are spending days writing something just to avoid minutes of code-behind, your approach is wrong. It is not required for Silverlight or WPF. I believe that line-of-business, data-driven, and forms-based applications are prime candidates for MVVM.

Games, entertainment websites, paint programs, and others may not make sense. Be sure you are using the right tool for the right job.

MVVM is not supposed to slow you down! All new patterns and frameworks come with a learning curve. The pattern is useful when it accelerates development, improves stability and performance, reduces risk, and so forth. Now, what are your thoughts and comments? Jump to Appendix A: Historical Patterns. Jump to Appendix B: Existing Frameworks. A very special thanks to the community members who took time out of their busy schedules to review and provide preliminary feedback and suggestions for this article: in alphabetical order.

This software architecture pattern was first described in the context of Smalltalk at Xerox in This pattern builds on MVC but places special constraints on the controller, now called the presenter. A general overview looks like this:. Here is how Microsoft describes: MVP. In , Martin Fowler published his description of the Presentation Model. There are a number of out of the box frameworks that exist which implement MVVM. In no particular order:. This is a very popular toolkit that contains support out of the box for base viewmodels, commands, messaging, and project templates to get started.

It supports both WPF and Silverlight projects. The stated goals for this framework are to enable concise declarative specification of user interfaces, enable easier separation of view and code, and provide a lean framework. Caliburn is a popular viewmodel-first framework that supports both WPF and Silverlight.