What's needed is a platform that makes it easy to build UIs using simple, time-tested, developer-approved design patterns. Sometimes we use complicated design patterns, which require writing a lot of code because the UI platform in use does not lend itself well to a simpler pattern. It is not always the design patterns at fault. The more complicated the patterns are, the more likely that shortcuts will be used later on which undermine all previous efforts to do things the right way. There are popular design patterns that can help to tame this unwieldy beast, but properly separating and addressing the multitude of concerns can be difficult. Public void InvokeCanExecuteChanged() => CanExecuteChanged?.Invoke(this, EventArgs.Volume 24 Number 02 Patterns - WPF Apps With The Model-View-ViewModel Design Pattern Public bool CanExecute(object parameter) => _canExecuteAction?.Invoke(parameter) ? true Public DelegateCommand(Action executeAction, Func canExecuteAction) Next, let’s go back and implement the CanExecute portions of the ICommand interface. Running our simple application we can see that clicking the button does change the name. Private void OnChangeName(object commandParameter) _changeNameCommand = new DelegateCommand(OnChangeName) Public ICommand ChangeNameCommand => _changeNameCommand Private readonly DelegateCommand _changeNameCommand We can now finish the code in the view model. For now, we will ignore the CanExecute parts of the interface and always allow the execution of the command. Public event EventHandler CanExecuteChanged įor instance, in this implementation, an Action delegate is invoked when the command is executed. Public bool CanExecute(object parameter) => true Public void Execute(object parameter) => _executeAction(parameter) Public DelegateCommand(Action executeAction) Unfortunately, WPF does not come with a default ICommand implementation suitable for use in a view model, however, the interface is simple enough to implement: public class DelegateCommand : ICommand Now we just need to assign a new command object to our ChangeNameCommand property in our view model. Next, we will add a button to the MainWindow and use a Binding to set its Command property to be the command in our view model. Bindings are declared in the view and link properties in the view back to properties in the view model. The Binding engine is what makes the MVVM pattern possible. Since the interaction between the view model and the model will largely depend on your specific application, for the rest of this article we will focus just on the interaction between the view and the view model. If you are using dependency injection, the model classes will typically be passed as interface constructor parameters in your view model. While the view model is concerned with pulling together the model’s data, the model classes perform the actual work of the application. Depending on your application, this is where the real work gets done. Model: Broadly speaking the model provides access to the data and services that your application needs. The view model is the connection between the view and model. ![]() This separation of concerns is one of the key tenets of MVVM. Unlike other design patterns, the view model should not know about its view. The view model class exposes the data to the view and provides commands to handle user interaction. ![]() In general, there is typically a one-to-one mapping between views and view model classes. View Model: These are the objects that provide the data and functionality for each of your views. The view may be quite dynamic and even handle some user interaction (see the Commands section below). Though it is certainly possible to construct the view entirely from code, the vast majority of the UI will (and should be) built with XAML. They may be Windows, User Controls, or Resource Dictionaries. ![]() For WPF, these are all of your XAML files. View: These are all of the UI elements, the pretty face of your application. ![]() Let’s look at the three pieces of MVVM: Model, View, and View Model. If you are just getting started with WPF, I suggest looking at this getting started guide. Though it is possible to create WPF applications without using the MVVM pattern, a little investment in learning can make building WPF applications much simpler. The Windows Presentation Framework (WPF) takes full advantage of the Model-View-ViewModel (MVVM) pattern. Getting Started: Model-View-ViewModel Pattern using Windows Presentation Framework
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