MSDN Magazine - February 2009 - (Page 75)

Figure 3 The RelayCommand Class public class RelayCommand : ICommand { #region Fields readonly Action _execute; readonly Predicate _canExecute; #endregion // Fields #region Constructors public RelayCommand(Action execute) : this(execute, null) { } public RelayCommand(Action execute, Predicate canExecute) { if (execute == null) throw new ArgumentNullException("execute"); _execute = execute; _canExecute = canExecute; } #endregion // Constructors #region ICommand Members [DebuggerStepThrough] public bool CanExecute(object parameter) { return _canExecute == null ? true : _canExecute(parameter); } public event EventHandler CanExecuteChanged { add { CommandManager.RequerySuggested += value; } remove { CommandManager.RequerySuggested -= value; } } public void Execute(object parameter) { _execute(parameter); } #endregion // ICommand Members } to it, are easy to implement by building on top of the existing application architecture. Now that you have a high-level understanding of what the demo application does, let’s investigate how it was designed and implemented. Relaying Command Logic Every view in the app has an empty codebehind file, except for the standard boilerplate code that calls InitializeComponent in the class’s constructor. In fact, you could remove the views’ codebehind files from the project and the application would still compile and run correctly. Despite the lack of event handling methods in the views, when the user clicks on buttons, the application reacts and satisfies the user’s requests. This works because of bindings that were established on the Command property of Hyperlink, Button, and MenuItem controls displayed in the UI. Those bindings ensure that when the user clicks on Figure 4 Inheritance Hierarchy the controls, ICommand objects exposed by the RelayCommand _saveCommand; ViewModel execute. You can think of the command object as an public ICommand SaveCommand adapter that makes it easy to consume a ViewModel’s functional{ ity from a view declared in XAML. get { When a ViewModel exposes an instance property of type if (_saveCommand == null) ICommand, the command object typically uses that ViewModel { _saveCommand = new RelayCommand(param => this.Save(), object to get its job done. One possible implementation pattern param => this.CanSave ); is to create a private nested class within the ViewModel class, so } return _saveCommand; that the command has access to private members of its containing } ViewModel and does not pollute the namespace. That nested class } implements the ICommand interface, and a reference to the containing ViewModel object is injected into its constructor. However, ViewModel Class Hierarchy creating a nested class that implements ICommand for each comMost ViewModel classes need the same features. They often need mand exposed by a ViewModel can bloat the size of the ViewModel to implement the INotifyPropertyChanged interface, they usually class. More code means a greater potential for bugs. need to have a user-friendly display name, and, in the case of workmsdnmagazine.com In the demo application, the RelayCommand class solves this problem. RelayCommand allows you to inject the command’s logic via delegates passed into its constructor. This approach allows for terse, concise command implementation in ViewModel classes. RelayCommand is a simplified variation of the DelegateCommand found in the Microsoft Composite Application Library (msdn.microsoft.com/library/cc707890). The RelayCommand class is shown in Figure 3. The CanExecuteChanged event, which is part of the ICommand interface implementation, has some interesting features. It delegates the event subscription to the CommandManager.RequerySuggested event. This ensures that the WPF commanding infrastructure asks all RelayCommand objects if they can execute whenever it asks the built-in commands. The following code from the CustomerViewModel class, which I will examine in-depth later, shows how to configure a RelayCommand with lambda expressions: February 2009 75 http://msdn.microsoft.com/library/cc707890 http://www.msdnmagazine.com

Table of Contents Feed for the Digital Edition of MSDN Magazine - February 2009

MSDN Magazine - February 2009 Contents Toolbox CLR Inside Out Data Points Cutting Edge Patterns In Practice Best Practices .Net Interop "Oslo" Basics Patterns Silverlight Under The Table Foundations Windows With C++ .NET Matters Going Places { End Bracket }

MSDN Magazine - February 2009

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