hardware id - Page 231 - Developer IT

Simple MVVM Walkthrough – Refactored

- by Sean Feldman

JR has put together a good introduction post into MVVM pattern. I love kick start examples that serve the purpose well. And even more than that I love examples that also can pass the real world projects check. So I took the sample code and refactored it slightly for a few aspects that a lot of developers might raise a brow. Michael has mentioned model (entity) visibility from view. I agree on that. A few other items that don’t settle are using property names as string (magical strings) and Saver class internal casting of a parameter (custom code for each Saver command). Fixing a property names usage is a straight forward exercise – leverage expressions. Something simple like this would do the initial job: class PropertyOf<T> { public static string Resolve(Expression<Func<T, object>> expression) { var member = expression.Body as MemberExpression; return member.Member.Name; } } With this, refactoring of properties names becomes an easy task, with confidence that an old property name string will not get left behind. An updated Invoice would look like this: public class Invoice : INotifyPropertyChanged { private int id; private string receiver; public event PropertyChangedEventHandler PropertyChanged; private void OnPropertyChanged(string propertyName) { if (PropertyChanged != null) { PropertyChanged(this, new PropertyChangedEventArgs(propertyName)); } } public int Id { get { return id; } set { if (id != value) { id = value; OnPropertyChanged(PropertyOf<Invoice>.Resolve(x => x.Id)); } } } public string Receiver { get { return receiver; } set { receiver = value; OnPropertyChanged(PropertyOf<Invoice>.Resolve(x => x.Receiver)); } } } For the saver, I decided to change it a little so now it becomes a “view-model agnostic” command, one that can be used for multiple commands/view-models. Updated Saver code now accepts an action at construction time and executes that action. No more black magic internal class Command : ICommand { private readonly Action executeAction; public Command(Action executeAction) { this.executeAction = executeAction; } public bool CanExecute(object parameter) { return true; } public event EventHandler CanExecuteChanged; public void Execute(object parameter) { // no more black magic executeAction(); } } Change in InvoiceViewModel is instantiation of Saver command and execution action for the specific command. public ICommand SaveCommand { get { if (saveCommand == null) saveCommand = new Command(ExecuteAction); return saveCommand; } set { saveCommand = value; } } private void ExecuteAction() { DisplayMessage = string.Format("Thanks for creating invoice: {0} {1}", Invoice.Id, Invoice.Receiver); } This way internal knowledge of InvoiceViewModel remains in InvoiceViewModel and Command (ex-Saver) is view-model agnostic. Now the sample is not only a good introduction, but also has some practicality in it. My 5 cents on the subject. Sample code MvvmSimple2.zip

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