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POGood or bad practice for Dialogs in wpf with MVVM?
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  1. POGood or bad practice for Dialogs in wpf with MVVM?
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    copied!<p>I lately had the problem of creating add and edit dialogs for my wpf app.</p> <p>All I want to do in my code was something like this. (I mostly use viewmodel first approach with mvvm)</p> <p>ViewModel which calls a dialog window:</p> <pre><code>var result = this.uiDialogService.ShowDialog("Dialogwindow Title", dialogwindowVM); // Do anything with the dialog result </code></pre> <p>How does it work?</p> <p>First, I created a dialog service:</p> <pre><code>public interface IUIWindowDialogService { bool? ShowDialog(string title, object datacontext); } public class WpfUIWindowDialogService : IUIWindowDialogService { public bool? ShowDialog(string title, object datacontext) { var win = new WindowDialog(); win.Title = title; win.DataContext = datacontext; return win.ShowDialog(); } } </code></pre> <p><code>WindowDialog</code> is a special but simple window. I need it to hold my content:</p> <pre><code>&lt;Window x:Class="WindowDialog" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" Title="WindowDialog" WindowStyle="SingleBorderWindow" WindowStartupLocation="CenterOwner" SizeToContent="WidthAndHeight"&gt; &lt;ContentPresenter x:Name="DialogPresenter" Content="{Binding .}"&gt; &lt;/ContentPresenter&gt; &lt;/Window&gt; </code></pre> <p>A problem with dialogs in wpf is the <code>dialogresult = true</code> can only be achieved in code. That's why I created an interface for my <code>dialogviewmodel</code> to implement it. </p> <pre><code>public class RequestCloseDialogEventArgs : EventArgs { public bool DialogResult { get; set; } public RequestCloseDialogEventArgs(bool dialogresult) { this.DialogResult = dialogresult; } } public interface IDialogResultVMHelper { event EventHandler&lt;RequestCloseDialogEventArgs&gt; RequestCloseDialog; } </code></pre> <p>Whenever my ViewModel thinks it's time for <code>dialogresult = true</code>, then raise this event.</p> <pre><code>public partial class DialogWindow : Window { // Note: If the window is closed, it has no DialogResult private bool _isClosed = false; public DialogWindow() { InitializeComponent(); this.DialogPresenter.DataContextChanged += DialogPresenterDataContextChanged; this.Closed += DialogWindowClosed; } void DialogWindowClosed(object sender, EventArgs e) { this._isClosed = true; } private void DialogPresenterDataContextChanged(object sender, DependencyPropertyChangedEventArgs e) { var d = e.NewValue as IDialogResultVMHelper; if (d == null) return; d.RequestCloseDialog += new EventHandler&lt;RequestCloseDialogEventArgs&gt; (DialogResultTrueEvent).MakeWeak( eh =&gt; d.RequestCloseDialog -= eh;); } private void DialogResultTrueEvent(object sender, RequestCloseDialogEventArgs eventargs) { // Important: Do not set DialogResult for a closed window // GC clears windows anyways and with MakeWeak it // closes out with IDialogResultVMHelper if(_isClosed) return; this.DialogResult = eventargs.DialogResult; } } </code></pre> <p>Now at least I have to create a <code>DataTemplate</code> in my resource file(<code>app.xaml</code> or something):</p> <pre><code>&lt;DataTemplate DataType="{x:Type DialogViewModel:EditOrNewAuswahlItemVM}" &gt; &lt;DialogView:EditOrNewAuswahlItem/&gt; &lt;/DataTemplate&gt; </code></pre> <p>Well thats all, I can now call dialogs from my viewmodels:</p> <pre><code> var result = this.uiDialogService.ShowDialog("Dialogwindow Title", dialogwindowVM); </code></pre> <p>Now my question, do you see any problems with this solution?</p> <p>Edit: for completeness. The ViewModel should implement <code>IDialogResultVMHelper</code> and then it can raise it within a <code>OkCommand</code> or something like this:</p> <pre><code>public class MyViewmodel : IDialogResultVMHelper { private readonly Lazy&lt;DelegateCommand&gt; _okCommand; public MyViewmodel() { this._okCommand = new Lazy&lt;DelegateCommand&gt;(() =&gt; new DelegateCommand(() =&gt; InvokeRequestCloseDialog( new RequestCloseDialogEventArgs(true)), () =&gt; YourConditionsGoesHere = true)); } public ICommand OkCommand { get { return this._okCommand.Value; } } public event EventHandler&lt;RequestCloseDialogEventArgs&gt; RequestCloseDialog; private void InvokeRequestCloseDialog(RequestCloseDialogEventArgs e) { var handler = RequestCloseDialog; if (handler != null) handler(this, e); } } </code></pre> <p>EDIT 2: I used the code from here to make my EventHandler register weak:<br> <a href="http://diditwith.net/2007/03/23/SolvingTheProblemWithEventsWeakEventHandlers.aspx" rel="noreferrer">http://diditwith.net/2007/03/23/SolvingTheProblemWithEventsWeakEventHandlers.aspx</a><br> (Website no longer exists, <a href="http://web.archive.org/web/20140319163345/http://diditwith.net/2007/03/23/SolvingTheProblemWithEventsWeakEventHandlers.aspx" rel="noreferrer">WebArchive Mirror</a>)</p> <pre><code>public delegate void UnregisterCallback&lt;TE&gt;(EventHandler&lt;TE&gt; eventHandler) where TE : EventArgs; public interface IWeakEventHandler&lt;TE&gt; where TE : EventArgs { EventHandler&lt;TE&gt; Handler { get; } } public class WeakEventHandler&lt;T, TE&gt; : IWeakEventHandler&lt;TE&gt; where T : class where TE : EventArgs { private delegate void OpenEventHandler(T @this, object sender, TE e); private readonly WeakReference mTargetRef; private readonly OpenEventHandler mOpenHandler; private readonly EventHandler&lt;TE&gt; mHandler; private UnregisterCallback&lt;TE&gt; mUnregister; public WeakEventHandler(EventHandler&lt;TE&gt; eventHandler, UnregisterCallback&lt;TE&gt; unregister) { mTargetRef = new WeakReference(eventHandler.Target); mOpenHandler = (OpenEventHandler)Delegate.CreateDelegate( typeof(OpenEventHandler),null, eventHandler.Method); mHandler = Invoke; mUnregister = unregister; } public void Invoke(object sender, TE e) { T target = (T)mTargetRef.Target; if (target != null) mOpenHandler.Invoke(target, sender, e); else if (mUnregister != null) { mUnregister(mHandler); mUnregister = null; } } public EventHandler&lt;TE&gt; Handler { get { return mHandler; } } public static implicit operator EventHandler&lt;TE&gt;(WeakEventHandler&lt;T, TE&gt; weh) { return weh.mHandler; } } public static class EventHandlerUtils { public static EventHandler&lt;TE&gt; MakeWeak&lt;TE&gt;(this EventHandler&lt;TE&gt; eventHandler, UnregisterCallback&lt;TE&gt; unregister) where TE : EventArgs { if (eventHandler == null) throw new ArgumentNullException("eventHandler"); if (eventHandler.Method.IsStatic || eventHandler.Target == null) throw new ArgumentException("Only instance methods are supported.", "eventHandler"); var wehType = typeof(WeakEventHandler&lt;,&gt;).MakeGenericType( eventHandler.Method.DeclaringType, typeof(TE)); var wehConstructor = wehType.GetConstructor(new Type[] { typeof(EventHandler&lt;TE&gt;), typeof(UnregisterCallback&lt;TE&gt;) }); IWeakEventHandler&lt;TE&gt; weh = (IWeakEventHandler&lt;TE&gt;)wehConstructor.Invoke( new object[] { eventHandler, unregister }); return weh.Handler; } } </code></pre>
 

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