I'm really scratching my head with this one. I have a mainwindow which opens a dialog. After the dialog closes, the CanExecute method on commands bound in the dialog are still executing. This is causing some serious problems in my application.
Example:
MainWindow has a button with a click handler. This is the click event handler:
private void Button_Click(object sender, RoutedEventArgs e)
{
DialogWindow window = new DialogWindow();
window.ShowDialog();
}
In the dialog I bind an items control to a static resource in the dialog window, and each item in the list has a command:
<Window.Resources>
<Collections:ArrayList x:Key="itemsSource">
<local:ItemViewModel Description="A"></local:ItemViewModel>
<local:ItemViewModel Description="B"></local:ItemViewModel>
<local:ItemViewModel Description="C"></local:ItemViewModel>
</Collections:ArrayList>
<DataTemplate DataType="{x:Type local:ItemViewModel}">
<Button Grid.Column="1" Command="{Binding Path=CommandClickMe}" Content="{Binding Path=Description}" Style="{StaticResource {x:Static ToolBar.ButtonStyleKey}}">
</Button>
</DataTemplate>
</Window.Resources>
<Grid>
<ToolBar ItemsSource="{StaticResource itemsSource}"></ToolBar>
</Grid>
This is the viewmodel:
public class ItemViewModel
{
private RelayWpfCommand<object> _commandClickMe;
public RelayWpfCommand<object> CommandClickMe
{
get
{
if (_commandClickMe == null)
_commandClickMe = new RelayWpfCommand<object>(obj => System.Console.Out.WriteLine("Hei mom"), obj => CanClickMe());
return _commandClickMe;
}
}
private bool CanClickMe()
{
return true;
}
public string Description { get; set; }
And this is the DelegateCommand implementation:
public class RelayWpfCommand<T> : ICommand
{
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
private readonly Predicate<T> _canExecute;
private readonly Action<T> _execute;
public RelayWpfCommand(Action<T> execute, Predicate<T> canExecute)
{
_execute = execute;
_canExecute = canExecute;
}
/// <summary>
/// Forces a notification that the CanExecute state has changed
/// </summary>
public void RaiseCanExecuteChanged()
{
CommandManager.InvalidateRequerySuggested();
}
public bool CanExecute(T parameter)
{
return _canExecute(parameter);
}
public void Execute(T parameter)
{
_execute(parameter);
}
bool ICommand.CanExecute(object parameter)
{
if (!IsParameterValidType(parameter))
return false;
return CanExecute((T)parameter);
}
void ICommand.Execute(object parameter)
{
if (!IsParameterValidType(parameter))
throw new ArgumentException(string.Format("Parameter must be of type {0}", typeof(T)));
Execute((T)parameter);
}
private static bool IsParameterValidType(object parameter)
{
if (parameter != null && !typeof(T).IsAssignableFrom(parameter.GetType()))
return false;
return true;
}
}
Now, If I close the dialog window and set a breakpoint in the CanExecute (I'm using Prism DelegateCommand with weak event subscription) method on the viewmodel, I notice that it triggers although the dialog has been closed. Why on earth is the binding between the button in the dialog and the command on the ViewModel still alive?
And I am checking if its being executed by closing the window and at a later time setting a breakpoint in the "CanClickMe" method in the viewmodel. It will get executed for a while, then suddenly stop (probably due to GC). This non-determenistic behaviour is causing problems because in the real application the viewmodel might already bee disposed.
You may use the WeakEvent Pattern to mitigate this problem. Please refer to the following Stackoverflow question: Is Josh Smith's implementation of the RelayCommand flawed?
I've seen this catch many times in different projects, I'm not sure whether this creepy bug lurks in your app too, but it's worth checking.
There is a known memory leak issue in WPF 3.5 (including SP1), basically you can encounter it if you are binding to something that isn’t a DependencyProperty or doesn’t implement INotifyPropertyChanged. And this is exactly what your code is about.
Just implement INotifyPropertyChanged on ItemViewModel and see how it goes. Hope this helps.
You could clear the CommandBindings Collection of your window, when it closes.
rather than having your command as a property, could you try the following:
public ICommand CommandClickMe
{
get
{
return new RelayWpfCommand<object>((obj)=>System.Console.Out.WriteLine("Hei mom"), obj => CanClickMe());
}
}
Related
This is casual and prototype code, hence me trying what I think should work, googling around if it doesn't, then asking here after perusing similar questions.
I have the following markup in my Shell view:
<StatusBarItem Grid.Column="0">
<TextBlock Text="{Binding StatusMessage}" />
</StatusBarItem>
<Separator Grid.Column="1" />
<StatusBarItem Grid.Column="2">
<ProgressBar Value="{Binding StatusProgress}" Minimum="0" Maximum="100" Height="16" Width="198" />
</StatusBarItem>
Then in ShellViewModel I have the following two properties and an event handler:
private string _statusMessage;
public string StatusMessage
{
get => _statusMessage;
set => SetProperty(ref _statusMessage, value);
}
private double _statusProgress;
public double StatusProgress
{
get => _statusProgress;
set => SetProperty(ref _statusProgress, value);
}
private void OnFileTransferStatusChanged(object sender, FileTransferStatusEventArgs fileTransferStatusEventArgs)
{
StatusMessage = fileTransferStatusEventArgs.RelativePath;
StatusProgress = fileTransferStatusEventArgs.Progress;
}
The event is raised periodically, i.e. every n iterations, from a file download helper class.
Now the strange thing is this, when the event handler updates the vm properties, on the Shell view, the TextBlock bound to StatusMessage updates and displays correctly, but the ProgressBar bound to StatusProgress does not, and remains blank. If I put a break-point in the event handler, I can see the StatusProgress property being properly updated in various values from 0 to 100, yet this does not reflect on the ProgressBar.
The idea of the event handler executing on another thread, which often causes UI update problems, occurred to me, but why is one UI element updating properly and the other not?
NOTE: I have been monumentally stupid and not tested the ProgressBar statically, i.e. just set the viewmodel's StatusProgress to a value and get the shell window to display, without going through the download loop. If I do this, the progress bar displays a length that more or less corresponds to its Value property. None of the layout change suggestions made in comments or answers changes this. Statically it is always visible and always displays a value.
EXAMPLE: I created a small example that believe duplicates the problem. In the example the progress bar doesn't update until the waited on task has completed, and I believe this is the case with my main question, but it was a long download, and I didn't wait for it to complete before noticing the progress bar wasn't updating.
Here is the StatusBar in `MainWindow.xaml:
<StatusBar DockPanel.Dock="Bottom" Height="20">
<StatusBar.ItemsPanel>
<ItemsPanelTemplate>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="2" />
<ColumnDefinition Width="200" />
</Grid.ColumnDefinitions>
</Grid>
</ItemsPanelTemplate>
</StatusBar.ItemsPanel>
<StatusBarItem Grid.Column="2">
<ProgressBar Value="{Binding StatusProgress}" Maximum="100" Minimum="0" Height="16" Width="198" />
</StatusBarItem>
</StatusBar>
With the code behind in MainWindow.xaml.cs:
public MainWindow()
{
InitializeComponent();
DataContext = new MainWindowViewModel();
}
public MainWindowViewModel ViewModel => (MainWindowViewModel)DataContext;
private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
{
ViewModel.Download();
}
And the code in the MainWindowViewModel:
private string _statusMessage = "Downloading something";
public string StatusMessage
{
get => _statusMessage;
set
{
if (value == _statusMessage) return;
_statusMessage = value;
OnPropertyChanged();
}
}
private int _statusProgress;
public int StatusProgress
{
get => _statusProgress;
set
{
if (value == _statusProgress) return;
_statusProgress = value;
OnPropertyChanged();
}
}
public void Download()
{
var dl = new FileDownloader();
dl.ProgressChanged += (sender, args) =>
{
StatusProgress = args.Progress;
};
dl.Download();
}
And finally the code for FileDownloader:
public class ProgressChangedEventArgs
{
public int Progress { get; set; }
}
public class FileDownloader
{
public event EventHandler<ProgressChangedEventArgs> ProgressChanged;
public void Download()
{
for (var i = 0; i < 100; i++)
{
ProgressChanged?.Invoke(this, new ProgressChangedEventArgs{Progress = i});
Thread.Sleep(200);
}
}
}
In the example, the progress bar remains blank, until FileDownloader finishes its loop, and then suddenly the progress bar shows full progress, i.e. complete.
What's happening
Anything that is not about UI should be done in tasks, because, if not, you're blocking the UI thread and the UI.
In your case, the download was happening on you UI thread, the latter was waiting for the download to finish before updating your UI.
Solution
You need to do two things to solve your problem:
remove the work from the UI thread.
make sure the work can communicate with you UI thread.
So, first, start the download work as a Task like this:
private ICommand _startDownloadCommand;
public ICommand StartDownloadCommand
{
get
{
return _startDownloadCommand ?? (_startDownloadCommand = new DelegateCommand(
s => { Task.Run(() => Download()); },
s => true));
}
}
and connect the button to the command like this:
<Button Command="{Binding StartDownloadCommand}" Content="Start download" Height="20"/>
Then have you download method as such:
public void Download()
{
Application.Current.Dispatcher.Invoke(() => { StatusMessage = "download started"; });
var dl = new FileDownloader();
dl.ProgressChanged += (sender, args) =>
{
Application.Current.Dispatcher.Invoke(() => { StatusProgress = args.Progress; });
};
dl.Download();
Application.Current.Dispatcher.Invoke(() => { StatusMessage = "download DONE"; });
}
The dispatch will have your property (on UI thread) updated from a non UI thread.
And yet, the DelegateCommand helper class:
public class DelegateCommand : ICommand
{
private readonly Predicate<object> _canExecute;
private readonly Action<object> _execute;
public event EventHandler CanExecuteChanged;
public DelegateCommand(Action<object> execute)
: this(execute, null) {}
public DelegateCommand(Action<object> execute,
Predicate<object> canExecute)
{
_execute = execute;
_canExecute = canExecute;
}
public bool CanExecute(object parameter) => _canExecute == null || _canExecute(parameter);
public void Execute(object parameter) => _execute(parameter);
public void RaiseCanExecuteChanged() => CanExecuteChanged?.Invoke(this, EventArgs.Empty);
}
Remarks
In order to implement the MVVM pattern I had this code behind:
public partial class MainWindow : IView
{
public IViewModel ViewModel
{
get { return (IViewModel)DataContext; }
set { DataContext = value; }
}
public MainWindow()
{
DataContext = new MainWindowViewModel();
}
}
public interface IViewModel {}
public interface IView {}
and this View:
<Window x:Class="WpfApp1.MainWindow"
d:DataContext="{d:DesignInstance local:MainWindowViewModel,
IsDesignTimeCreatable=True}"
xmlns:local="clr-namespace:WpfApp1"
and this ViewModel:
public class MainWindowViewModel: INotifyPropertyChanged, IViewModel
This happens, because StatusBarItem default style sets its HorizontalContentAlignment to Left, which leads ProgressBar to get only a small amount of space horizontally.
You can make the ProgressBar to fill the StatusBarItem completely by setting StatusBarItem's HorizontalContentAlignment to Stretch or you can set the Width of the ProgressBar.
ProgressBar is a DispatcherObject, and DispatcherObject can be only accessed by the Dispatcher it is associated with.
If I understand your question well your OnFileTransferStatusChanged is being triggered on a background thread, so since you're not accessing controls using a Dispatcher (or from the UI thread) you're not guaranteed that the code will work.
The problem is that binding from a non-UI thread usually works until it doesn't - e.g. on a non-dev machine.
like first answer
be sure to be on the main UI thread, because OnFileTransferStatusChanged is on another thread. use this in your event
Application.Current.Dispatcher.Invoke(prio, (ThreadStart)(() =>
{
StatusMessage = fileTransferStatusEventArgs.RelativePath;
StatusProgress = fileTransferStatusEventArgs.Progress;
}));
I made few changes to your sample as your file downloaded is working on UI thread and application just freezes you can see it by changing focus to other application and trying to get back - window will not appear nor update.
changes:
private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
{
Task.Factory.StartNew(() => ViewModel.Download());
}
forces download to execute in new thread.
public MainWindow()
{
InitializeComponent();
DataContext = ViewModel = new MainWindowViewModel();
}
public MainWindowViewModel ViewModel { get; }
removed cast and access to UI thread only property DataContext.
Now I can see progress bar filling up.
You can't see any changes because your Main thread AKA UI Thread is busy Sleeping
and it does not have time to update your UI
Let Task handle your lengthy job and Main thread for Updating UI
Wrap your code inside Task and you can see your progress bar progressing.
private async void ButtonBase_OnClick(object sender, RoutedEventArgs e)
{
await Task.Run(() =>
{
_viewModel.Download();
});
//_viewModel.Download(); //this will run on UI Thread
}
In WPF application together with MVVMLight Toolkit, I would like to see your opinion, what is the best way to implement if I need to Cancel the Window Close event.
In Window.Closing event I can set the e.Cancel = true, which prevents closing the form. To identify if the Close is allowed, or should be prevented is in the ViewModel context.
One solution could be if I define an Application variable, and I can query this in the normal event handler in view code behind?
thanks
With MVVM Light you got EventToCommand:
So you could in xaml wire up the closing event to the VM.
<Window ...
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
xmlns:command="http://www.galasoft.ch/mvvmlight">
<i:Interaction.Triggers>
<i:EventTrigger EventName="Closing">
<command:EventToCommand Command="{Binding ClosingCommand}"
PassEventArgsToCommand="True" />
</i:EventTrigger>
</i:Interaction.Triggers>
and in the VM:
public RelayCommand<CancelEventArgs> ClosingCommand { get; private set; }
ctor() {
ClosingCommand = new RelayCommand<CancelEventArgs>(args => args.Cancel = true);
}
If you do not want to pass CancelEventArgs to the VM:
You could always take the similar approach with a Behavior and just use a simple bool from the VM(bind this bool to the Behavior) to indicate the closing event should be cancelled.
Update:
Download Link for following example
To do this with a Behavior you could just have a Behavior such as:
internal class CancelCloseWindowBehavior : Behavior<Window> {
public static readonly DependencyProperty CancelCloseProperty =
DependencyProperty.Register("CancelClose", typeof(bool),
typeof(CancelCloseWindowBehavior), new FrameworkPropertyMetadata(false));
public bool CancelClose {
get { return (bool) GetValue(CancelCloseProperty); }
set { SetValue(CancelCloseProperty, value); }
}
protected override void OnAttached() {
AssociatedObject.Closing += (sender, args) => args.Cancel = CancelClose;
}
}
Now in xaml:
<i:Interaction.Behaviors>
<local:CancelCloseWindowBehavior CancelClose="{Binding CancelClose}" />
</i:Interaction.Behaviors>
Where CancelClose is a bool property from the VM which indicates if the Closing event should be cancelled or not. In the attached example I have a Button to toggle this bool from the VM that should let you test the Behavior
You could to control this using Messages, for instance:
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
Messenger.Default.Register<CloseApplicationMessage>(this, m => Close());
Loaded += MainWindowLoaded;
Closing += MainWindowClosing;
}
private void MainWindowClosing(object sender, CancelEventArgs e)
{
//Ask for saving
var closingMessage = new ClosingApplicationMessage();
Messenger.Default.Send(closingMessage);
if (closingMessage.Cancel)
e.Cancel = true;
}
...
The mvvm message:
public class ClosingApplicationMessage
{
public bool Cancel { get; set; }
}
In this way, in any place you are listening to the ClosingApplicationMessage, you can control when the application is going to close, and may to cancel it.
Hope this helps...
This is probably overkill on the MVVM pattern but it's new to me and I'm interested to see if it is possible.
If I attach to the MouseMove event for a Window and do DragMove, I can move a bordeless window. Can I achieve this by some other method in MVVM or should I just accept adding this code to the Window's codebehind?
This is pure UI logic and doesn't belong in a ViewModel. The only reason you wouldn't want to put this in your code-behind would be for re-use and that is better solved with a custom Window derived control.
Personally I think any solution using MVVM would not make this code any better. Also, this is typically something that's view related and hasn't got anything to do with the data you're displaying.
IMHO, unless this is something that effects your data (aka the Model) then it is View code and should be in the View's code-behind and not in the Model.
I'm going to actually answer your question. The answer is yes. I'm using Cinch to assist me in the event binding and view model creation. The solution uses DragMove, but not as part of the code-behind (which is what I believe you are asking).
Event binding in the XAML:
<Window
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
xmlns:cinchV2="clr-namespace:Cinch;assembly=Cinch.WPF"
...>
<i:Interaction.Triggers>
<i:EventTrigger EventName="MouseLeftButtonDown">
<cinchV2:EventToCommandTrigger Command="{Binding MouseLeftButtonDown}" />
</i:EventTrigger>
</i:Interaction.Triggers>
<Grid>
...
</Grid>
</Window>
In the ViewModel:
[ExportViewModel("MainViewModel")]
[PartCreationPolicy(CreationPolicy.NonShared)]
internal sealed class MainViewModel : ViewModelBase
{
public SimpleCommand<object, EventToCommandArgs> MouseLeftButtonDown { get; private set; }
[ImportingConstructor]
public MainViewModel(IUIVisualizerService uiVisualizerService)
{
...
MouseLeftButtonDown = new SimpleCommand<object, EventToCommandArgs>(OnMouseLeftButtonDown);
}
private static void OnMouseLeftButtonDown(EventToCommandArgs e)
{
((Window)e.Sender).DragMove();
}
}
Fairly simple, right? Any events that come from the UI contain the View as the sender. So, here, we simply call the method on the view within the event handler in the ViewModel.
The project I'm working on uses no code-behind (even if it is not recommended in MVVM).
I know that I am a little late to the question, but this is what I have been using for sometime now and it works like a charm.
DashboardViewModel viewModel;
public DashboardView()
{
InitializeComponent();
viewModel = new DashboardViewModel();
viewModel.RequestClose += (s, e) => Application.Current.Dispatcher.Invoke(this.Close);
viewModel.RequestMinimize += (s, e) => Application.Current.Dispatcher.Invoke(() => { this.WindowState = WindowState.Minimized; });
DataContext = viewModel;
}
and something like this in your viewModel
#region Public Event Handlers
public event EventHandler<EventArgs> RequestClose;
public event EventHandler<EventArgs> RequestMinimize;
#endregion
Using the ICommand interface...
#region ICommand Members
public ICommand CloseCommand { get; private set; }
public ICommand MinimizeCommand { get; private set; }
#endregion
Configure the commands...
private void SetupCommands()
{
CloseCommand = new RelayCommand(CloseApplication);
MinimizeCommand = new RelayCommand(MinimizeApplication);
}
Here is the RelayCommand class.
public class RelayCommand : ICommand
{
#region Private Readonly Properties
private readonly Action<object> executeCommand;
private readonly Predicate<object> canExecute;
#endregion
#region Constructors
public RelayCommand(Action<object> execute) : this(execute, null)
{
}
public RelayCommand(Action<object> execute, Predicate<object> canExecute)
{
if (execute == null)
throw new ArgumentNullException("execute");
this.executeCommand = execute;
this.canExecute = canExecute;
}
#endregion
#region Public ICommand Members
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)
{
executeCommand(parameter);
}
#endregion
}
And some example methods...
private void MinimizeApplication(object obj)
{
RequestMinimize(this, new EventArgs());
}
private void CloseApplication(object obj)
{
RequestClose(this, new EventArgs());
}
Hope this helps!
I know it's an old question. However I prepared another simple implementation. Use following behavior to make window moveable:
public class WindowMoveBehavior : Behavior<Grid>
{
protected override void OnAttached()
{
base.OnAttached();
AssociatedObject.MouseLeftButtonDown += AssociatedObject_MouseLeftButtonDown;
}
protected override void OnDetaching()
{
AssociatedObject.MouseLeftButtonDown -= AssociatedObject_MouseLeftButtonDown;
base.OnDetaching();
}
private void AssociatedObject_MouseLeftButtonDown(object sender, System.Windows.Input.MouseButtonEventArgs e)
{
Window.GetWindow(AssociatedObject).DragMove();
}
}
Xaml example:
<Style x:Key="CustomWindowStyle" TargetType="{x:Type Window}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Window}">
<Grid>
<i:Interaction.Behaviors>
<behaviors:WindowMoveBehavior/>
</i:Interaction.Behaviors>
<!-- different controls and content -->
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
11 years passed, but maybe someone is still interested in case to drag window using MVVM. This tricky solution is based on window's property "Tag" - almost no one use it but it's time to find out it's strength :) So all you need is System.Windows.Interactivity nuget, no Cinch or events!
Xaml:
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
<Window ... Tag="{Binding WindowTag}">
<i:Interaction.Triggers>
<i:EventTrigger EventName="MouseLeftButtonDown">
<i:InvokeCommandAction Command="{Binding DragMoveWindowCommand}" />
</i:EventTrigger>
</i:Interaction.Triggers>
Let's find out your current window and move it. In ViewModel:
private object _windowTag;
public object WindowTag
{
get
{
return _windowTag;
}
set
{
_windowTag = value;
OnPropertyChanged("WindowTag");
}
}
private RelayCommand _dragMoveWindowCommand;
public RelayCommand DragMoveWindowCommand
{
get
{
return _dragMoveWindowCommand ??
(_dragMoveWindowCommand = new RelayCommand(obj =>
{
try
{
var window = WindowService.GetCurrentWindowByTag(WindowTag = 1);
window.DragMove();
}
catch (Exception ex)
{
}
}));
}
}
public static Window GetCurrentWindowByTag(object tag)
{
return (from Window w in App.Current.Windows
where w.Tag == tag
select w)?.FirstOrDefault();
}
Enjoy!)
I'm having some difficulty with Context Menu commands on my View Model.
I'm implementing the ICommand interface for each command within the View Model, then creating a ContextMenu within the resources of the View (MainWindow), and using a CommandReference from the MVVMToolkit to access the current DataContext (ViewModel) Commands.
When I debug the application, it appears that the CanExecute method on the command is not being called except at the creation of the window, therefore my Context MenuItems are not being enabled or disabled as I would have expected.
I've cooked up a simple sample (attached here) which is indicative of my actual application and summarised below. Any help would be greatly appreciated!
This is the ViewModel
namespace WpfCommandTest
{
public class MainWindowViewModel
{
private List<string> data = new List<string>{ "One", "Two", "Three" };
// This is to simplify this example - normally we would link to
// Domain Model properties
public List<string> TestData
{
get { return data; }
set { data = value; }
}
// Bound Property for listview
public string SelectedItem { get; set; }
// Command to execute
public ICommand DisplayValue { get; private set; }
public MainWindowViewModel()
{
DisplayValue = new DisplayValueCommand(this);
}
}
}
The DisplayValueCommand is such:
public class DisplayValueCommand : ICommand
{
private MainWindowViewModel viewModel;
public DisplayValueCommand(MainWindowViewModel viewModel)
{
this.viewModel = viewModel;
}
#region ICommand Members
public bool CanExecute(object parameter)
{
if (viewModel.SelectedItem != null)
{
return viewModel.SelectedItem.Length == 3;
}
else return false;
}
public event EventHandler CanExecuteChanged;
public void Execute(object parameter)
{
MessageBox.Show(viewModel.SelectedItem);
}
#endregion
}
And finally, the view is defined in Xaml:
<Window x:Class="WpfCommandTest.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfCommandTest"
xmlns:mvvmtk="clr-namespace:MVVMToolkit"
Title="Window1" Height="300" Width="300">
<Window.Resources>
<mvvmtk:CommandReference x:Key="showMessageCommandReference" Command="{Binding DisplayValue}" />
<ContextMenu x:Key="listContextMenu">
<MenuItem Header="Show MessageBox" Command="{StaticResource showMessageCommandReference}"/>
</ContextMenu>
</Window.Resources>
<Window.DataContext>
<local:MainWindowViewModel />
</Window.DataContext>
<Grid>
<ListBox ItemsSource="{Binding TestData}" ContextMenu="{StaticResource listContextMenu}"
SelectedItem="{Binding SelectedItem}" />
</Grid>
</Window>
To complete Will's answer, here's a "standard" implementation of the CanExecuteChanged event :
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
(from Josh Smith's RelayCommand class)
By the way, you should probably consider using RelayCommand or DelegateCommand : you'll quickly get tired of creating new command classes for each and every command of you ViewModels...
You have to keep track of when the status of CanExecute has changed and fire the ICommand.CanExecuteChanged event.
Also, you might find that it doesn't always work, and in these cases a call to CommandManager.InvalidateRequerySuggested() is required to kick the command manager in the ass.
If you find that this takes too long, check out the answer to this question.
Thank you for the speedy replies. This approach does work if you are binding the commands to a standard Button in the Window (which has access to the View Model via its DataContext), for example; CanExecute is shown to be called quite frequently when using the CommandManager as you suggest on ICommand implementing classes or by using RelayCommand and DelegateCommand.
However, binding the same commands via a CommandReference in the ContextMenu
do not act in the same way.
In order for the same behaviour, I must also include the EventHandler from Josh Smith's RelayCommand, within CommandReference, but in doing so I must comment out some code from within the OnCommandChanged Method. I'm not entirely sure why it is there, perhaps it is preventing event memory leaks (at a guess!)?
public class CommandReference : Freezable, ICommand
{
public CommandReference()
{
// Blank
}
public static readonly DependencyProperty CommandProperty = DependencyProperty.Register("Command", typeof(ICommand), typeof(CommandReference), new PropertyMetadata(new PropertyChangedCallback(OnCommandChanged)));
public ICommand Command
{
get { return (ICommand)GetValue(CommandProperty); }
set { SetValue(CommandProperty, value); }
}
#region ICommand Members
public bool CanExecute(object parameter)
{
if (Command != null)
return Command.CanExecute(parameter);
return false;
}
public void Execute(object parameter)
{
Command.Execute(parameter);
}
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
private static void OnCommandChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
CommandReference commandReference = d as CommandReference;
ICommand oldCommand = e.OldValue as ICommand;
ICommand newCommand = e.NewValue as ICommand;
//if (oldCommand != null)
//{
// oldCommand.CanExecuteChanged -= commandReference.CanExecuteChanged;
//}
//if (newCommand != null)
//{
// newCommand.CanExecuteChanged += commandReference.CanExecuteChanged;
//}
}
#endregion
#region Freezable
protected override Freezable CreateInstanceCore()
{
throw new NotImplementedException();
}
#endregion
}
However, binding the same commands via a CommandReference in the
ContextMenu do not act in the same way.
That's a bug in CommandReference implementation. It follows from these two points:
It is recommended that the implementers of ICommand.CanExecuteChanged hold only weak references to the handlers (see this answer).
Consumers of ICommand.CanExecuteChanged should expect (1) and hence should hold strong references to the handlers they register with ICommand.CanExecuteChanged
The common implementations of RelayCommand and DelegateCommand abide by (1). The CommandReference implementation doesn't abide by (2) when it subscribes to newCommand.CanExecuteChanged. So the handler object is collected and after that CommandReference no longer gets any notifications that it was counting on.
The fix is to hold a strong ref to the handler in CommandReference:
private EventHandler _commandCanExecuteChangedHandler;
public event EventHandler CanExecuteChanged;
...
if (oldCommand != null)
{
oldCommand.CanExecuteChanged -= commandReference._commandCanExecuteChangedHandler;
}
if (newCommand != null)
{
commandReference._commandCanExecuteChangedHandler = commandReference.Command_CanExecuteChanged;
newCommand.CanExecuteChanged += commandReference._commandCanExecuteChangedHandler;
}
...
private void Command_CanExecuteChanged(object sender, EventArgs e)
{
if (CanExecuteChanged != null)
CanExecuteChanged(this, e);
}
In order for the same behaviour, I must also include the EventHandler
from Josh Smith's RelayCommand, within CommandReference, but in doing
so I must comment out some code from within the OnCommandChanged
Method. I'm not entirely sure why it is there, perhaps it is
preventing event memory leaks (at a guess!)?
Note that your approach of forwarding subscription to CommandManager.RequerySuggested also eliminates the bug (there's no more unreferenced handler to begin with), but it handicaps the CommandReference functionality. The command with which CommandReference is associated is free to raise CanExecuteChanged directly (instead of relying on CommandManager to issue a requery request), but this event would be swallowed and never reach the command source bound to the CommandReference. This should also answer your question as to why CommandReference is implemented by subscribing to newCommand.CanExecuteChanged.
UPDATE: submitted an issue on CodePlex
An easier solution for me, was to set the CommandTarget on the MenuItem.
<MenuItem Header="Cut" Command="Cut" CommandTarget="
{Binding Path=PlacementTarget,
RelativeSource={RelativeSource FindAncestor,
AncestorType={x:Type ContextMenu}}}"/>
More info: http://www.wpftutorial.net/RoutedCommandsInContextMenu.html
Does anyone know how I can force CanExecute to get called on a custom command (Josh Smith's RelayCommand)?
Typically, CanExecute is called whenever interaction occurs on the UI. If I click something, my commands are updated.
I have a situation where the condition for CanExecute is getting turned on/off by a timer behind the scenes. Because this is not driven by user interaction, CanExecute is not called until the user interacts with the UI. The end result is that my Button remains enabled/disabled until the user clicks on it. After the click, it is updated correctly. Sometimes the Button appears enabled, but when the user clicks it changes to disabled instead of firing.
How can I force an update in code when the timer changes the property that affects CanExecute? I tried firing PropertyChanged (INotifyPropertyChanged) on the property that affects CanExecute, but that did not help.
Example XAML:
<Button Content="Button" Command="{Binding Cmd}"/>
Example code behind:
private ICommand m_cmd;
public ICommand Cmd
{
if (m_cmd == null)
m_cmd = new RelayCommand(
(param) => Process(),
(param) => EnableButton);
return m_cmd;
}
// Gets updated from a timer (not direct user interaction)
public bool EnableButton { get; set; }
Calling System.Windows.Input.CommandManager.InvalidateRequerySuggested() forces the CommandManager to raise the RequerySuggested event.
Remarks: The CommandManager only pays attention to certain conditions in determining when the command target has changed, such as change in keyboard focus. In situations where the CommandManager does not sufficiently determine a change in conditions that cause a command to not be able to execute, InvalidateRequerySuggested can be called to force the CommandManager to raise the RequerySuggested event.
I was aware of CommandManager.InvalidateRequerySuggested() a long time ago, and used it, but it wasn't working for me sometimes. I finally figured out why this was the case! Even though it doesn't throw like some other actions, you HAVE to call it on the main thread.
Calling it on a background thread will appear to work, but sometimes leave the UI disabled. I really hope this helps somebody, and saves them the hours I just wasted.
A workaround for that is binding IsEnabled to a property:
<Button Content="Button" Command="{Binding Cmd}" IsEnabled="{Binding Path=IsCommandEnabled}"/>
and then implement this property in your ViewModel. This also makes it a bit easier for the UnitTesting to work with the properties rather than commands to see if the command can be executed at a certain point of time.
I, personally, find it more convenient.
Probably this variant will suit you:
public interface IRelayCommand : ICommand
{
void UpdateCanExecuteState();
}
Implementation:
public class RelayCommand : IRelayCommand
{
public event EventHandler CanExecuteChanged;
readonly Predicate<Object> _canExecute = null;
readonly Action<Object> _executeAction = null;
public RelayCommand( Action<object> executeAction,Predicate<Object> canExecute = null)
{
_canExecute = canExecute;
_executeAction = executeAction;
}
public bool CanExecute(object parameter)
{
if (_canExecute != null)
return _canExecute(parameter);
return true;
}
public void UpdateCanExecuteState()
{
if (CanExecuteChanged != null)
CanExecuteChanged(this, new EventArgs());
}
public void Execute(object parameter)
{
if (_executeAction != null)
_executeAction(parameter);
UpdateCanExecuteState();
}
}
Using simple:
public IRelayCommand EditCommand { get; protected set; }
...
EditCommand = new RelayCommand(EditCommandExecuted, CanEditCommandExecuted);
protected override bool CanEditCommandExecuted(object obj)
{
return SelectedItem != null ;
}
protected override void EditCommandExecuted(object obj)
{
// Do something
}
...
public TEntity SelectedItem
{
get { return _selectedItem; }
set
{
_selectedItem = value;
//Refresh can execute
EditCommand.UpdateCanExecuteState();
RaisePropertyChanged(() => SelectedItem);
}
}
XAML:
<Button Content="Edit" Command="{Binding EditCommand}"/>
Thanks guys for the tips. Here's a bit of code on how to marshal that call from a BG thread to the UI thread:
private SynchronizationContext syncCtx; // member variable
In the constructor:
syncCtx = SynchronizationContext.Current;
On the background thread, to trigger the requery:
syncCtx.Post( delegate { CommandManager.InvalidateRequerySuggested(); }, null );
Hope that helps.
-- Michael
To update only a single GalaSoft.MvvmLight.CommandWpf.RelayCommand you could use
mycommand.RaiseCanExecuteChanged();
and for me i've created an Extension method:
public static class ExtensionMethods
{
public static void RaiseCanExecuteChangedDispatched(this RelayCommand cmd)
{
System.Windows.Application.Current.Dispatcher.Invoke(DispatcherPriority.Normal, new Action(() => { cmd.RaiseCanExecuteChanged(); }));
}
public static void RaiseCanExecuteChangedDispatched<T>(this RelayCommand<T> cmd)
{
System.Windows.Application.Current.Dispatcher.Invoke(DispatcherPriority.Normal, new Action(() => { cmd.RaiseCanExecuteChanged(); }));
}
}