I refer excellent tutorial of Josh Smith to work with treeview.
https://www.codeproject.com/Articles/26288/Simplifying-the-WPF-TreeView-by-Using-the-ViewMode
I try to modified with this code to add, remove, rename item to this treeview but I don't know why it not update
Rename item command
#region RenameCommand
/// <summary>
/// Returns the command used to execute a search in the family tree.
/// </summary>
public ICommand RenameCommand
{
get { return _renameCommand; }
}
private class RenameFamilyTreeCommand : ICommand
{
readonly FamilyTreeViewModel _familyTree;
public RenameFamilyTreeCommand(FamilyTreeViewModel familyTree)
{
_familyTree = familyTree;
}
public bool CanExecute(object parameter)
{
return true;
}
event EventHandler ICommand.CanExecuteChanged
{
// I intentionally left these empty because
// this command never raises the event, and
// not using the WeakEvent pattern here can
// cause memory leaks. WeakEvent pattern is
// not simple to implement, so why bother.
add { }
remove { }
}
public void Execute(object parameter)
{
//MessageBox.Show("Rename command");
_familyTree._rootPerson.Children[0].Children[0].Header = "Hello";
if (_familyTree._rootPerson.Children[0] == null)
return;
// Ensure that this person is in view.
if (_familyTree._rootPerson.Children[0].Parent != null)
_familyTree._rootPerson.Children[0].Parent.IsExpanded = true;
_familyTree._rootPerson.Children[0].IsSelected = true;
}
}
#endregion // RenameCommand
Add item command
#region AddCommand
/// <summary>
/// Returns the command used to execute a search in the family tree.
/// </summary>
public ICommand AddCommand
{
get { return _addCommand; }
}
private class AddFamilyTreeCommand : ICommand
{
public FamilyTreeViewModel _familyTree;
public AddFamilyTreeCommand(FamilyTreeViewModel familyTree)
{
_familyTree = familyTree;
}
public bool CanExecute(object parameter)
{
return true;
}
event EventHandler ICommand.CanExecuteChanged
{
// I intentionally left these empty because
// this command never raises the event, and
// not using the WeakEvent pattern here can
// cause memory leaks. WeakEvent pattern is
// not simple to implement, so why bother.
add { }
remove { }
}
public void Execute(object parameter)
{
Person newPerson = new Person();
newPerson.Header = "New Person";
newPerson.Name = "1.1.1.75";
PersonViewModel newPersonViewModel = new PersonViewModel(newPerson);
////_rootPerson.Children.Add(newPersonViewModel);
//_rootPerson.Children.Add(newPersonViewModel);
//if (newPersonViewModel.Parent != null)
// newPersonViewModel.Parent.IsExpanded = true;
//newPersonViewModel.IsSelected = true;
_familyTree._rootPerson.Children[0].Children.Add(newPersonViewModel);
if (_familyTree._rootPerson.Children[0] == null)
return;
// Ensure that this person is in view.
if (_familyTree._rootPerson.Children[0].Parent != null)
_familyTree._rootPerson.Children[0].Parent.IsExpanded = true;
_familyTree._rootPerson.Children[0].IsSelected = true;
}
}
#endregion // AddCommand
Add command working fine but it's seem to be GUI not update. Rename command is not working but GUI is updated. I don't know reason why, And it's hard to access person class (use parent, person, children,..)
Is there anyone successfully update add, rename, remove command to Josh Smith project.
p/s: I debug by messagebox.show and see binding command for add and rename are working well, But the problem is I don't know what exactly to use Add, remove, rename person in Josh Smith project
Adding items is not reflected in the UI, because the source collection Person.Children doesn't implement INotifyCollectionChanged.
Whenever you need dynamic collections, where add, remove or move operations should update the binding target, you should use the ObservableCollection<T>, which implements INotifyCollectionChanged.
Similar applies to the Person.Name property. If you want a property's change to be reflected to the UI, then your view model must implement INotifyPropertyChanged and raise the INotifyPropertyChanged.PropertyChanged event whenever the binding source (the view model property) has changed.
Generally, when a class serves as a binding source for data binding, then this class must implement INotifyPropertyChanged (if this interface is not implemented, then the performance of data binding becomes very bad).
When the modification of a property should update the UI (binding.target) by invoking the data binding, then the modified property must raise the INotifyPropertyChanged.PropertyChanged event.
When the modification of a collection should update the UI (binding target) by invoking the data binding, then the modified collection must implement INotifyCollectionChanged and raise the INotifyCollectionChanged.CollectionChanged event. ObservableCollection provides a default implementation of INotifyCollectionChanged.
The following example follows the above rules. The changes made to the Person class should fix your issues. Changes to the data model will now be reflected in the TreeView:
public class Person : INotifyPropertyChanged
{
private ObservableCollection<Person> _children = new ObservableCollection<Person>();
public ObservableCollection<Person> Children
{
get { return _children; }
}
private string name
public string Name
{
get => this.name;
set
{
this.name = value;
OnPropertyChanged();
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
Actually exploring the Command Pattern and finds it pretty interesting. I'm writing a WPF Windows App following the MVVM Architectural Pattern.
I've begun with these post which explain the basics.
Basic MVVM and ICommand usuage example
Simplify Distributed System Design Using the Command Pattern, MSMQ, and .NET
Now that I was able to break user actions into commands, I thought this could be great to inject the commands that I want. I noticed that the commands are found into the ViewModel in the first referenced article, So I thought that would be great if I could use them along Ninject and actually inject my command into my view model using a binding that would look like the following:
kernel
.Bind<ICommand>()
.To<RelayCommand>()
.WithConstructorArgument("execute", new Action<object>(???));
But then, what to put in here ???. The expected answer is a method. Great! I just need a method to be put in there.
Because the first article simply initialize its commands within the ViewModel constructor, it is easy to say what method should be executed on the command execute call.
But from within the CompositionRoot? This is no place to put a method that will do anything else than bind types together through whatever DI container you're using!
So now, I've come across the Interceptor Pattern using Ninject Extensions. This looks like it could suits my requirements, and there is a bit of confusion here, if I may say. Not that the articles are confusing, they're not. I'm confused!
Using Ninject.Extensions.Interception Part 1 : The Basics
Using Ninject.Extensions.Interception Part 2 : Working With Interceptors
Also, there is this answer from BatteryBackupUnit who always sets great answers.
Ninject - How to implement Command Pattern with Ninject?
But now, I can't see how to glue it all up together! Humbly, I'm lost.
So here's my code so far.
RelayCommand
public class RelayCommand : ICommand {
public RelayCommand(Action<object> methodToExecute, Predicate<object> canExecute) {
if(methodToExecute == null)
throw new ArgumentNullException("methodToExecute");
if(canExecute == null)
throw new ArgumentNullException("canExecute");
this.canExecute = canExecute;
this.methodToExecute = methodToExecute;
}
public bool CanExecute(object parameter) {
return canExecute != null && canExecute(parameter);
}
public event EventHandler CanExecuteChanged {
add {
CommandManager.RequerySuggested += value;
canExecuteChanged += value;
}
remove {
CommandManager.RequerySuggested -= value;
canExecuteChanged -= value;
}
}
public static bool DefaultCanExecute(object parameter) { return true; }
public void Execute(object parameter) { methodToExecute(parameter); }
public void OnCanExecuteChanged() {
var handler = canExecuteChanged;
if(handler != null) handler(this, EventArgs.Empty);
}
public void Destroy() {
canExecute = _ => false;
methodToExecute = _ => { return; };
}
private Predicate<object> canExecute;
private Action<object> methodToExecute;
private event EventHandler canExecuteChanged;
}
CategoriesManagementViewModel
public class CategoriesManagementViewModel : ViewModel<IList<Category>> {
public CategoriesManagementViewModel(IList<Category> categories
, ICommand changeCommand
, ICommand createCommand
, ICommand deleteCommand) : base(categories) {
if(changeCommand == null)
throw new ArgumentNullException("changeCommand");
if(createCommand == null)
throw new ArgumentNullException("createCommand");
if(deleteCommand == null)
throw new ArgumentNullException("deleteCommand");
this.changeCommand = changeCommand;
this.createCommand = createCommand;
this.deleteCommand = deleteCommand;
}
public ICommand ChangeCommand { get { return changeCommand; } }
public ICommand CreateCommand { get { return createCommand; } }
public ICommand DeleteCommand { get { return deleteCommand; } }
private readonly ICommand changeCommand;
private readonly ICommand createCommand;
private readonly ICommand deleteCommand;
}
I wonder, would it be better off using Property Injection, though I tend not to use it all?
Let's say I have CategoriesManagementView that calls another window, let's say CreateCategoryView.Show(), and then the CreateCategoryView takes over until the user is back to the management window.
The Create Command then needs to call CreateCategoryView.Show(), and that is what I tried from within the CompositionRoot.
CompositionRoot
public class CompositionRoot {
public CompositionRoot(IKernel kernel) {
if(kernel == null) throw new ArgumentNullException("kernel");
this.kernel = kernel;
}
//
// Unrelated code suppressed for simplicity sake.
//
public IKernel ComposeObjectGraph() {
BindCommandsByConvention();
return kernel;
}
private void BindCommandsByConvention() {
//
// This is where I'm lost. I can't see any way to tell Ninject
// what I want it to inject into my RelayCommand class constructor.
//
kernel
.Bind<ICommand>()
.To<RelayCommand>()
.WithConstructorArgument("methodToExecute", new Action<object>());
//
// I have also tried:
//
kernel
.Bind<ICommand>()
.ToConstructor(ctx =>
new RelayCommand(new Action<object>(
ctx.Context.Kernel
.Get<ICreateCategoryView>().ShowSelf()), true);
//
// And this would complain that there is no implicit conversion
// between void and Action and so forth.
//
}
private readonly IKernel kernel;
}
Perhaps I am overcomplicating things, that is generally what happends when one gets confused. =)
I just wonder whether the Ninject Interception Extension could be the right tool for the job, and how to use it effectively?
I created a simple example of a command interacting with an injected service. might not compile since i'm going from memory. Maybe this can help you.
public class TestViewModel
{
private readonly IAuthenticationService _authenticationService;
public DelegateCommand SignInCommand { get; private set; }
public TestViewModel(IAuthenticationService authenticationService) //Inject auth service
{
_authenticationService = authenticationService
SignInCommand = new DelegateCommand(OnSignInRequest)
}
private void OnSignInRequest(Action<bool> isSuccessCallback)
{
var isSuccess = _authenticationService.SignIn();
isSuccessCallback(isSuccess);
}
}
}
Can Execute of a ICommand while a Context menu open
With the continuation of the above query, Still its not able to achieve.
Is there any way to raise context menu command's CanExcute while open the context menu. This need to be done in the context menu itself and here am not able to access the viewmodel.
Any idea on this?
public static BaseCommand SaveCommand
{
get
{
if (saveCommand == null)
saveCommand = new BaseCommand(OnSaveCommandClicked, OnSaveCommandCanExcute);
return saveCommand;
}
}
where BaseCommand is derived from ICommand.
public class BaseCommand : ICommand
{
private Predicate<object> _canExecute;
private Action<object> _method;
public event EventHandler CanExecuteChanged;
public BaseCommand(Action<object> method)
: this(method, null)
{
}
public BaseCommand(Action<object> method, Predicate<object> canExecute)
{
_method = method;
_canExecute = canExecute;
}
public bool CanExecute(object parameter)
{
if (_canExecute == null)
{
return true;
}
return _canExecute(parameter);
}
public void Execute(object parameter)
{
_method.Invoke(parameter);
}
}
Inside BaseCommand, replace the CanExecuteChanged to look like the following. And it will work the way you want it.
public event EventHandler CanExecuteChanged
{
add
{
CommandManager.RequerySuggested += value;
}
remove
{
CommandManager.RequerySuggested -= value;
}
}
I have to use this Command in a WPF application (i dont like it really but if i have to -> i have to ):
http://wpftutorial.net/DelegateCommand.html
But the main problem I have here is that I don t want to have in nearly every line of my code a call to the
RaiseCanExecuteChanged()
method. So what could I do to do that auto like RoutedUICommand does.
I have a lot of databindings and as example if Foo.FooProp != null Command can execute. But I want as less code as possible and so I would have to register events everywhere or update commands all over my application....
When I use a DelegateCommand, I just manually raise the CanExecuteChanged in the PropertyChange event whenever a property the command relies on changes.
Here's an example I did a few days ago where the CanExecute was based off the IsLoading and IsValid properties:
public MyViewModel()
{
this.PropertyChanged += MyViewModel_PropertyChanged;
}
void MyViewModel_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
switch (e.PropertyName)
{
case "IsLoading":
case "IsValid":
((DelegateCommand)MyCommand).RaiseCanExecuteChanged();
break;
}
}
public ICommand MyCommand
{
get
{
if (_myCommand == null)
_myCommand = new DelegateCommand(Run, CanRun);
return _myCommand;
}
}
public bool CanRun()
{
return this.IsValid && !IsLoading;
}
I find this keeps the logic easy to follow and maintain, and it only checks the CanExecuteChanged() method when the relevant properties change.
You could implement a form of DelegateCommand which invokes the delegates added to CanExecuteChanged everytime there is a change of possible consequence in the UI. This example uses CommandManager.RequerySuggested.
public class AutoDelegateCommand : DelegateCommand, ICommand
{
public AutoDelegateCommand(Action<object> execute)
: base(execute)
{
}
public AutoDelegateCommand(Action<object> execute, Predicate<object> canExecute)
: base(execute, canExecute)
{
}
event EventHandler ICommand.CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
}
I think I've seen an example like this before, perhaps in the MVVMLight toolkit?
Short Version
Calls to CommandManager.InvalidateRequerySuggested() take far longer to take effect than I would like (1-2 second delay before UI controls become disabled).
Long Version
I have a system where I submit tasks to a background-thread based task processor. This submit happens on the WPF UI thread.
When this submit happens, the object that manages my background thread does two things:
It raises a "busy" event (still on the UI thread) that several view models respond to; when they receive this event, they set an IsEnabled flag on themselves to false. Controls in my views, which are databound to this property, are immediately grayed out, which is what I would expect.
It informs my WPF ICommand objects that they should not be allowed to execute (again, still on the UI thread). Because there is nothing like INotifyPropertyChanged for ICommand objects, I am forced to call CommandManager.InvalidateRequerySuggested() to force WPF to reconsider all of my command objects' CanExecute states (yes, I actually do need to do this: otherwise, none of these controls become disabled). Unlike item 1, though, it takes a significantly longer time for my buttons/menu items/etc that are using ICommand objects to visually change to a disabled state than it does for the UI controls that have their IsEnabled property manually set.
The problem is, from a UX point of view, this looks awful; half of my controls are immediately grayed out (because their IsEnabled property is set to false), and then a full 1-2 seconds later, the other half of my controls follow suit (because their CanExecute methods are finally re-evaluated).
So, part 1 of my question:
As silly as it sounds to ask, is there a way I can make CommandManager.InvalidateRequerySuggested() do it's job faster? I suspect that there isn't.
Fair enough, part 2 of my question:
How can I work around this? I'd prefer all of my controls be disabled at the same time. It just looks unprofessional and awkward otherwise. Any ideas? :-)
CommandManager.InvalidateRequerySuggested() tries to validate all commands, which is totally ineffective (and in your case slow) - on every change, you are asking every command to recheck its CanExecute()!
You'd need the command to know on which objects and properties is its CanExecute dependent, and suggest requery only when they change. That way, if you change a property of an object, only commands that depend on it will change their state.
This is how I solved the problem, but at first, a teaser:
// in ViewModel's constructor - add a code to public ICommand:
this.DoStuffWithParameterCommand = new DelegateCommand<object>(
parameter =>
{
//do work with parameter (remember to check against null)
},
parameter =>
{
//can this command execute? return true or false
}
)
.ListenOn(whichObject, n => n.ObjectProperty /*type safe!*/, this.Dispatcher /*we need to pass UI dispatcher here*/)
.ListenOn(anotherObject, n => n.AnotherObjectProperty, this.Dispatcher); // chain calling!
The command is listening on NotifyPropertyChanged events from object that affect whether it can execute, and invokes the check only when a requery is needed.
Now, a lot of code (part of our in-house framework) to do this:
I use DelegateCommand from Prism, that looks like this:
/// <summary>
/// This class allows delegating the commanding logic to methods passed as parameters,
/// and enables a View to bind commands to objects that are not part of the element tree.
/// </summary>
public class DelegateCommand : ICommand
{
#region Constructors
/// <summary>
/// Constructor
/// </summary>
public DelegateCommand(Action executeMethod)
: this(executeMethod, null, false)
{
}
/// <summary>
/// Constructor
/// </summary>
public DelegateCommand(Action executeMethod, Func<bool> canExecuteMethod)
: this(executeMethod, canExecuteMethod, false)
{
}
/// <summary>
/// Constructor
/// </summary>
public DelegateCommand(Action executeMethod, Func<bool> canExecuteMethod, bool isAutomaticRequeryDisabled)
{
if (executeMethod == null)
{
throw new ArgumentNullException("executeMethod");
}
_executeMethod = executeMethod;
_canExecuteMethod = canExecuteMethod;
_isAutomaticRequeryDisabled = isAutomaticRequeryDisabled;
this.RaiseCanExecuteChanged();
}
#endregion
#region Public Methods
/// <summary>
/// Method to determine if the command can be executed
/// </summary>
public bool CanExecute()
{
if (_canExecuteMethod != null)
{
return _canExecuteMethod();
}
return true;
}
/// <summary>
/// Execution of the command
/// </summary>
public void Execute()
{
if (_executeMethod != null)
{
_executeMethod();
}
}
/// <summary>
/// Property to enable or disable CommandManager's automatic requery on this command
/// </summary>
public bool IsAutomaticRequeryDisabled
{
get
{
return _isAutomaticRequeryDisabled;
}
set
{
if (_isAutomaticRequeryDisabled != value)
{
if (value)
{
CommandManagerHelper.RemoveHandlersFromRequerySuggested(_canExecuteChangedHandlers);
}
else
{
CommandManagerHelper.AddHandlersToRequerySuggested(_canExecuteChangedHandlers);
}
_isAutomaticRequeryDisabled = value;
}
}
}
/// <summary>
/// Raises the CanExecuteChaged event
/// </summary>
public void RaiseCanExecuteChanged()
{
OnCanExecuteChanged();
}
/// <summary>
/// Protected virtual method to raise CanExecuteChanged event
/// </summary>
protected virtual void OnCanExecuteChanged()
{
CommandManagerHelper.CallWeakReferenceHandlers(_canExecuteChangedHandlers);
}
#endregion
#region ICommand Members
/// <summary>
/// ICommand.CanExecuteChanged implementation
/// </summary>
public event EventHandler CanExecuteChanged
{
add
{
if (!_isAutomaticRequeryDisabled)
{
CommandManager.RequerySuggested += value;
}
CommandManagerHelper.AddWeakReferenceHandler(ref _canExecuteChangedHandlers, value, 2);
}
remove
{
if (!_isAutomaticRequeryDisabled)
{
CommandManager.RequerySuggested -= value;
}
CommandManagerHelper.RemoveWeakReferenceHandler(_canExecuteChangedHandlers, value);
}
}
bool ICommand.CanExecute(object parameter)
{
return CanExecute();
}
void ICommand.Execute(object parameter)
{
Execute();
}
#endregion
#region Data
private readonly Action _executeMethod = null;
private readonly Func<bool> _canExecuteMethod = null;
private bool _isAutomaticRequeryDisabled = false;
private List<WeakReference> _canExecuteChangedHandlers;
#endregion
}
/// <summary>
/// This class allows delegating the commanding logic to methods passed as parameters,
/// and enables a View to bind commands to objects that are not part of the element tree.
/// </summary>
/// <typeparam name="T">Type of the parameter passed to the delegates</typeparam>
public class DelegateCommand<T> : ICommand
{
#region Constructors
/// <summary>
/// Constructor
/// </summary>
public DelegateCommand(Action<T> executeMethod)
: this(executeMethod, null, false)
{
}
/// <summary>
/// Constructor
/// </summary>
public DelegateCommand(Action<T> executeMethod, Func<T, bool> canExecuteMethod)
: this(executeMethod, canExecuteMethod, false)
{
}
/// <summary>
/// Constructor
/// </summary>
public DelegateCommand(Action<T> executeMethod, Func<T, bool> canExecuteMethod, bool isAutomaticRequeryDisabled)
{
if (executeMethod == null)
{
throw new ArgumentNullException("executeMethod");
}
_executeMethod = executeMethod;
_canExecuteMethod = canExecuteMethod;
_isAutomaticRequeryDisabled = isAutomaticRequeryDisabled;
}
#endregion
#region Public Methods
/// <summary>
/// Method to determine if the command can be executed
/// </summary>
public bool CanExecute(T parameter)
{
if (_canExecuteMethod != null)
{
return _canExecuteMethod(parameter);
}
return true;
}
/// <summary>
/// Execution of the command
/// </summary>
public void Execute(T parameter)
{
if (_executeMethod != null)
{
_executeMethod(parameter);
}
}
/// <summary>
/// Raises the CanExecuteChaged event
/// </summary>
public void RaiseCanExecuteChanged()
{
OnCanExecuteChanged();
}
/// <summary>
/// Protected virtual method to raise CanExecuteChanged event
/// </summary>
protected virtual void OnCanExecuteChanged()
{
CommandManagerHelper.CallWeakReferenceHandlers(_canExecuteChangedHandlers);
}
/// <summary>
/// Property to enable or disable CommandManager's automatic requery on this command
/// </summary>
public bool IsAutomaticRequeryDisabled
{
get
{
return _isAutomaticRequeryDisabled;
}
set
{
if (_isAutomaticRequeryDisabled != value)
{
if (value)
{
CommandManagerHelper.RemoveHandlersFromRequerySuggested(_canExecuteChangedHandlers);
}
else
{
CommandManagerHelper.AddHandlersToRequerySuggested(_canExecuteChangedHandlers);
}
_isAutomaticRequeryDisabled = value;
}
}
}
#endregion
#region ICommand Members
/// <summary>
/// ICommand.CanExecuteChanged implementation
/// </summary>
public event EventHandler CanExecuteChanged
{
add
{
if (!_isAutomaticRequeryDisabled)
{
CommandManager.RequerySuggested += value;
}
CommandManagerHelper.AddWeakReferenceHandler(ref _canExecuteChangedHandlers, value, 2);
}
remove
{
if (!_isAutomaticRequeryDisabled)
{
CommandManager.RequerySuggested -= value;
}
CommandManagerHelper.RemoveWeakReferenceHandler(_canExecuteChangedHandlers, value);
}
}
bool ICommand.CanExecute(object parameter)
{
// if T is of value type and the parameter is not
// set yet, then return false if CanExecute delegate
// exists, else return true
if (parameter == null &&
typeof(T).IsValueType)
{
return (_canExecuteMethod == null);
}
return CanExecute((T)parameter);
}
void ICommand.Execute(object parameter)
{
Execute((T)parameter);
}
#endregion
#region Data
private readonly Action<T> _executeMethod = null;
private readonly Func<T, bool> _canExecuteMethod = null;
private bool _isAutomaticRequeryDisabled = false;
private List<WeakReference> _canExecuteChangedHandlers;
#endregion
}
/// <summary>
/// This class contains methods for the CommandManager that help avoid memory leaks by
/// using weak references.
/// </summary>
internal class CommandManagerHelper
{
internal static void CallWeakReferenceHandlers(List<WeakReference> handlers)
{
if (handlers != null)
{
// Take a snapshot of the handlers before we call out to them since the handlers
// could cause the array to me modified while we are reading it.
EventHandler[] callees = new EventHandler[handlers.Count];
int count = 0;
for (int i = handlers.Count - 1; i >= 0; i--)
{
WeakReference reference = handlers[i];
EventHandler handler = reference.Target as EventHandler;
if (handler == null)
{
// Clean up old handlers that have been collected
handlers.RemoveAt(i);
}
else
{
callees[count] = handler;
count++;
}
}
// Call the handlers that we snapshotted
for (int i = 0; i < count; i++)
{
EventHandler handler = callees[i];
handler(null, EventArgs.Empty);
}
}
}
internal static void AddHandlersToRequerySuggested(List<WeakReference> handlers)
{
if (handlers != null)
{
foreach (WeakReference handlerRef in handlers)
{
EventHandler handler = handlerRef.Target as EventHandler;
if (handler != null)
{
CommandManager.RequerySuggested += handler;
}
}
}
}
internal static void RemoveHandlersFromRequerySuggested(List<WeakReference> handlers)
{
if (handlers != null)
{
foreach (WeakReference handlerRef in handlers)
{
EventHandler handler = handlerRef.Target as EventHandler;
if (handler != null)
{
CommandManager.RequerySuggested -= handler;
}
}
}
}
internal static void AddWeakReferenceHandler(ref List<WeakReference> handlers, EventHandler handler)
{
AddWeakReferenceHandler(ref handlers, handler, -1);
}
internal static void AddWeakReferenceHandler(ref List<WeakReference> handlers, EventHandler handler, int defaultListSize)
{
if (handlers == null)
{
handlers = (defaultListSize > 0 ? new List<WeakReference>(defaultListSize) : new List<WeakReference>());
}
handlers.Add(new WeakReference(handler));
}
internal static void RemoveWeakReferenceHandler(List<WeakReference> handlers, EventHandler handler)
{
if (handlers != null)
{
for (int i = handlers.Count - 1; i >= 0; i--)
{
WeakReference reference = handlers[i];
EventHandler existingHandler = reference.Target as EventHandler;
if ((existingHandler == null) || (existingHandler == handler))
{
// Clean up old handlers that have been collected
// in addition to the handler that is to be removed.
handlers.RemoveAt(i);
}
}
}
}
}
I have then written a ListenOn extension method, that 'binds' the command to a property, and invokes its RaiseCanExecuteChanged:
public static class DelegateCommandExtensions
{
/// <summary>
/// Makes DelegateCommnand listen on PropertyChanged events of some object,
/// so that DelegateCommnand can update its IsEnabled property.
/// </summary>
public static DelegateCommand ListenOn<ObservedType, PropertyType>
(this DelegateCommand delegateCommand,
ObservedType observedObject,
Expression<Func<ObservedType, PropertyType>> propertyExpression,
Dispatcher dispatcher)
where ObservedType : INotifyPropertyChanged
{
//string propertyName = observedObject.GetPropertyName(propertyExpression);
string propertyName = NotifyPropertyChangedBaseExtensions.GetPropertyName(propertyExpression);
observedObject.PropertyChanged += (sender, e) =>
{
if (e.PropertyName == propertyName)
{
if (dispatcher != null)
{
ThreadTools.RunInDispatcher(dispatcher, delegateCommand.RaiseCanExecuteChanged);
}
else
{
delegateCommand.RaiseCanExecuteChanged();
}
}
};
return delegateCommand; //chain calling
}
/// <summary>
/// Makes DelegateCommnand listen on PropertyChanged events of some object,
/// so that DelegateCommnand can update its IsEnabled property.
/// </summary>
public static DelegateCommand<T> ListenOn<T, ObservedType, PropertyType>
(this DelegateCommand<T> delegateCommand,
ObservedType observedObject,
Expression<Func<ObservedType, PropertyType>> propertyExpression,
Dispatcher dispatcher)
where ObservedType : INotifyPropertyChanged
{
//string propertyName = observedObject.GetPropertyName(propertyExpression);
string propertyName = NotifyPropertyChangedBaseExtensions.GetPropertyName(propertyExpression);
observedObject.PropertyChanged += (object sender, PropertyChangedEventArgs e) =>
{
if (e.PropertyName == propertyName)
{
if (dispatcher != null)
{
ThreadTools.RunInDispatcher(dispatcher, delegateCommand.RaiseCanExecuteChanged);
}
else
{
delegateCommand.RaiseCanExecuteChanged();
}
}
};
return delegateCommand; //chain calling
}
}
You then need the following extension to NotifyPropertyChanged
/// <summary>
/// <see cref="http://dotnet.dzone.com/news/silverlightwpf-implementing"/>
/// </summary>
public static class NotifyPropertyChangedBaseExtensions
{
/// <summary>
/// Raises PropertyChanged event.
/// To use: call the extension method with this: this.OnPropertyChanged(n => n.Title);
/// </summary>
/// <typeparam name="T">Property owner</typeparam>
/// <typeparam name="TProperty">Type of property</typeparam>
/// <param name="observableBase"></param>
/// <param name="expression">Property expression like 'n => n.Property'</param>
public static void OnPropertyChanged<T, TProperty>(this T observableBase, Expression<Func<T, TProperty>> expression) where T : INotifyPropertyChangedWithRaise
{
observableBase.OnPropertyChanged(GetPropertyName<T, TProperty>(expression));
}
public static string GetPropertyName<T, TProperty>(Expression<Func<T, TProperty>> expression) where T : INotifyPropertyChanged
{
if (expression == null)
throw new ArgumentNullException("expression");
var lambda = expression as LambdaExpression;
MemberExpression memberExpression;
if (lambda.Body is UnaryExpression)
{
var unaryExpression = lambda.Body as UnaryExpression;
memberExpression = unaryExpression.Operand as MemberExpression;
}
else
{
memberExpression = lambda.Body as MemberExpression;
}
if (memberExpression == null)
throw new ArgumentException("Please provide a lambda expression like 'n => n.PropertyName'");
MemberInfo memberInfo = memberExpression.Member;
if (String.IsNullOrEmpty(memberInfo.Name))
throw new ArgumentException("'expression' did not provide a property name.");
return memberInfo.Name;
}
}
where INotifyPropertyChangedWithRaise is this (it estabilishes standard interface for raising NotifyPropertyChanged events):
public interface INotifyPropertyChangedWithRaise : INotifyPropertyChanged
{
void OnPropertyChanged(string propertyName);
}
Last piece of puzzle is this:
public class ThreadTools
{
public static void RunInDispatcher(Dispatcher dispatcher, Action action)
{
RunInDispatcher(dispatcher, DispatcherPriority.Normal, action);
}
public static void RunInDispatcher(Dispatcher dispatcher, DispatcherPriority priority, Action action)
{
if (action == null) { return; }
if (dispatcher.CheckAccess())
{
// we are already on thread associated with the dispatcher -> just call action
try
{
action();
}
catch (Exception ex)
{
//Log error here!
}
}
else
{
// we are on different thread, invoke action on dispatcher's thread
dispatcher.BeginInvoke(
priority,
(Action)(
() =>
{
try
{
action();
}
catch (Exception ex)
{
//Log error here!
}
})
);
}
}
}
This solution is a reduced version of the solution proposed by Tomáš Kafka(thanks to Tomas for describing his solution in detail)in this thread.
In Tomas's solution he had
1) DelegateCommand
2) CommandManagerHelper
3) DelegateCommandExtensions
4) NotifyPropertyChangedBaseExtensions
5) INotifyPropertyChangedWithRaise
6) ThreadTools
This solution has
1) DelegateCommand
2) DelegateCommandExtensions method and NotifyPropertyChangedBaseExtensions method in Delegate Command itself.
Note Since our wpf application follows MVVM pattern and we handle commands at viewmodel level which executes in UI thread we don't need to get the reference to UI disptacher.
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq.Expressions;
using System.Reflection;
using System.Windows.Input;
namespace ExampleForDelegateCommand
{
public class DelegateCommand : ICommand
{
public Predicate<object> CanExecuteDelegate { get; set; }
private List<INotifyPropertyChanged> propertiesToListenTo;
private List<WeakReference> ControlEvent;
public DelegateCommand()
{
ControlEvent= new List<WeakReference>();
}
public List<INotifyPropertyChanged> PropertiesToListenTo
{
get { return propertiesToListenTo; }
set
{
propertiesToListenTo = value;
}
}
private Action<object> executeDelegate;
public Action<object> ExecuteDelegate
{
get { return executeDelegate; }
set
{
executeDelegate = value;
ListenForNotificationFrom((INotifyPropertyChanged)executeDelegate.Target);
}
}
public static ICommand Create(Action<object> exec)
{
return new SimpleCommand { ExecuteDelegate = exec };
}
#region ICommand Members
public bool CanExecute(object parameter)
{
if (CanExecuteDelegate != null)
return CanExecuteDelegate(parameter);
return true; // if there is no can execute default to true
}
public event EventHandler CanExecuteChanged
{
add
{
CommandManager.RequerySuggested += value;
ControlEvent.Add(new WeakReference(value));
}
remove
{
CommandManager.RequerySuggested -= value;
ControlEvent.Remove(ControlEvent.Find(r => ((EventHandler) r.Target) == value));
}
}
public void Execute(object parameter)
{
if (ExecuteDelegate != null)
ExecuteDelegate(parameter);
}
#endregion
public void RaiseCanExecuteChanged()
{
if (ControlEvent != null && ControlEvent.Count > 0)
{
ControlEvent.ForEach(ce =>
{
if(ce.Target!=null)
((EventHandler) (ce.Target)).Invoke(null, EventArgs.Empty);
});
}
}
public DelegateCommand ListenOn<TObservedType, TPropertyType>(TObservedType viewModel, Expression<Func<TObservedType, TPropertyType>> propertyExpression) where TObservedType : INotifyPropertyChanged
{
string propertyName = GetPropertyName(propertyExpression);
viewModel.PropertyChanged += (PropertyChangedEventHandler)((sender, e) =>
{
if (e.PropertyName == propertyName) RaiseCanExecuteChanged();
});
return this;
}
public void ListenForNotificationFrom<TObservedType>(TObservedType viewModel) where TObservedType : INotifyPropertyChanged
{
viewModel.PropertyChanged += (PropertyChangedEventHandler)((sender, e) =>
{
RaiseCanExecuteChanged();
});
}
private string GetPropertyName<T, TProperty>(Expression<Func<T, TProperty>> expression) where T : INotifyPropertyChanged
{
var lambda = expression as LambdaExpression;
MemberInfo memberInfo = GetmemberExpression(lambda).Member;
return memberInfo.Name;
}
private MemberExpression GetmemberExpression(LambdaExpression lambda)
{
MemberExpression memberExpression;
if (lambda.Body is UnaryExpression)
{
var unaryExpression = lambda.Body as UnaryExpression;
memberExpression = unaryExpression.Operand as MemberExpression;
}
else
memberExpression = lambda.Body as MemberExpression;
return memberExpression;
}
}}
Explanation of the solution:
Normally when we bind a UI element(Button)to the ICommand implementation the WPF Button registers for a Event "CanExecuteChanged" in ICommand implementation .If your Icommand implementation for "CanExecuteChanged" hook to the CommandManager's RequesySuggest event(read this article http://joshsmithonwpf.wordpress.com/2008/06/17/allowing-commandmanager-to-query-your-icommand-objects/) then when ever CommandManager detects conditions that might change the ability of a command to execute(changes like Focus shifts and some keyboard events) , CommandManager’s RequerySuggested event occurs which in turn will cause Button'e delegate to be called since we hooked the buttos's delgate to CommandManager’s RequerySuggested in the implementation of "CanExecuteChanged" in our DelegateCommand .
But the problem is that ComandManager is not able to always detect the changes. Hence the solution it to raise "CanExecuteChanged" when our command implementation(DelegateCommand) detects there is a change.Normally when we declare the delagate for ICommand's CanExecute in our viewmodel we bind to properties declared in our viewmodel and our ICommand implementation can listen for "propertychanged" events on these properties. Thats what the "ListenForNotificationFrom" method of the DelegateCommand does. In case the client code does not register for specific property changes the DelegateCommand by defaults listens to any property change on the view model where command is declared and defined.
"ControlEvent" in DelegateCommand which is list of EventHandler that stores the Button's
"CanExecuteChange EventHandler" is declared as weak reference to avoid memory leaks.
How will ViewModel use this DelegateCommand
There are 2 ways to use it.
(the second usage is more specific to the properties that you want the Command to listen to.
delegateCommand = new DelegateCommand
{
ExecuteDelegate = Search,
CanExecuteDelegate = (r) => !IsBusy
};
anotherDelegateCommand = new DelegateCommand
{
ExecuteDelegate = SearchOne,
CanExecuteDelegate = (r) => !IsBusyOne
}.ListenOn(this, n => n.IsBusyOne);
A detailed ViewModel
public class ExampleViewModel
{
public SearchViewModelBase()
{
delegateCommand = new DelegateCommand
{
ExecuteDelegate = Search,
CanExecuteDelegate = (r) => !IsBusy
};
anotherDelegateCommand = new DelegateCommand
{
ExecuteDelegate = SearchOne,
CanExecuteDelegate = (r) => !IsBusyOne
}.ListenOn(this, n => n.IsBusyOne);
}
private bool isBusy;
public virtual bool IsBusy
{
get { return isBusy; }
set
{
if (isBusy == value) return;
isBusy = value;
NotifyPropertyChanged(MethodBase.GetCurrentMethod());
}
}
private bool isBusyOne;
public virtual bool IsBusyOne
{
get { return isBusyOne; }
set
{
if (isBusyOne == value) return;
isBusyOne = value;
NotifyPropertyChanged(MethodBase.GetCurrentMethod());
}
}
private void Search(object obj)
{
IsBusy = true;
new SearchService().Search(Callback);
}
public void Callback(ServiceResponse response)
{
IsBusy = false;
}
private void Search(object obj)
{
IsBusyOne = true;
new SearchService().Search(CallbackOne);
}
public void CallbackOne(ServiceResponse response)
{
IsBusyOne = false;
}
private void NotifyPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
private void NotifyPropertyChanged(MethodBase methodBase)
{
string methodName = methodBase.Name;
if (!methodName.StartsWith("set_"))
{
var ex = new ArgumentException("MethodBase must refer to a Property Setter method.");
throw ex;
}
NotifyPropertyChanged(methodName.Substring(4));
}
}
Tomas has a nice solution, but pls note there's a serious bug in that the CanExecute will not always fire when bound to a Button due to this :
// Call the handlers that we snapshotted
for (int i = 0; i < count; i++)
{
EventHandler handler = callees[i];
handler(null, EventArgs.Empty);
}
The 'null' parameter passed in causes issues with the CanExecuteChangedEventManager (used by the WPF Button class to listen to changes on any Command bound to it). Specifically, the CanExecuteChangedEventManager maintains a collection of weak events that need to be invoked to determine if the command Can-Execute() but this collection is keyed by the 'sender'.
The fix is simple and works for me - change the signature to
internal static void CallWeakReferenceHandlers(ICommand sender, List<WeakReference> handlers)
{
....
handler(sender, EventArgs.Empty);
}
Sorry I haven't described it too well - in a bit of a rush to catch up with my dev now after taking a few hours to figure this out !
I would suggest looking into ReactiveUI and specifically at the ICommand implementation it provides, ReactiveCommand. It uses a different approach than DelegateCommand/RelayCommand which are implemented with delegates for CanExecute that must be actively evaluated. ReactiveCommand's value for CanExecute is pushed using IObservables.
is there a way I can make CommandManager.InvalidateRequerySuggested() do it's job faster?
Yes, there is way to make it work faster!
Implement Command to keep / cache CanExecuteState in a boolean variable.
Implement RaiseCanExecuteChanged method to recalculate CanExecuteState and if it really changed to raise CanExecuteChanged event.
Implement CanExecute method to simply return CanExecuteState.
When InvalidateRequerySuggested method is invoked Command subscribers will only read CanExecuteState variable by invoking CanExecute method and check if it changed or not. That's almost zero overhead. All Commands will be disabled / enabled almost the same time.
All work will be done in RaiseCanExecuteChanged method that will be called only once for a Command and only for a limited set of Commands.
Try writing your own binding that calls your RaiseCanExecuteChanged() within converts? it is easier
Just to clarify:
You want to fire an update of CanExecute when Command property changed
Create your own binding class that detect changes in the Command property and then calls RaiseCanExecuteChanged()
Use this binding in CommandParameter
Worked for me.