bool CanExecute(object parameter);
void
Execute(object parameter);
event EventHandler CanExecuteChanged;
The Execute method is only invoked when CanExecute returns true. In case the CanExecute method returns false then the binding control is disabled automatically.
In order to know the CanExecute value, listen to the CanExecuteChanged event, that may vary based on the parameter passed.
Using Code
Use of ICommand
The ICommand interface is generally used in the MVVM architecture. Here in the Button control the Command property is bound to the "UpdateCommand". Since UpdateCommand is nothing but an ICommand instance, while loading the window it will check the CanExecute return value and if it returns true then it will enable the button control and the Execute method is ready to be used otherwise the button control is disabled.
<Button x:Name=
"btnUpdate"
Width=
"100"
Height=
"20"
HorizontalAlignment=
"Center"
Grid.Row=
"1"
Content=
"Update"
Command=
"{Binding Path=UpdateCommand}"
/>
Passing a parameter to the CanExecute and Execute methods
A parameter can be passed through the "CommandParameter" property. Once the button is clicked the selected address value is passed to the ICommand.Execute method.
The CommandParameter is sent to both CanExecute and Execute events.
<Button x:Name=
"btnUpdate"
Width=
"100"
Height=
"20"
HorizontalAlignment=
"Center"
Grid.Row=
"1"
Content=
"Update"
Command=
"{Binding Path=UpdateCommand}"
CommandParameter=
"{Binding ElementName=lstPerson, Path=SelectedItem.Address}"
>
</Button>
Tasks of CanExecuteChanged
CanExecuteChanged notifies any command sources (like a Button or CheckBox) that are bound to that ICommand that the value returned by CanExecute has changed. Command sources care about this because they generally need to update their status accordingly (eg. a Button will disable itself if CanExecute() returns false).
Listen CanExecuteChanged
The CommandManager.RequerySuggested event is raised whenever the CommandManager thinks that something has changed that will affect the ability of commands to execute. This might be a change of focus, for example. This event happens to fire a lot.
public
event
EventHandler CanExecuteChanged
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
ICommand in MVVM
Model
Create a folder named Model and a class named Person. Implement INotifyPropertyChanged and override the PropertyChanged event. Define two properties, Name and Address.
public
event
PropertyChangedEventHandler PropertyChanged;
public
void
OnPropertyChanged(
string
propertyName)
if
(PropertyChanged !=
null
)
PropertyChanged(
this
,
new
PropertyChangedEventArgs(propertyName));
public
string
Name
return
name;
name = value;
OnPropertyChanged(
"Name"
);
public
string
Address
return
address;
address = value;
OnPropertyChanged(
"Address"
);
ViewModel
Create a folder named ViewModel and a class named PersonViewModel. Initialize a List object of type person. Expose this object through the Persons property. Also expose an ICommand instance in this class.
Define a class Updater and implement ICommand interface.
_personList =
new
List<Person>()
new
Person(){Name=
"Prabhat"
, Address=
"Bangalore"
},
new
Person(){Name=
"John"
,Address=
"Delhi"
}
public
IList<Person> Persons
get
{
return
_personList; }
set
{ _personList = value; }
private
ICommand mUpdater;
public
ICommand UpdateCommand
if
(mUpdater ==
null
)
mUpdater =
new
Updater();
return
mUpdater;
mUpdater = value;
class
Updater : ICommand
#region ICommand Members
public
bool
CanExecute(
object
parameter)
return
true
;
public
event
EventHandler CanExecuteChanged
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
public
void
Execute(
object
parameter)
#endregion
Create a view folder and a Person.xaml class. Bind the command to the button control. Once the button is clicked, the command parameter is passed to the Execute method. You can also bind a command to any control that supports the command property. The TextBox is bound with listView selectedItem.
<Grid.ColumnDefinitions>
<ColumnDefinition Width=
"100"
></ColumnDefinition>
<ColumnDefinition Width=
"*"
></ColumnDefinition>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height=
"auto"
></RowDefinition>
<RowDefinition Height=
"auto"
></RowDefinition>
</Grid.RowDefinitions>
<Label x:Name=
"lblName"
Content=
"Name"
Grid.Row=
"0"
Grid.Column=
"0"
VerticalAlignment=
"Top"
></Label>
<TextBox x:Name=
"txtName"
Grid.Row=
"0"
Grid.Column=
"1"
VerticalAlignment=
"Top"
Text=
"{Binding ElementName=lstPerson, Path=SelectedItem.Name}"
>
</TextBox>
<Label x:Name=
"lblAddress"
Content=
"Address"
Grid.Row=
"1"
Grid.Column=
"0"
VerticalAlignment=
"Top"
></Label>
<TextBox x:Name=
"txtAddress"
Grid.Row=
"1"
Grid.Column=
"1"
VerticalAlignment=
"Top"
Text="{Binding ElementName=lstPerson
Path=SelectedItem.Address}"></TextBox>
</Grid>
<Button x:Name=
"btnUpdate"
Width=
"100"
Height=
"20"
HorizontalAlignment=
"Center"
Grid.Row=
"1"
Content=
"Update"
Command=
"{Binding Path=UpdateCommand}"
CommandParameter=
"{Binding ElementName=lstPerson, Path=SelectedItem.Address}"
></Button>
<ListView x:Name=
"lstPerson"
Grid.Row=
"2"
ItemsSource=
"{Binding Persons}"
>
<ListView.View>
<GridView>
<GridViewColumn Header=
"Name"
DisplayMemberBinding=
"{Binding Name}"
/>
<GridViewColumn Header=
"Address"
Width=
"200"
DisplayMemberBinding=
"{Binding Address}"
/>
</GridView>
</ListView.View>
</ListView>
</Grid>
App.xaml.cs
Override the OnStartup method of the Application class. Initialize a view and viewModel class and wrap a viewmodel to the view by the DataContext property.
base
.OnStartup(e);
View.Person person =
new
View.Person();
PersonViewModel personViewModel =
new
PersonViewModel();
person.DataContext = personViewModel;
person.Show();
Summary
What is ICommand in WPF.
How ICommand used in MVVM architecture.
MVVM architecture flow.
How to pass parameter in CanExecute and Execute method.
How to listen to CanExecuteChanged event.
Point Of Interest
Instead of events, commands have been used in the MVVM architecture. We cannot pass a parameter to the event but we can pass a parameter through a command.