启用DelegateCommand

Question

Little puzzled

If i have a button in a wpf window and bind it to a DelegateCommand

<Button Grid.Row="1"  Content="Remove" Command="{Binding CommandDelete }" />

And in my simple viewmodel i attach

CommandDelete = new DelegateCommand<string>(OnDeleteExecute, OnDeleteCanExecute);

If the button is to be enabled when i choose an item from a list i can bind the list selecteditem to property in my viewmodel

Report selectedReport;
            public Report SelectedReport
            {
                get { return this.selectedReport ;}
                set { this.selectedReport = value;}
            }
private bool OnDeleteCanExecute(string commandParameter)
            {
                return (this.selectedReport != null);
            }

This seems fine to me so far and the only thing missing is to raise a CanExecute event for the specific button in the setter for SelectedReport

CommandDelete.RaiseCanExecuteChanged();

This works but my question is if i have 10 button do i need to call RaiseCanExecuteChanged for each and every button each time a item is selected or is there as smarter way

Answer 1

Usually I place my RaiseCanExecuteChanged() in the PropertyChanged event for that class.

For example, if DeleteCommand.CanExecute is based on the SelectedReport property, I'll hook into the PropertyChanged event of the ViewModel and raise the CanExecuteChanged event anytime SelectedReport changes.

void MyViewModel_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    if (e.PropertyName == "SelectedReport")
        DeleteCommand.RaiseCanExecuteChanged();
}

This keeps all the logic in one central place and ensures the CanExecuteChanged gets raised whenever one of the parameters changes.

The other alternative is to switch from using a DelegateCommand to a RelayCommand , which automatically raise it's CanExecuteChanged when a property changes. I would assume there's a performance difference, however have never noticed one.