ICommand - RelayCommand CanExecute method does not update button IsEnabled property

Question

I have the following implementation of an RelayCommand in my viewModel:

RelayCommand _resetCounter;

private void ResetCounterExecute()
{
    _data.ResetCounter();
}

private bool CanResetCounterExecute()
{
    if (_data.Counter > 0)
    {
        return true;
    }
    else
    {
        return false;
    }

}

public ICommand ResetCounter
{ 
    get 
    {

        if (_resetCounter == null)
        {
            _resetCounter = new RelayCommand(this.ResetCounterExecute,this.CanResetCounterExecute);
        }
        return _resetCounter; 
    }
}

By calling _data.ResetCounter(); in the ResetCounterExecute method i reset the counter value to 0 in my model.

And this is the implementation of my RealyCommand Class that i use based on samples.

internal class RelayCommand : ICommand
{
    readonly Action _execute;
    readonly Func<bool> _canExecute;

    public RelayCommand(Action execute)
        : this(execute, null)
    {
    }

    public RelayCommand(Action execute, Func<bool> canExecute)
    {
        if (execute == null)
            throw new ArgumentNullException("execute");

        _execute = execute;
        _canExecute = canExecute;
    }

    [DebuggerStepThrough]
    public bool CanExecute(object parameter)
    {
        return _canExecute == null ? true : _canExecute();
    }

    public event EventHandler CanExecuteChanged
    {
        add
        {
            if (_canExecute != null)
                CommandManager.RequerySuggested += value;
        }
        remove
        {
            if (_canExecute != null)
                CommandManager.RequerySuggested -= value;
        }
    }

    public void Execute(object parameter)
    {
        _execute();
    }
}

In XAML i bind the comman to a button:

<Button Name="btnResetCount" Content="Reset" Command="{Binding Path=CounterViewModel.ResetCounter}" Click="btnResetCount_Click">

My Problem is that the button just gets enabled once i click on any control in the UI. But what i need is that the button gets enabled once my _data.Counter > 0 applies. So from my research it seems that i need to implement CommandManager.InvalidateRequerySuggested(); or use the RelayCommand.RaiseCanExecuteChanged() .

I would like to know if this two ways are the only ways to notify the UI to refresh the bindings.

Also i would like to ask how i would have to implement the RelayCommand.RaiseCanExecuteChanged() in my current case. Where and how should i raise it to ensure that the UI changes the button state if the condition is given.

Thanks in advance.

Answer 1

when using CommandManager.RequerySuggested you can force CommandManager to invoke RequerySuggested event by calling CommandManager.InvalidateRequerySuggested()

or you may perhaps implement RaiseCanExecuteChanged . this may be more reliable method to trigger the same.

example

internal class RelayCommand : ICommand
{
    ...

    public event EventHandler CanExecuteChanged;

    public void RaiseCanExecuteChanged()
    {
        EventHandler handler = CanExecuteChanged;
        if (handler != null)
            handler(this, EventArgs.Empty);
    }

    ...
}

and when you want to invalidate or _data.Counter changes, call

ResetCounter.RaiseCanExecuteChanged();

additionally you may also want to read How does CommandManager.RequerySuggested work?